DMMs

Posts related to DMMs:
7/9/12
Increasing DMM Measurement Throughput

The vast majority of electronic tests involve using a digital multimeter (DMM) at one time or another. There are a variety of ways to reduce DMM measurement times to improve overall test throughput. Of course, test time improvements sometimes require compromises in other areas, but knowing the tradeoffs involved in throughput improvements and identifying what is important in your specific test situation will help you determine which trade-offs make the most sense.

Auto zero: Accuracy versus test time
Auto zero is a DMM feature that helps you improve accuracy. When you use the auto zero feature, the DMM makes an additional zeroing measurement with each measurement you make, thereby eliminating the
offsets of the amplifier and integration stages inside the DMM. However, turning this feature off cuts the measurement time in half. These offsets are initially calibrated out, but the offsets can drift slightly with a change in temperature. Therefore, if your measurements are taken in an environment with a stable temperature, or if there are several measurements taken in a short period of time (temperature changes occur over longer periods of time), the improvements in throughput by turning auto zero off will far outweigh any slight compromise in accuracy. For example, with auto zero off in a stable environment, the Agilent 34410A/11A DMMs typically adds only an additional 0.0002% of range +2 μV to the DC voltage accuracy specification. Note that with auto zero off, any range, function, or integration time setting change can cause a single auto zero cycle to be performed on the first reading using the new setting. Consequently, turning auto zero off and constantly changing settings defeats the time savings advantage. Check your DMM auto zero operation to be sure of the circumstances leading to an advantage from this change.

34410A DMM

Reduce the number of changes
Changing functions or measurement ranges also requires extra time in most DMMs. Try to group your measurements to minimize function changes and range changes. For example, if you make some voltage measurements and some resistance measurements, try to do all of the voltage measurements together and all the resistance measurements together instead of changing back and forth from one function to the other. Also, try to group your low-voltage measurements together and your high-voltage measurements together to minimize range changing. Voltage ranges above 10 V use a mechanical attenuator that takes time to switch in and out. Grouping your measurements by function and range will reduce your measurement times considerably.

Auto range variations
Auto range time can sometimes contribute to longer test times, but not always. The time to auto range varies with the DMM design. DMMs using flash A/D converters and parallel gain amplifiers can actually reduce test times by using auto ranging, since the time to change ranges is zero. In these cases, the time to issue a range change command from a host computer and parse the command in the instrument will be slower. Manual ranging of integrating DMMs is still the fastest way to take a measurement. Manual ranging also allows you to keep the DMM on a fixed range, which eliminates unwanted zero measurements and prevents the mechanical attenuator from needlessly actuating. Note that the I/O speed and range command parse time for the Agilent 34410/11A DMM is significantly faster than the auto range algorithm.

Integration time versus noise
Integration time is another parameter over which you have direct control, but there is a clear tradeoff. DMMs integrate their measurements over a set period of time: the integration time. The biggest benefit to choosing a longer integration time is it eliminates unwanted noise from contributing to your measurement, especially AC mains line voltage noise. However, longer integration times obviously increase your measurement times. For example, if the integration time is set to an integral number of power line cycles (NPLCs) such as 1, 2, 10, or 100, the power line noise contribution will be minimized due to averaging over a longer period of time and due to increasing the normal mode rejection (NMR). With an NPLC setting of 10 in a 60-Hz environment, the integration time is 166 ms (200 ms for a 50-Hz line). The larger the integral NPLC value, the larger the NMR (for example, 60 Hz rejection), but the longer the measurement time.

DMMs are used in virtually all electronic test systems; therefore, making conscious choices about how to make DMM measurements can save large amounts of test time, thereby increasing throughput. Here is a helpful checklist for better throughput:
  • If appropriate, turn auto zero off
  • Minimize function and range changes
    • Group similar measurement functions together (DCV, DC ohms, ACV, etc.)
    • Use fixed ranges instead of auto range, if appropriate
    • Shorten integration time with consideration for noise rejection, resolution, and accuracy

For more info on Agilent DMMs click here



6/4/12
Measuring the Triple Point of Water

In this post we take a look at a colleague of mine cover what the triple point of water is, how to measure it, and how to compensate for the error in a Standard Platinum Resistance Thermometer (SPRT). The video is long but interesting especially if you are into Metrology.





To learn more about the 34420A Micro-OHM Meter click here



4/16/12
Understanding Switch Types for Automated Test

Switches serve as the central nervous system in automated test. They interface between the DUT and the test instruments routing signals to and from the DUT. In this post we will look at different low frequency switch types and discuss some of their pros and cons to help you choose the right type of switch for your measurement needs.

The types of switch we will be looking at in this post include: armature relays, reed relays, solid-state switches, and mercury-wetted relays. The following is a quick overview of each.

Armature relays — Because of their ruggedness, cost, and ability to handle higher currents and voltages, armatures are the most commonly used relays. Armature relays usually have low resistance. They generally have slower switch times, and they are somewhat more susceptible to arcing and switch bounce than the other types. Some armature relays are sealed; others are not.

Typical Lifetime: 10M
Typical speed: 250/s

Reed relays — When you need to switch at high speeds, reed relays typically are a good choice. In general, reed relays switch much faster than armature relays, have very low contact resistance and offer the added benefit of being hermetically sealed. They do not have the capacity to carry as high voltages and currents as armature relays.

Typical Lifetime: 10M
Typical speed: 2000/s

Reed Relay
Solid-state switches — Solid-state switches can cover low to high power switching applications, for instance they can be used for switching ac line voltages. The two main advantages of solid-state switches is their speed and no moving parts. Solid-state switches have the fastest switch times of all the switches. Since they have no moving parts there is no arcing or switch-bounce problems. They basically have an infinite lifetime as long as they are used within their power ranges. However, they generally have the highest "on" resistance of the switches and their isolation and crosstalk specs are typically the worst of the group.

