Economical thermocouple amplifier circuit

Question

I'm looking for the most economical circuit that reasonably accurate circuit for amplifying the voltage produced by a thermocouple (so it can be read by an ADC). For the sake of argument, component count and manual calibration time is not an issue.

It's a K type thermocouple (41uV/ºC) and I'm hoping for an accuracy of around 25ºC from 100ºC to 1000ºC. 1 sample per second is plenty. The ADC is 10 bit, and there is a regulated 12v, 5v, 3.3v and ground supply.

Could it be more economical with the above requirements to use cheap op-amps instead of a purpose made instrumentation amp?

As I understand, the problem with cheap op-amps is their input offset voltage and drift. But am I correct in saying:

• Many cheap op-amps can be offset-nulled with a variable resistor?
• and the offset drift is relatively negligible even for a thermocouple? (15uV/ºC in the LM741)

If so, the question remains what circuit could be used. Would 3 op-amps be necessary? or could it be done with fewer?

Last but not least, suggestions for which op-amp would be appreciated.

Answer 1

Would 3 op-amps be necessary?

Not in theory. In theory, you could just connect one end of the thermocouple to ground and then just feed the other end to a non-inverting amplifier. The problem, though, is noise pickup. Thermocouples have long wires, and those long wires act as antennas, picking up all sorts of junk. In most circuits, this wouldn't be a problem, but because thermocouples have such low voltages, the junk can easily overwhelm your actual temperature signal. By building an instrumentation amplifier, with 3 op-amps, you can remove (most of) this noise.

You may be able to get away with a single op-amp differential amplifier, but the large resistor values you'd need to use to get good input impedance would create a large amount of Johnson noise, which would wind up in your signal.

If you don't want to go for a proper amplifier, you'd need to use three op-amps. However, the matched resistors you'd need, plus the op-amps, may end up costing more than an instrumentation amplifier that uses, say, a single gain-setting resistor.

Also, have you thought about your cold-junction compensation? One of the issues with thermocouples is that they measure differential temperature; e.g. you have one junction at temperature A and another at temperature B, the thermocouple voltage is (some constant K) * (A - B).

If you want to find out the absolute temperature of A, you need to know the temperature B.

Now, from your requirements you may actually be able to get away with a cheap hack. You can just assume that B is, say, 25C (roughly room temperature) and as long as B doesn't go outside the range 12.5C-37.5C, the temperature you get for A will be within 25C of A's actual temperature. You have enough error tolerance that I'd consider this viable.

If, though, the ambient temperature your circuit must operate in can go outside that temperature range, you will need to incorporate cold junction compensation. This consists, basically, of generating a voltage with the same temperature coefficient as your thermocouple, but relative to absolute temperature; in other words, you have ((some constant K) * (A - B)) + C. C would be equal to your constant K times B; as such it cancels out B and you end up just with ((some constant K) * (A - B)) + K*B = K*A - K*B + K*B = K*A.

The typical method for generating this voltage is via a diode. This is best done on an IC, and as such you may find that a thermocouple amplifier with a built-in cold junction compensator will do you much better than an op-amp, and in fact may cost less.

Answer 2

There is yet another completely different option, which is to not use a amplifier at all. Instead, use a high resolution delta-sigma A/D. This completely gets around the amplifier offset voltage issue since there is no amplifier. Some low pass filtering would be a good idea, and you have to carefully consider layout and the thermal gradients accross your board, but then again you'd have to do that with any amplifier too.

Modern delt-sigma A/Ds have such high resolution that reading thermocouples with them directly is feasible. Let's say you believe you can get 20 bits of resolution. That's about 3 µV resolution even if the A/D is running from a 3.3 V reference. Some can go to 2 V reference and still maintain the resolution, but either way that's still a small fraction of one °C. A much more significant source of error will be measuring the temperature of the junctions on the board accurately enough to do the cold junction compensation properly. In any case, all this is well within you 25°C accuracy spec.

Answer 3

You really don't want to do this circuit "manually". While the circuit isn't very difficult, there are a lot of things you have to consider in order to make this work correctly.

Several companies, like Analog Devices and Maxim, make chips specifically for this kind of thing-- that include the ADC and all of the cold-junction compensation stuff. The MAX31855 from Maxim is a little 8-pin chip that takes power, the TC inputs, and a SPI port for interfacing direct to the MCU. You can't get more simple than that! And it is not very expensive either.