Sunday, February 26, 2012

Accurately Measuring Battery Capacity

In this post we will look at solution for measuring and characterizing the capacity of rechargeable batteries for devices such smart phones, portable game systems, and wireless sensors. The following video made by a colleague of mine explains the challenges of measuring battery capacity and an ideal solution made of three products: N6781A SMU unit for battery drain analysis, N6705B DC power analyzer mainframe, and the 14585A analysis software. I have blogged about the N6781A in the past. One feature that I want to point out that the video doesn't cover is its seamless current measurement ranging. This is a patented Agilent only technology that allows the N6781A to seamlessly move through its measurement ranges without any discontinuity or glitches in its output. This means you get 18 bits of measurement resolution no matter if you are measuring microamps or amps.


The video demonstrated measuring the battery's capacity by using the N6781A as a load, meaning it simulated the device that the battery would be powering. Since the N6781A is 2-quadrants and can regulate its output at 0 V it can also act as a zero ohm measurement shunt to measure and characterize the battery's capacity while powering the device it will be used in. To use the N6781A as a zero ohm shunt you simply connect it in series with the battery and the device it is powering so the current flows through the N6781A. The below figure shows the zero ohm setup.

Notice in the above figure that the N6781A has a built-in separate DVM for measuring the battery's voltage even though it is not directly connected to the battery. 

In this post we have been focusing on characterizing a battery's capacity, of course the Above zero ohm shunt setup could be used to characterized the device's power needs too. And the N6781A can also be used as a source to simulate the battery to characterize the device's power needs. If you have any questions regarding this post feel free to email me and if you have any comments or personal insights to share please use the "Comments" section below.


Tuesday, February 21, 2012

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.


Monday, February 13, 2012

Simulating High Bandwidth Power Supply Transients

A power supply transient can be defined as an unintended variation of the supply’s amplitude or current. Example terms that refer to supply transients include power surge, spikes, dropouts, interrupts, etc. Power supply transients are caused by things like a sudden sharp load change or when external energy is coupled into the supply, such as when a lightening strike occurs near the supply.

Simulating power supply transients is needed to ensure the device will work to spec in its intended operating environment and ensure it is reliable. Example applications for power supply transient testing is in automotive, aircraft, and satellite electronics. In this blog post will look at a low cost solution for simulating high bandwidth power supply transients using general purpose test equipment and some simple analog circuitry.

With modern high performance power supplies, transients with rise or fall times as fast as ~ 1ms can be simulated. The below images show a captured 10 V transient pulse on top of a 12 VDC level created with Agilent's N6705B DC Power Analyzer. You can see the measured rise time of the pulse is ~ 300 us.



To simulate power supply transients or power arbitrary waveforms with rise and fall times less than 300 us you can use the below solution, which we will call the Active Variable Resistive solution or AVR for short.


From the above AVR schematic, the two DC sources and the waveform generator can be implemented with general purpose test equipment. The MOSFET Switches and Choke are implemented using basic circuit components and should be chosen based on power levels and other considerations that we will discuss.

To go through the AVR system’s theory of operation lets use a simple example transient. Lets say we have a 10 V nominal DC power supply level powering our DUT (load) and we want to create a 10 V transient pulse with a 1 ms pulse width on top of our power supply level.

To start we would set DC source 1 to a 10 V level to serve as our nominal DC power supply level. The waveform generator would be set to a negative voltage value, like -5V, to turn on the P-type MOSFET switch so it acts as a short. In turn the negative voltage from the waveform generator would ensure the N-type MOSFET switch is reversed bias or off. Since the waveform generator’s low is floating, it is tied to the same node as the high of the load so it is at the same potential as the sources of the MOSFETs which are also connected to the same node as the load high. The choke is added to increase the impedance to ground when a transient is generated since there is a small amount of capacitance between the waveform generator’s low and ground. DC source 2 is set to 20 V, but no current is flowing out of it because the N-type switch is off. The current condition of the AVR system is DC source 1 is supplying power to the DUT at 10 V. To create the transient pulse we program the waveform generator for a single 1 ms pulse. The amplitude of the pulse should be enough to fully turn on the N-type switch, for this example we will say 5 V. When the pulse is triggered it will turn the N-type switch on and the P-type switch off. DC source 2 will quickly pull the load node up to 20 V and DC source 1 is effectively removed from the DUT. After 1 ms the waveform generator’s output will go low again and turn off the N-type switch and turn on the P-type switch. This removes DC source 2 from the load and puts DC source 1 back in the load circuit. At this point we have generated our 10 V transient pulse on top of our 10 V power supply level and we are now back at our initial conditions.

To generate a negative pulse with the AVR we would set DC source 2 for the nominal DC level and set DC source 1 to the bottom level of the interrupt, which could be any level between 0 V and the nominal DC level. Set the waveform generator so that the initial condition of the N-type switch is on and the P-type switch is off. Then, using the waveform generator, send a negative pulse to toggle the switches to create the interrupt. To create more complex power waveforms you would use the MOSFETs as variable resistors instead of switches. In this case ensure you use high power MOSFETs and heat dissipation as needed. Also for more complex waveforms a two channel waveform generator would be ideal so the two MOSFETs are not tied to the same waveform.

I implemented the solution using the following test equipment and components:
•DC source 1: N6705B DC Power Analyzer with N6762A Module
•DC source 2: N6705B DC Power Analyzer with N6762A Module
•MOSFET Switches: Si4410BDY and Si4925BDY
•Waveform Generator: 33522A Fg / Awg

Using the implemented AVR solution I created the below example 15 V 1 ms transient pulse on a 10 VDC level. The first scope shot shows the whole pulse and the second shows the rise time. Both rise and fall times were less than 6 us.


The pulse transient was generated into a 10 Ohm load with a 0.1 uF, 1 uF, and 10 uF capacitors in parallel. This was done to simulate a real world DUT supply input that presents a low impedance to ground for AC signals. Below is an interrupt created with the solution into the same load. The DC level is 25 V. The interrupt pulse is 20 V with a width of 500 us. The resulting fall and rise time of the interrupt is about 7 us.


Simulating power supply transients with a modern high performance power supply allows you to create waveforms with rise and fall times as fast as 300 us. To achieve higher bandwidths a high cost power system is typically employed by standards and quality labs. In this post we discussed and demonstrated a low cost alternative solution for creating higher bandwidth power supply transient waveforms right on the bench. If you have any questions, comments, or add-ons to this post please leave them in the "comments" section below.



Monday, February 6, 2012

Power Supply Analysis App for Scopes

The following video demonstrates a power supply analysis application that enables automatic, consistent and fast characterization of Switch Mode Power Supply (SMPS) using InfiniiVision 3000 X-Series, 6000 or 7000 Series oscilloscopes.