Typical Lifetime: Inifinite
Typical speed: 4000/s


Mercury-wetted relays — Of all the switches mercury-wetted relays are the least common. These switches use liquid mercury inside to avoid switch bounce. They also have a long life compared to the other mechanical switches and have very low contact resistance. However, they are position-sensitive due to the liquid mercury, and must be mounted in the correct orientation to operate properly. They can also be expensive. These switches are used in applications were switch bounce cannot be tolerated and a low "on" resistance is needed. Today there are not many test and measurement companies who provide mercury-wetted relay cards or products so you typically have to integrate them into your test system on some type of custom platform.

Typical Lifetime: 10M
Typical speed: 50/s

Mercury-Wetted Relay
The best way to ensure your relays have a long life is by ensuring that they are used within their power ratings. But with mechanical relays, namely armature and reed, the lifetime can also be dependent on the type of load they are used with. This is due to their susceptibility to arcing which occurs from the electric field at the relay when it is opened or closed. Relay manufacturers specify how long their relays will last, but the expected lifetime will vary depending on the loads they are subjected to. For resistive loads, manufacturers’ specifications are typically fairly accurate. On the other hand, if you are using capacitive or inductive load, your relay life span will be shorter than the manufacturers specification. How much shorter depends on the type of loads you are switching. One technique you can use to better manage the lifetime of your switch relays is derating. Derating gives you a realistic picture of how long your relay will last. Loads can be classified into four general groups:

Resistive loads — Relay manufacturers assume you will be using resistive loads when they rate their relays. The load is a simple resistive element, and it is assumed that the current flow through the contacts will be fairly constant, although some increase may occur due to arcing during “make” or “break.” Ideally, a relay with a purely resistive load can be operated at its stated voltage and current ratings and attain its full lifetime. Industry practice, however, is to derate to 75 percent of the relay’s stated capacity.

Inductive loads — Switching inductive loads is difficult, primarily because current tends to continue to flow in inductors, even as contacts are being broken. The stored energy in inductors induces arcing; arc-suppression schemes are frequently used. When you are switching inductive loads, you typically will want to derate relay contacts to 40 percent of the resistive load rating.

Capacitive loads — Capacitors resemble short circuits when they are charging, so the in-rush current from a capacitive load can be very high. Series resistors are often used to limit in-rush current; without a limiting resistor, contact welding may occur. Its common today for switch card makers to integrate current protection resistors into their products. These protection resistors can be switched in or out of the current path as needed. When you are switching capacitive loads, you typically will want to derate your relay to 75 percent of the resistive rating.

Motor loads — When an electric motor starts up, it has very low impedance and requires a large in-rush current to begin building a magnetic field and begin rotating. Once it is running, it generates a back electromagnetic force (emf), which can cause a large inductive spike when the switch is opened. The result is a large in-rush current at “turn-on” and arcing at “turn-off.” When you are switching a motor load, typical industry practice is to derate to 20 percent of the resistive rating.


In this post we looked at an overview of different switch relay types for low frequency automated test purposes. From there we discussed switch lifetime, focusing on armature and reed relay types. If you have any questions on this post send me an email and if you have anything to add use the comments section below.

Click here to check out Agilent's switching solutions



2/21/12
Using a DMM as a Low Frequency Analyzer

Just about every electrical engineer and technician has a DMM on their bench. They are used for their high resolution and high accuracy voltage, current, and resistance measurement capabilities. One capability that most high performance DMMs have that users are typically not aware of is their low frequency digitizing capability. In this post we will look at combining a DMM's high resolution measurement capability with its digitizing capability to create a Low Frequency Analyzer (LFA). We will also look at some free software for performing LFA functionality with a DMM.

Most modern high performance DMMs provide a digitizing capability built-in. The sample rate is adjustable and ranges from around 1 S/s at greater than 20 bits of resolution to 50 KS/s at 14 bits of resolution. At 50 KS/s we can analyze signal frequency components below 25 KHz. This makes the DMM a useful solution for analyzing signals in applications such as:
  • Measuring total harmonic distortion on power line signals. Click here to check out an article on this application.
  • Vibration measurement and analysis.
  • Audio frequency analysis.
The challenge is there is no useful way to analyze the digitized measurements on the DMM itself. To make the digitized measurements available for analysis we have to pull them off the DMM and post process them. If we want to use our DMM as LFA that post processing includes performing a Fast Fourier Transform (FFT) on the measurements so we can view the signal's frequency components.

Let's use Agilent's 34411A high performance DMM as our example DMM we want to use as a LFA. The 34411A provides two ways to access its digitizer functionality and retrieve the readings. The first is using its LXI web interface via a LAN connection to the instrument. This method provides the advantage of needing no custom software. All you need is a web browser. There is a past GPETE post that shows a video using the 34411A as a digitizer via its LXI web interface. To check out the video click here. Once you have the readings from the web interface you can then transfer them to a program like Excel for FFT analysis. Click here to learn how to do FFT analysis in Excel.

The second way is to use custom software to connect to the 34411A, set it up as a digitizer, retrieve the measurements, and analyze them. Popular test and measurement software environments like Matlab, LabView, and VEE provide instrument drivers, FFT libraries, and plotting capabilities to make this easy as possible for experienced programmers. But if your not an experience programmer or you just don't have time to put together some custom software, I created two free programs that allow you to use the following DMMs as low cost LFAs: 34411A, 34410A, and the L4411A. Both programs use LAN to connect and control the DMMs. The first program is based off of Matlab and is entitled "Dynamic Signal Analyzer 34411A." It can be downloaded from Matlab Central (link: http://www.mathworks.com/matlabcentral/fileexchange/35161). As an example I used Dynamic Signal Analyzer 34411A to capture a 60 Hz power line signal for analysis using the 34411A. The resulting frequency domain and time domain plots from the program can be seen below.
Frequency spectrum from 60 Hz power line signal

The second LFA program was done using Agilent VEE and it is called "34411A LFA." The great thing about VEE is you can run a VEE program without using any for pay software. All you need to do to run a VEE program is to download its free run time environment. If you are interested in the VEE program just shoot me an email, tell me about your application, and I will send you the program with some instructions (neil_forcier@agilent.com). Keep in mind that both of these programs are offered "as is" and are not supported by Agilent (just me). 

