Thursday, December 16, 2010

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). 

Friday, December 3, 2010

Creating a Differential Signal with a Function / Arbitrary Waveform Generator

Single-ended signals are referenced to a common level, such as ground, that they can share with other signals, so a single-ended signal requires only a single path or wire. Differential signals are made up of a pair of paths that are both dedicated to a single signal at any given time. One path is used at a higher potential than the other. Differential signals do add complexity, since they require two wires instead of just one, but they provide a number of performance advantages over single-ended signals.
Differential signal advantages include better signal-to-noise ratio, fewer timing errors, and less crosstalk. These advantages make differential signals common in applications such as ADC inputs, instrumentation amplifiers, measurement sensors (like accelerometers), and communication signals. When engineers design and test devices that use differential signals, simulating the differential signals for testing can be challenging. These challenges are caused by the fact that most function/arbitrary waveform generators (FAWGs) have single-ended outputs; instruments that can generate differential signals tend to be fairly costly. In this post I will explain two ways to create a low cost differential signal: using a FAWG with some custom hardware and using a 2-channel FAWG
One way to use custom hardware at the output of a single-ended source to create a differential signal is to use a differential amplifier circuit design as shown it the figure.

The resistors in the differential circuit were chosen to achieve a gain value of 1. I set the DC offset to 0 V. When building the circuit, be careful to keep signal paths and wiring as short as possible to keep parasitic reactive affects low for better signal integrity.
FAWG’s with two single-ended channels (isolated from ground) can have their channels combined into a single differential signal channel. To do this, you need to tie together the two "low" or "common" connections of each channel. The "high" of one channel must be used as the high signal path of the differential channel and the "high" of the other channel must be used as the inverse return wire or low signal path, as shown in the figure.

In addition to the two channels, it is also a lot easier to do this on a two-channel FAWG that has channel tracking capability, like Agilent’s 33522A two-channel function/arb waveform generator. This feature gives you the ability to create an inverted mirror image of the output signal from channel one onto channel two, which is exactly what is needed to create a differential signal. Also, this capability means you only have to set up the arb or built-in waveform on one channel and the inverted version of the waveform automatically tracks to the other channel. Without this feature you would have to setup an arb or built-in waveform on both channels and try to output them in sync using triggering.
As an example I measured and captured three signals with a differential input high-resolution digitizer. The example signal we used was a squarewave at 500 KHz. The figure below shows a digitizer screen shot of the signals. The three signals:
  1. Signal in yellow is a differential signal from the output of the differential amplifier connected to the single-ended FAWG
  2. Signal in green is the differential signal output created by the two channels from the 33522A
  3. Signal in purple is the output of the single-ended FAWG before the differential amplifier input

As you can see from the figure there is quite a bit of ringing on the differential signal created with the custom hardware. When I built my diff amp circuit I was careful to keep wiring as short as possible and I provided a large ground plane. Now with further time and engineer effort I could probably further improve the signal integrity of the circuit. But the point of this example is to show you can save test time and achieve better signal quality by using a 2-chan FAWG with tracking to create a differential signal. Also the cost of a 2-chan FAWG is still typically much cheaper than a differential output waveform generator.