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THE BASICS OF THERMOCOUPLES

by Bob Perrin

Start ý A Tail of Two Metals ý Breaking the Loop ý Measuring Temperature ý Practical Matters ý Circuits ý Closing Time ý Sources and PDF

CIRCUITS

When designing a thermocouple interface, there are only a few pieces of information you need to know:

• what type of thermocouple will be used
• what is the full range of temperatures the hot junction will be exposed to
• what is the full range of temperatures the cold junction will be exposed to
• what is the temperature resolution required for your application
• does your system require galvanic isolation
• what type of cold junction compensation will be used

If the answer to the last question requires the analog addition of a voltage from a commercial cold junction compensation IC, then the manufacturer of the IC will probably supply you with an adequate reference design. If you plan to do the cold-junction compensation either physically (by an ice bath) or in software (by measuring the cold junctionýs temperature with another device), then you must build or buy a data-acquisition system.

Galvanic isolation is an important feature in many industrial applications. Because thermocouples are really just long loops of wire, they will often pick up high levels of common-mode noise. In some applications, the thermocouples may be bonded to equipment that is at line voltage (or higher).

In this case, galvanic isolation is required to keep high-voltage AC out of your data acquisition system. This type of isolation is usually accomplished in one of two waysýusing either an opto-isolator or a transformer. Both systems require the thermocouple signal conditioner to allow its ground to float with respect to earth ground. Figure 3a and 3b outlines these schemes.

Because the focus of this article is on the interface to the thermocouple, Iýll have to leave the details of implementing galvanic isolation to another article.

Given the tiny voltage levels produced by a thermocouple, the designer of the signal-conditioning module should focus carefully on noise rejection. Using the common-mode rejection (CMR) characteristics of a differential amplifier is a good place to start. Figure 4 shows a simple yet effective thermocouple interface.

(Click here for figure)

Figure 4ýThe common-mode filter and common-mode rejection characteristics pay off in thermocouple amplifiers.

The monolithic instrumentation amplifier (in-amp) is a $2ý$5 part (depending on grade and manufacturer). These are usually 8-pin DIP or SOIC devices. In-amps are simple differential amplifiers. The gain is set with a single external resistor. The input impedance of an in-amp is typically 10 gigaohms.

Certainly you can use op-amps, or even discrete parts to build a signal conditioner. However, all the active components on a monolithic in-amp are on the same dice and are kept more-or-less isothermal. This means in-amp characteristics behave nicely over temperature. Good CMR, controllable gain, small size, and high input impedance make in-amps perfect as the heart of a thermocouple conditioning circuit.

Temperature tends to change relatively slowly. So, if you find your system has noise, you can usually install supplementary low-pass filters. These can be implemented in hardware or software. In many systems, itýs not uncommon to take 128 measurements over 1 s and then average the results. Digital filters are big cost reducers in production systems.

Another problem often faced when designing thermocouple circuits is nulling amplifier offset. You can null the amplifier offset in a variety of ways [2], but my favorite is by chopping the input. Figure 5 shows how this process can be accomplished.

Figure 5ýAn input chopper like a CD4052 is all that is necessary to null signal conditioner offsets.

Thermocouples have such small signal levels, gains on the order of 1000 V/V are not uncommon, which means an op-amp or in-amp with a voltage offset of even 1 mV will have an offset at the output on the order of volts.

The chopper in Figure 5 allows the microcontroller to reverse the polarity of the thermocouple. To null the circuit, the microcontroller will take two measurements then subtract them.

First, set the chopper so the ADC measures GAIN (Vsensor + Voffset). Second, set the chopper so the ADC measures GAIN (ýVsensor + Voffset).

Subtract the second measurement from the first and divide by two. The result is GAIN*Vsensor. As you can see, this is exactly the quantity we are interested in. The in-ampýs offset has been removed from the measurement.

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