Electronic – Type J Thermocouple Signal Conditioning to 0-5V

It will be easier to see how to do this after distilling your requirements down to a real spec. You want 16-22 mV to map to 0-5 V. That's a gain of 833 centered around 19 mV. Let's presume you have a well regulated 5 V supply available.

First, leave a little margin. Let's say we'll use a opamp with up to 1 mV offset voltage, and you don't want either extreme of the signal to result in full high or low output. Leaving 1.5 mV room at each end sounds reasonable. That means the input range is 9 mV instead of just the 6 mV you actually care about. The target gain is therefore 5 V / 9 mV = 550.

That's a lot for a single opamp stage. In practise I'd probably use two stages, but it will be easier to show and explain as a single stage amplifier:

The gain is governed by R3 and the parallel combination of R1 and R2, and the point around which the gain applies by the voltage divider of R1 and R2. Everything is ratiometric, so we can pick one resistance. Let's make R3 1 MΩ and see what the others come out to.

R1 and R2 divide the 5 V supply to make the input gain pivot point, which is 19 mV. The ratio of R1/R2 is therefore 4.981V/19mV = 262.

Next we look at the output impedance of the voltage divider, which works against R3 to set the gain. Since the target gain is 550, R3/(R1//R2) = 549, or (R1//R2) = R3 / 549 = 1821 Ω. This gives us two equations to find the two reistances R1 and R2. Solving yields R1 = 479 kΩ and R2 = 1828 Ω.

In reality you're not going to find resistors with exactly these values, and they will have some tolerance anyway. Find the nearest 1% values, then go back and check what input voltage range you end up with that results in 0-5 volts out. Don't forget to run the min/max test with the resistors the full 1% off in the worst direction, and don't forget about the opamp input offset voltage. The nominal gain may have to be reduced to guarantee the minimum 16-22 mV always maps to within the 0-5 V output range. Actually, I'd try to stay away from the top and bottom 50 mV or so of the output range, even with a "rail to rail" CMOS opamp. I've shown you the method, so this checking and re-calculating is your job.

1. You can use any amplifier you like. You will get better performance if you use a modern $5 dual op-amp than a 2-cent/amplifier LM324. Sub-zero means nothing to me. If you want to the amplifier to have guaranteed characteristics below 0°C, then you should buy one that is guaranteed for that temperature range. Below -55°C you may have to qualify them yourselves. For temperatures down to -20°C it's unlikely that anything untoward will occur other than possibly slightly worse performance for a chip guaranteed over the commercial temperature range. A single 5V supply will impose significant limitations on the input common mode voltage. You can add pullup resistors to the output of an LM324 to get a bit more range (you probably do not want to do this on the output amplifier). Note that the left two amplifiers can saturate at either rail, so even if the output should be within range that doesn't mean that the amplifier will work properly. I presume your common mode input voltage will be close to 2.5V so this may not be a problem. LM158 and LM124 ancient designs are particularly crappy amplifiers for low level DC- large offset voltage and large offset drift with temperature.

2. See above. Offset voltage, offset voltage drift. Probably useful range of input. Closed loop gain accuracy will likely be lower too, because of op-amp gain.

3. If you could use an analog switch with your micro to auto-zero the amplifier you could improve performance. A SPDT switch is sufficient (eg. 1/2 74HC4066). Just transfer one input to be shorted to the other at ~2.5V and measure the amplified offset voltage. Average over many measurements and subtract from the measured signal.