In this post we talk about how high performance DMMs, with their high resolution and digitizing capabilities, can be used as a low cost LFA. We talk about how we can access the digitized measurements for post processing and analysis. Finally we looked at two free programs available to you for using the following Agilent DMMs as LFAs: 34411A, 34410A, and the L4411A. If you have any comments or personal experiences to add to this post use the comments section below.



1/6/12
Triggering a DMM with No Hands

Making a DMM measurement on a really really small pin on a chip is not an easy task. You drink a little less coffee that morning and carefully position your probe tip on the pin. Then your frozen scared to move your head to look at the DMM's display in fear the probe tip will slip off before you catch a glimpse of the reading. Has this ever happen to you? In this post we will look at easy way that a colleague of mine came up with to capture a DMM reading on the display without moving your head or your hands.

All you need is a DMM with external triggering capability, a mechanical switch, and some cabling. Here is the setup:

  • Set the DMM for manual external triggering. This means the DMM will only take a measurement on an external trigger event.
  • Position the switch on the ground under your bench by your feet.
  • Wire up the switch to the external trigger input of the DMM
From there position your probe tip on the tiny measurement pin and without taking your eyes off the measurement or moving your hands, toggle the switch with your foot. Remove your probes from the measurement point and look up to view your captured measurement on the DMM display.

When you close the switch with your foot the external triggering input of the DMM is pulled low, which in turn triggers a measurement on the DMM. To avoid the hassle of ordering a switch you could always cannibalize it from an old piece of electronics. For instance, my colleague used an old mouse. They wired a BNC adapter to a switch connected to one of the mouse buttons. The below figures show the mouse setup.




If you have any helpful comments or personal incites to add to this post just use the "Post a Comment" section below.




12/9/11
Digitizing with a DMM

The following video demonstrates how to use one of Agilent's high performance DMMs as a low frequency digitizer and plot the result without writing any code. This can be useful in applications like DC to AC inverter distortion analysis. Where you want to capture the resulting AC voltage or current waveform from the inverter output to analyze its distortion in the time or frequency domain.


The video demonstrates collecting the measurement data from memory using the DMM's web interface and then transferring to Excel by cutting and pasting. Another easy way to do this without any code is by using Agilent's free Command Expert software. Command Expert allows you to create an Excel spreadsheet that will connect to the DMM, configure the measurement, execute the measurement, collect the measurement data, and plot it. Click here to check out my post on Command Expert.

Click here to go to the 34411A DMM product page



11/28/11
Using Offset Compensation when Making Resistance Measurements

Here is a short straight forward video showing how the offset compensation (OCOMP) feature on DMMs is used. This feature is great when making resistance measurements on active circuits or any DUT that has stray voltage. Enjoy!



Click here to check out more test and measurement video tutorials on YouTube

Click here for more info on Agilent DMMs



11/11/11
Comprehensive Look at Sources of Error in DC Voltage Measurements Part 2

This is part 2 of a 2 part post that takes a comprehensive look at all of the factors that can lead to errors in a DC voltage measurement with a DMM and how to eliminate them so you can achieve the highest accuracy possible in your measurement. In part 2 we will cover the following topics: loading errors,  power-line noise, injected current noise, and ground loop errors. If you are a seasoned DMM measurement veteran and you feel I missed something in the following sections please add it as a comment.

Loading Errors Due to Input Resistance — Measurement loading errors occur when the resistance of the DUT is an appreciable percentage of the DMM’s own input resistance. The figure below shows this error source. To reduce the effects of loading errors, and to minimize noise pickup, see if your DMM allows you to set its input resistance to a higher value. For instance, Agilent 34401A’s input resistance can be set from 10 M to > 10 G for the 100 mVdc, 1 Vdc, and 10 Vdc ranges.

Ri should be much larger than Rs or loading error will be a factor in the measurement
Power-Line Noise — This type of noise is caused by the powerline voltage signal (50 Hz or 60 Hz) being coupled onto the measurement setup either from the DUT, the DMM, or both. This noise appears as an AC ripple summed on top of the DC level you are measuring. To eliminate this common noise source DMM designers use integrating or averaging measurement time settings that are integer multiples of the powerline noise's period. Remember if you integrate over a sine wave you get zero. This is typically called normal mode rejection or NMR. If you set the integration time to an integer value of the powerline cycles (PLCs) of the spurious input, these errors (and their harmonics) will  average out to approximately zero. For instance, the Agilent 34401A provides three integration times to reject power-line frequency noise (and power-line frequency harmonics). When you apply power to the DMM, it measures the power-line frequency (50 Hz or 60 Hz), and then determines the proper integration time. The table below shows the noise rejection achieved with various configurations. For better resolution and increased noise rejection, select a longer integration time.



Noise Caused by Injected Current — Residual capacitances in the DMM’s power transformer cause small currents to flow from the LO terminal to earth ground. The frequency of the injected current is the power line frequency or possibly harmonics of the power line frequency. The injected current is dependent upon the power line configuration and frequency. With Connection A (see figure below), the injected current flows from the earth connection provided by the circuit to the LO terminal of the DMM, adding no noise to the measurement. However, with Connection B, the injected current flows through the resistor R, thereby adding noise to the measurement. With Connection B, larger values of R will worsen the problem.


The measurement noise caused by injected current can be significantly reduced by setting the integration time of the DMM to 1 power line cycle (PLC) or greater.

Ground Loop Error — I have done two posts on this topic and the links for them are below. The second being the more thorough one.

Understanding Ground Loop Error in Voltage Measurements


If you think I missed anything or if you have a question please leave it in a comment




11/7/11
Comprehensive Look at Sources of Error in DC Voltage Measurements Part 1

In this two part post we will (or at least attempt to) take a comprehensive look at all of the factors that can lead to errors in a DC voltage measurement with a DMM and how to eliminate them so you can achieve the highest accuracy possible in your measurement. In part one we will cover radio frequency interference, thermal EMF errors, noise caused by magnetic fields, and common mode rejection. If you are a seasoned DMM measurement veteran and you feel I missed something in the following sections please add it as a comment.

