Help with opamp selection for thermocouple ADC

I have an ADC which takes an input signal in the range of +/- 64mV relative to ground. I am using this to read a thermocouple for measuring exhaust gas temperature, and it works fine if I use a probe with an ungrounded hot junction; I tie one side of the cold junction to ground at the ADC and measure the other side. In order to use a more common grounded probe, which is my goal, I need a differential stage before the ADC.

The +/- 64mV range is just about right for my purposes, so I do not want any gain, nor do I want to switch ADCs if I can help it. If it simplifies the analog stage I can deal with a 0-64mV input and output range ( i.e. I do not need to measure probe temperatures below the "cold" junction temp, although it wouldn't be undesirable to be able to do so.) I do need to reliably read values very near 0 volts.

I understand how to use an opamp to convert the difference in voltage between the two halves of the cold junction to a ground-referenced voltage. I am having trouble selecting the correct opamp. I want to minimize errors due to offset voltage or nonlinearity, but if possible I don't want to spend $8/ea for the parts.

I am using a single +5v supply.

Any advice will be appreciated.

Thanks

Unless you can be sure that your thermocouple's common mode voltage is above 0 volts your circuit won't work. An op amp must have a power supply greater than the range of input voltages (at least all the ones I've used). For example you won't be able to buffer a signal that varies 64 mV above or below -3V with an op amp that has a 0 to +5V supply.

Typically you use a grounded thermocouple when you're trying to measure the temperature of some hot metal chunk in a somewhat noisy environment. Problems arise if there are several different types of metal between your hot junction and your adc. These different metals form thermocouple junctions that will affect what you read from your hot junction.

I'm afraid you only have 3 options.

1...Continue to use an ungrounded thermocouple. They make some nice expensive ones that are in a stainless steel sheath. These may be too fat for your application.

2...Verify that the metals between your ground point and the thermocouple always produce a positive common mode voltage between 0 and 5 volts. If someone changes the type of metal used somewhere in the system you might get hozed.

3...Bite the bullet and design in a negative supply for your op amp. If you have something like a Max232 chip in your system you can sometimes tap -10V off of that. You won't need more than a few milliamps of current.

Something else to consider is your op amp will have some minimum offset voltage, typically hundreds of microvolts. Depending on the type of thermocouple and type of op amp used this can result in an error of several tens of degrees. If you have a micro in your system, you can get around this by shorting the thermocouple from time to time and seeing what your adc reads. Save this value and subtract it from your thermocouple reading before doing any kind of temperature lookup.

Mark