Radio Frequency Interference -- Most voltage-measuring instruments can generate false readings in the presence of large, high-frequency signal sources such as nearby radio and television transmitters, computer monitors, and cellular telephones. Especially when the high frequency energy is coupled to the multimeter on the system cabling. This effect can be severe when the cabling is 1/4, 1/2, or any integer multiple of the high frequency wavelength. You probably have experienced this type of effect first hand if you ever placed a mobile phone near speaker wiring and heard bursts of noise from the speaker that were certainly not part of the intended audio experience. To reduce interference, try to minimize the exposure of the system cabling to high-frequency RF sources. You can add shielding to the cabling or use shielded cabling.  If the measurement is extremely sensitive to RFI radiating from the DMM or your DUT, use a common mode choke in the system cabling, as shown in the figure below, to attenuate DMM emissions. Often you can see this same EMI reducing method being used on the data cable for your computer monitor.


Thermal EMF Errors -- Thermoelectric voltages, the most common source of error in low level voltage measurements, are generated when circuit connections are made with dissimilar metals at different temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the junction temperature. It is a good idea to take the necessary precautions to minimize thermocouple voltages and temperature variations in low level voltage measurements. The best connections are formed using copper-to-copper crimped connections. The figure below shows common thermoelectric voltages for connections between dissimilar metals.

Agilent benchtop DMMs use copper alloy for their input connectors
Noise Caused by Magnetic Fields -- When you make measurements near magnetic fields, take precautionary steps to avoid inducing voltages in the measurement connections. Voltage can be induced by either movement of the input connection wiring in a fixed magnetic field, or by a varying magnetic field. An unshielded, poorly dressed input wire moving in the earth’s magnetic field can generate several millivolts. The varying magnetic field around the ac power line can also induce voltages up to several hundred millivolts. Be especially careful when working near conductors carrying large currents. Where possible, route cabling away from magnetic fields, which are commonly present around electric motors, generators, televisions and computer monitors. In addition, when you are operating near magnetic fields, be certain that the input wiring has proper strain relief and is tied down securely. Use twisted-pair connections to the multimeter to reduce the noise pickup loop area, or dress the wires as closely together as possible.

For more on magnetic coupling and other spurious coupling issues in measurements check out the post Ground Loops and Other Spurious Coupling Mechanisms and How to Prevent Them

Common Mode Rejection (CMR) -- Ideally, a DMM is completely isolated from earth-referenced circuits.  However, there is finite resistance between the DMM’s input LO terminal and earth ground. This can cause errors when measuring low voltages that are floating relative to earth ground. Check out the post Understanding Common Mode DMM Specifications for more information on CMR.

Stay tuned for part 2 next week!




9/19/11
DMM Resistance Measurement Considerations

The digital multimeter or DMM offers two methods for measuring resistance: 2–wire and 4–wire ohms. For both methods, the test current flows from the input HI terminal and then through the resistor being measured. For 2–wire ohms, the voltage drop across the resistor being measured is sensed internal to the multimeter. Therefore, test lead resistance is also measured. For 4–wire ohms, separate "sense" connections are required. Since no current flows in the sense leads, the resistance in these leads does not give a measurement error. In this blog post I will discuss some general considerations and tips when making DMM resistance measurements.

4–Wire Ohms Measurements
4-wire ohm measurement use the HI and LO DMM leads as well as the HI-Sense leads (that is why they are called "4-wire"), the setup for a 4-wire ohms measurement is shown below. 

The sense leads essentially extend the DMM measurement to the DUT junctions instead of the HI and LO terminals . This eliminates the voltage drop across the HI and LO leads caused by the test current. Since the sense leads are high impedance there is essentially no current flow into the sense inputs. The 4–wire ohms method provides the most accurate way to measure small resistances. Test lead resistances and contact resistances are automatically reduced using this method. Four–wire ohms is often used in automated test applications where resistive and/or long cable lengths, numerous connections, or switches exist between the DMM and the DUT.

Removing Test Lead Resistance Errors
Modern DMMs offer a built-in function, often labeled as "Null" or "Math", for eliminating test lead error.To use the Math function on a DMM you short the test leads to together. The Math function will then make a resistance measurement of the test leads and store it. The DMM will then mathematically subtract the measured lead resistance for subsequent resistance measurements to cancel out the lead resistance error.

Minimizing Power Dissipation Effects
When measuring resistors designed for temperature measurements (or other resistive devices with large temperature coefficients), be aware that the DMM will dissipate some power in the device–under–test. If power dissipation is a problem, you should select the DMM's next higher measurement range to reduce the errors to acceptable levels. The following table shows examples of Agilent's 34410A and 34411A DMMs source current for various measurement ranges.


Errors in High Resistance Measurements
When you are measuring large resistances, significant errors can occur due to insulation resistance and surface cleanliness. You should take the necessary precautions to maintain a "clean" high–resistance system. Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and "dirty" surface films. Nylon and PVC are relatively poor insulators (10^9 Ω) when compared to PTFE (Teflon) insulators (10^13 Ω). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 MΩ resistance in humid conditions.

Click here to check out the DMMs that Agilent offers


7/26/11

Using a PXI DMM & VI Source to Make Parametric Measurements


6/28/11

PXI DMM Speed Showdown: Agilent vs Brand X


Below is a video demonstrating how Agilent's M9183A PXI DMM is over 10 times faster in a transactional test setup compared to Brand X's PXI DMM (you can probably guess who Brand X is). Transactional testing refers to automated test setups where the DMM and switching has to be reconfigured for each measurement or test point on the DUT. That means at each test point the DMM is configured, armed (*INIT command), and the measurement is fetched from memory into the test software.Everything in the test is the same except the DMMs so each test setup has the same switching, same PXI chassis, same code, and the DMM measurement aperture time is the same. Check out the video below.

The reason that Brand X's DMM is so much slower is because its measurement arming time is ~ 6 ms! Agilent's M9183A measurement arming time is < 100 us. With an arm time of 6 ms Brand X is wasting the benefit gained by the high speed PXI bus. In fact, with such a large arm time a 'box' DMM using USB, LAN, or GPIB is faster. For instance Agilent's 34410A DMM takes only ~ 2 ms to make a measurement (that is including IO latency). 


For more info on Agilent's PXI DMM click here




5/15/11

Understanding Common Mode DMM Specifications


Here is quick 5 min video that a colleague of mine made that does a great job of explaining what the Common Mode DMM spec is, for both DC and AC.






4/12/11

Understanding Resistance Measurement Errors

This post covers resistance measurement error external to the DMM and how to prevent it. Quality bench-top DMMs offer two methods for measuring resistance: 2-wire and 4-wire ohms. For both methods, the test current flows from the input HI terminal and then through the resistance being measured and finally into the LO terminal. For 2-wire ohms, the voltage drop across the resistor being measured is sensed internal to the DMM. Therefore, test lead resistance is also measured. For 4-wire ohms, separate “sense” connections are required. Since no current (or very little) flows in the sense leads, the resistance in these leads does not give a measurement error. Errors for dc voltage measurements (see Agilent Application Note 1389-1) also apply to resistance measurements. However, there are additional error sources that are unique to resistance measurements.


Power Dissipation Effects — When measuring resistors designed for temperature measurements, such as thermistors and RTDs, be aware that the DMM’s stimulus current will dissipate some power in the DUT. This dissipated power will raise the temperature of the DUT and therefore change its resistance such that its resistance value no longer represents the ambient temperature of the environment it is in. If power dissipation is a problem, select the DMM’s next higher measurement range to reduce the amount of stimulus current used to make the measurement and therefore reduce the temperature error to an acceptable level. The figure below shows the stimulus current for Agilent’s 34401A at each resistance measurement range.


Settling Time Effects — Capacitive and inductive elements are a part of every measurement setup. The amount of setting time that is needed to make an accurate measurement depends on the values of the reactive elements and the time constants associated with their current paths. Quality DMMs like Agilent’s 34401A, have the ability to insert automatic measurement settling delays. These delays are adequate for resistance measurements with less than 200 pF of combined cable and device capacitance, which is particularly important when measuring resistances above 100 k. Settling time errors are most pronounced after a connection change, such as a switch closure in a test system, or after a range change in the DMM. In these types of scenarios allow a settle time that matches your accuracy needs. To check for setting time errors, make resistance measurements (on a known resistance) immediately after and at set time intervals after a range or connection change. Repeat this a number times and calculate the average and standard deviation for each set of measurements in a particular time interval. Once the average and standard deviation measurements of each time interval begin to match you can determine what your minimum setting time is.


High-Resistance Measurement Errors — When you measure large resistances, significant errors can occur due to insulation resistance and surface cleanliness. You should take the necessary precautions to maintain a "clean" high-resistance system. Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and "dirty" surface films. Nylon and PVC are relatively poor insulators (10 ^9 ohms) compared to PTFE Teflon insulators (10^13 ohms). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 M resistance in humid conditions.





4/5/11

Ground Loops and Other Spurious Coupling Mechanisms and How to Prevent Them Part 2

This is a two part blog post is based off a great tutorial I read titled “Tutorial on Ground Loops” by P. M. Bellan. A link to the tutorial can be found at the end of the post. The subject of the two part post is something you won’t find in textbooks from your EE courses, spurious coupling mechanisms in circuits and test systems and how to prevent them. A spurious coupling mechanism is any outside unwanted voltage or current source entering a signal path. There are five main spurious coupling mechanisms: direct conduction, capacitive coupling, inductive coupling, radiated electromagnetic field pickup, and ground loops. The part one post focused on the first four types of spurious coupling (click here for part one). In this post we will focus strictly on ground loops.

Ground Loop Theory
Kirchoff's law tells us the total DC voltage in a complete circuit (Vloop) is zero. When considering AC components in a circuit, Kirchoff's law is superseded by Faraday's law. With Faraday's law Vloop = - dF/dt. F is the magnetic flux in the loop. The magnetic flux is given by F = BA. B is the magnetic field linking in the loop and the area of the loop is A. Faraday's law is what makes the basis for Transformers, inductors, and also ground loops. The make up of a ground loop signal depends on linked spurious time-dependent magnetic flux produced by nearby external circuits (fluorescent lighting, switching power supply, motor, computer) as well as the resistance and inductance in the loop.

Ground Loop Examples
The figure below shows and example of a measurement setup where a ground loop is created. The ground loop is formed by the the measurement instrument being connected to ground, the sensor or device under test (DUT) is connected to ground, and both instrument low sides are connected together via the shielding in the coax cable.This creates a loop (shown in green) for spurious time-dependent magnetic flux to enter (shown in blue) and creates unwanted current flow (shown with green arrows).
Used by permission from P. M. Bellan
Eliminating the ground loop in the above example is just a matter of locating and eliminating the inadvertent short (assuming you know it exists). What if we are making a measurement on a DUT that is suppose to be grounded? In this case we can look to our measurement or test instrument to brake the ground loop. For instance most high quality DMMs and power supplies have their low side connector isolated from ground with the option to short it to a ground potential. In this case you would want to ensure the low side connection of the instrument remains isolated from ground to prevent a ground loop from being created.Of course the isolation does have some finite impedance for DC and AC that you may want to check in the instrument's specifications when performing sensitive measurements. Some test and measurement instruments, such as scopes, typically have the low side connector tied to ground. In this case you would need a differential circuit (differential probe for scopes), isolation transformer, or optical isolation to break the ground loop. These types of ground isolation methods introduce their on limitations on the measurement.
The next example involves a multi-channel measurement setup and each channel shares a common low side connection. This setup may be a instrument, such as a digitizer, with multiple channels or a complex test system with switching. This time a loop is created between the low sides of two measurement channels as shown in the figure below. 
Used by permission from P. M. Bellan
One common way ground loops of this type are broken is by disconnecting the shielding or low side return from either the instrument or the DUT. This method will brake the ground loop but it will also degrade the signal since it brakes the signal path return. 
The third example ground loop involves two test and measurement instruments connected together. As shown in the figure below, each instrument is connected to ground and the loop is completed by each instrument's low side being connected together.
Used by permission from P. M. Bellan

This type of ground loop can be broken if one of the instrument's low side is isolated from ground.

Methods for Eliminating and Reducing Ground Loop Error
  • Breaking the loop: As mentioned throughout the post, there are multiple ways to break the loop to eliminate ground loop error including remove an unintentional short, use an instrument with an isolated low side, use a differential circuit / probe, use an isolation transformer, or use optical isolators depending on your application.
  • Increasing inductance of loop path to decrease loop current: By adding inductance, and therefore impedance, into the ground loop you can lower the ground loop current without affecting the desired signal. This can be accomplished by wrapping cabling, whether it be twisted pair or coax cable, around shield beads, a ferrite core, or an iron-core transformer. The more turns the more inductance is added to the loop. For a real world example of this look at the video cord on your computer monitor it will most likely have an iron-core around it. This method is more effective for higher frequency or sudden transient ground loop signals. 
  • Eliminating the area of the ground loop: Make the area of the loop smaller by using shorter signal cabling, shorter power cables, twisting power cables together, and using AC power outlets that are adjacent to each other
  • Removing linked magnetic flux: If possible remove noisy devices out of the test measurement area. This could mean moving a test away from a motor or replacing a switching supply with a linear supply.





3/30/11
Ground Loops and Other Spurious Coupling Mechanisms

This is a two part blog post is based off a great tutorial I read titled “Tutorial on Ground Loops” by P. M. Bellan. A link to the tutorial can be found at the end of the post. The subject of the two part post is something you won’t find in textbooks from your EE courses, spurious coupling mechanisms in circuits and test systems and how to prevent them. A spurious coupling mechanism is any outside unwanted voltage or current source entering a signal path. There are five main spurious coupling mechanisms: direct conduction, capacitive coupling, inductive coupling, radiated electromagnetic field pickup, and ground loops. This first post will focus on the first four types of spurious coupling and the second will focus strictly on ground loops.


Direct Conduction:
Direct conduction is caused when an unwanted conductive path is created between a circuit or signal path of interest and a neighboring signal path. This type of spurious coupling is shown in the figure below. The arrows in red represent the current from a neighboring signal entering the circuit of interest through an unwanted conductive path. The cause of this could be an un-insulated crossed wire, a shorted PCB run, sloppy solder job, or a malfunctioning switch in a test system. The only way to prevent it is to identify the low resistance path and correct it.
Used by permission from P. M. Bellan


Capacitive Coupling:
Capacitive coupling, also known as stray capacitance, occurs due to the capacitance that exists between two otherwise isolated adjacent signal paths (see figure below). The current coupled from one signal path onto the other signal path can be represented by I = C * dv / dt. Where “I” is the unwanted current coupled into the signal path, “C” is the stray capacitance that exists between the adjacent signal paths, and “dv/dt” is the rate of change in voltage on the adjacent signal path. The closer the signal paths are together the higher the value of “C” is and therefore the higher the “I” value in the adjacent signal path. Also the larger the rate of change of the voltage (dv/dt) the higher the coupled “I” value in the adjacent signal path. Higher signal frequencies mean higher dv/dt. High impedance signal paths are especially vulnerable to this type of spurious coupling because they are low current already so any added current can have a big effect. In test systems capacitive coupling affects can be seen when using low frequency switch cards close or over their upper frequency limits. The same is true when using dense interface and bus connection types in test systems at their upper frequency limits. In these types of cases the capacitive coupling is often referred to as crosstalk. This capacitive coupling can be reduced by using shielded cable and connectors, larger spacing on PCB runs, and higher frequency rated switching products in your signal paths.
Used by permission from P. M. Bellan


Inductive Coupling:
Whenever current flows through a conductor it creates circular lines of magnetic flux (remember the Right Hand Rule). Any signal path adjacent to a current carrying signal path will have magnetic lines of flux coupled onto it creating a current flow in the signal path equivalent to the amount of linked flux lines that are in contact with it. This same principal is how transformers work. One way to reduce inductive coupling affects is to space out adjacent signal paths because the more space between the signal paths the weaker any linked magnetic flux lines will be. More practical ways include using twisted pair wiring or shielded cable. Twisted pair wiring essentially cancels out magnetic lines of flux by creating an alternating pattern where the magnetic flux lines oppose each other. Twisted pair wiring can be purchased or you can make it yourself. To make twisted pair take two wires of the same length. Secure one end of each wire in a vise and place the other ends inside a drill where the drill bit goes. Stretch the wires out, fire off the drill, and you have twisted pair. Shielded cable has a conductor as its outer shell which catches any flux lines created by the internal conductor essentially blocking them from coupling on adjacent signal paths.


Radiated Electromagnetic Field Pickup:
In today’s wireless world we are surrounded by transmitters. On top of that devices, such as electric motors, can act like transmitter even though that was not their design intention. These intentional and non-intentional transmitters fill every part of our environment with electromagnetic waves. Radiated electromagnetic field pickup is when your circuit or signal path acts like an antenna and picks up radiated electromagnetic waves. To get a first hand example of this kind of spurious coupling, place your mobile phone close to a stereo speaker (make sure the speaker is on). From there send a text message or some other data transmission and you will hear interference on the speaker created by the transmitted packets coming from the phone. The amount and frequency range of electromagnetic waves a circuit or signal path picks up depends on its length compared to the electromagnetic wave lengths in the area. For instance if your circuit or signal path is close to a transmitter that is operating at frequencies whose wavelengths are the same length, twice the length, or four times the length of your circuit or signal path the pickup will be much stronger. To reduce radiated electromagnetic field pickup use shielded cabling and conductive casing around any exposed signal paths. The idea is you want to create a Faraday Cage around your signal paths.


Stay tuned for part 2 next week


12/16/10
Transducers for making Temperature Measurements with a DMM

Four types of transducers are commonly used for making temperature measurements with a DMM: resistance temperature detectors (RTDs), thermistors, IC sensors and thermocouples. Each type has advantages and disadvantages. The follow is a quick summary of each.

Use thermistors for better sensitivity
A thermistor is a temperature transducer who's resistance changes as a function of temperature. They consist of semiconductor materials and provide excellent sensitivity, but their temperature range is limited, commonly from -80°C to 150°C. Thermistors have highly nonlinear temperature-resistance relationships; therefore, their conversion algorithms are complex. Modern DMMs make thermistor measurements by storing a table of thermistor resistance values and the temperatures they map to. Agilent DMMs like the 34401A, 34410A, and 34411A use the standard Hart-Steinhart approximation to provide accurate conversions, with a typical resolution of .08°C. Check out Agilent’s E2308A thermistor temperature probe

Use RTDs for more accuracy
Resistance temperature detectors (RTDs) work a lot like thermsistors in the sense that they are resistive temperature transducers. They provide high accuracy over a range of roughly -200 to 500°C. There is very little conversion complexity for an RTD since it is so intrinsically linear. Modern DMMs such as the Agilent 34410A provide measurement for the IEC751standard RTD, which has a sensitivity of .0385%/°C.

IC temperature sensors are an easy to use linear solution
Many vendors provide probes that produce a voltage proportional to temperature in degrees C or F. The probes typically use an IC temperature sensor such as the National Semiconductor LM135 series. A temperature IC can cover temperatures from -50°C to +150°C. You can easily compute the temperature from the probe output shown on the DMM display. For example, 270 mV is 27°C.

Thermocouples offer a wide measurement range and ruggedness
Thermocouples can measure the broadest range of temperature, from -210°C to 1100°C, and their rugged construction makes them ideal for harsh environments. Thermocouple temperature measurements are based off the Seebeck Voltage. Take two dissimilar metals and connect one end of each metal together and a small voltage will be present at the open end of the metals, this is the Seebeck voltage. This voltage is a function of the temperature at the junction of the two metals. This relationship between the temperature and the voltage produced by the two dissimilar metals is how thermocouple temperature measurements are made. If you directly connect a thermocouple wire to a DMM connector (which is metal) you create more thermocouple junctions which will distort the original intended measurement junction. Because of this a reference junction is needed between the DMM and the thermocouple cabling. This adds complexity to the measurement. There are DAQ instruments out there with a built-in reference junction like the Agilent 34970A or 34972A. For more technical details on thermocouple temperature measurements reference junctions check out the application note link below. 
    

By far the most common way I make temperature measurements is with thermocouple because of its ruggedness and its range. For instance I can use the same thermocouple wire to measure the temperature in an environmental chamber, to measure the ambient room temperature, or to measure the heat on a processor chip without having to worry about does its range cover what I am measuring or being delicate when I attach it to a surface. Below is a table that runs through each transducer’s pros and cons (click on to enlarge). 






11/28/10
Tips for Making True RMS AC Measurements with a DMM

True RMS responding DMMs measure the "heating" potential of an applied voltage. Power dissipated in a resistor is proportional to the square of an applied voltage, independent of the waveshape of the signal. Todays general purpose DMMs can accurately measures true RMS voltage or current, as long as the wave shape contains negligible energy above the meter’s effective bandwidth (more on this in a bit). Most DMM's ACV and ACI functions measure the AC–coupled true rms value (DC is rejected). For symmetrical waveforms like sinewaves, triangle waves, and square waves, the ac–coupled and ac+dc values are equal, since these waveforms do not contain a dc offset. However, for non–symmetrical waveforms (such as pulse trains) there is a dc voltage content, which is rejected by ac–coupled true RMS measurements. DC rejection is desirable in certain applications such as when you want to measure the ac ripple present on DC power supplies. For situations where you want to know the AC+DC true rms value, you can determine it by combining results from dc and ac measurements, as shown below:

A common misconception is that "since an ac multimeter is true rms, its sine wave accuracy specifications apply to all waveforms." Actually, the shape of the input signal can dramatically affect measurement accuracy for any multimeter, especially when that input signal contains high–frequency components which exceed the instrument’s bandwidth. As an example, consider a pulse train, one of the most challenging waveforms for a multimeter. The pulse–width of that waveform largely determines its high–frequency content. The frequency spectrum of an individual pulse is determined by its Fourier Integral. The frequency spectrum of the pulse train is the Fourier Series that samples along the Fourier Integral at multiples of the input pulse repetition frequency (PRF).

The below figure shows the Fourier Integral of two different pulses: one of broad width (200 μs); the other narrow (6.7 μs). Agilent's 34410A/11A series of DMMs have an effective AC measurement bandwidth of 300 KHz.  If we used one of these DMMs to measure the RMS ACV value of both pulses in the figure the measured value of the broader pulse will be more accurate than the measured value of the narrow pulse since its frequency components outside of the DMM's bandwidth are larger in amplitude.
When making true RMS measurements on non-symmetrical waveforms, accuracy drops as the crest factor and/or the frequency of the waveform increases (for more info on crest factor click here). Here is a list of other tips when making true RMS AC measurements:
1. For maximum accuracy, measure as close to full scale as you can. You might need to override auto scaling in some cases. Be careful with high-crest-factor signals not to overload and saturate the meter’s input circuitry.
2. Be sure to select your DMMs appropriate low-frequency filter to allow for the fundamental to be captured. The lower the filter the longer the measurement will take.
3. You may not want to use the first measured value because many DMMs have a large-value DC-blocking capacitor in the input path. You need to allow this capacitor to charge, especially when you are measuring low-frequency signals or when you are switching between measurement points that have a large DC offset.
4. When you measure AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will serve as an antenna and the DMM will measure these unwanted signals as well. Reduce the area of the “antenna,” use
good shielding techniques, and make sure the AC source and the DMM are connected to the same electrical outlet to minimize ground loops.
5. AC loading errors: The input impedance of a DMM is often in the region of 10 MΩ in parallel with 100 pF. The cabling you use to connect signals to the multimeter adds additional capacitance and loading. As frequency increases, loading will change. For example, at 1 kHz, the input resistance will now be closer to 850 kΩ, and at 100 kHz it will be closer to 16 kΩ.





9/13/10
Largest Modular Instrument Introduction Ever


Today at Autotestcon Agilent is launching the largest modular instrument introduction in test and measurement history! Agilent is introducing 47 products in both PXI and AXIe (more on AXIe form factor click here), with the bulk being in PXI. The products range from RF switches to pulse pattern generators to DMMs. If we rewind back a couple decades, Agilent was the pioneer in modular instrumentation with their extensive VXI portfolio. While still staying in the modular instrumentation arena, over the last decade they focused more on high accuracy ‘box’ instruments. With this introduction it is clear that Agilent plans on playing a bigger role in the modular instrumentation industry. You can see a list of Agilent’s modular instrument introductions in the top figure (click on it to enlarge).
Since this is a GPETE blog I wanted to fill you in on the details on Agilent’s new PXI DMM family. The family consists of the M9182A (shown in lower figure) and the M9183A. Both are 6.5 digit DMMs, the M9183A provides higher speed performance and added features. Here is a quick rundown on specs and features:



  •      6.5 digit resolution
  •      4.5 digits at 4,500 rdg/s (82) and 20,000 rdg/s (83)
  •     Measurement capability: dcV/I, acV/I, 2/4-wire Ω, Freq, °C/F, and capacitance (83 only)
  •     DCV basic 1 year accuracy: 30 ppm
  •     Compatibility: cPCI, PXI-H, PXI-1
Surprisingly there are not really many PXI DMMs out there. National Instruments is by far the leader in this space with about 90% of the PXI DMM market so Agilent has an uphill battle. If you’re a consumer of PXI DMMS this is good news! Fierce competition means lower prices, more promotions, and more innovation for you as the end user. For more info on Agilent’s new PXI DMMs see the links below.




9/7/10
Understanding Ground Loop Error in Voltage Measurements


A true ground potential is something that only exists on paper or in simulations. In the real world there is no such thing as a true ground which in test and measurement leads to ground loop errors. Ground loops present problems when measuring low level signals such as thermocouple measurements. When measuring voltages in circuits where the DMM and the device-under-test are both referenced to a common earth ground, a ground loop is formed. As shown in the figure, any voltage difference between the two ground reference points (Vground) causes a current to flow through the LO measurement lead. This causes an error voltage (VL) which leads to inaccuracies in the DMM’s measurement.
When considering ground loops just in terms of DC, as long as Ri is a large value (meaning air between the two potentials) the error will be fairly insignificant when measuring mV and up. Agilent DMMs such as the 34401A, the 34410A, and the 34411A have a Ri of 10 Gohm at 80% humidity. 80% humidity is high for a lab environment so in most settings the actual Ri is much greater than 10 Gohms. Error caused by DC ground loops can be further reduced by keeping the ground path of low level signals as short as possible.
The bigger source of noise and error from ground loops is the AC component. The DMMs impedance to ground is lower with AC because of the capacitive component, Ci, in parallel with Ri. The capacitive component results from the windings in the transformer inside the DMM. Referring to the Z calculation at the bottom of the figure, as the frequency increases the Z isolation of the DMM to ground begins to decrease. Now in most low frequency settings the ground loop noise is from the power line so it is 60 or 50 Hz. The effect of AC power line ground loop noise can be reduced by setting the DMM’s measurement integration time to 1 or more power line cycles (for 60 Hz that is 16.67 ms). If your testing environment consists of high frequency signals, high speed digital signals, or noisy components like relays or motor it is best to put any sensitive voltage measurements on a separate ground potential if possible.

For the ground loop Wikipedia page click here



7/16/10
Calculating DMM Accuracy and Resolution

The figure below shows the accuracy specifications for Agilent’s 34410A 6 ½ digit DMM from the data sheet. Let’s assume we are using the 34410A in the 10 V range, it was calibrated within the last 24 hrs, and we are measuring an exact 5 VDC source. We would use the parameters circled in red in the figure and the following formula to calculate the accuracy and useful resolution of our measurement: +/- (% of reading + % of range). For our hypothetical measurement the 34410A would measure: +/- ((5.0 x .000015) + (10.0 x .000004))--> 5.000115 V to 4.999885 V. Since the last digit does not give use any useful information the 34410A provides 6 ½ digits of resolution and the display would read between 5.00012 V and 4.99989 V.

Now that was pretty easy right? One important variable we need to add to the mix is measurement sample time. Sample time is how long the DMM’s internal ADC samples the voltage before integrating the measurement and displaying the result. In the accuracy calculation that we just did, it was assumed we were using the max sample time of 10 PLCs. PLC stands for power line cycle. In the US we use 60 Hz for AC power so 1 PLC is 16.667 ms (1/60). In other parts of the world 50 Hz is sometimes used so 1 PLC would be 20 ms. The DMM’s sample time units are typically based on PLCs. The more PLCs you sample over the better the accuracy you get (To a point). Of course longer sample times equal less throughput so as always there is a tradeoff. Let’s take the same measurement scenario using the 34410A we used above but this time we want to be able to make 10,000 measurements in 1 second. To do this we must set the DMM’s sample rate to 0.006 PLCs. Since our measurement sample time is so much lower we add another uncertainty part to our accuracy calculation. This is due to power line noise and our integration time is smaller so less random noise is being cancelled out. For the 34410A we add the RMS Noise Adder shown in the figure below circled in red.

Accuracy at the 10 V range for a 5 V measurement at 10,000 meas/s: ((5.0 x .000015) + (10.0 x .000004) + (10.0 x .000012))--> 5.00024 V to 4.99976 V. Once again the last digit in this calculation provides no information so there are only 5 digits of useful resolution. The 34410A’s display would read between 5.0002 V and 4.9998 V.
Be careful when purchasing a DMM that is advertised as having high resolution at high measurement speeds. You want to look at its specs and calculate its accuracy at that speed to ensure the advertised resolution is actually “useful” resolution if it isn’t your just paying for extra digits of random noise.
For more information on understanding DMM accuracy and resolution click here

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