I have this thermocouple conditioning circuit. Let's say my gain is 100. Minimum low level for AD8237 is 20mV. As K type thermocouple outputs 40uV per 1C, I will have 4mV per 1 degree C.
This means I will not see change on the output while temperature goes from ambient to ambient + 5C.
Suppose, I don't want to loose these 5 degrees at the bottom of my range. What do I do? I think, I have to inject some offset into the +IN input? If so, how do I do it? What are the other possibilities?
I would like to avoid using split supply.
Best Answer
Note that the 20mv error is a measure of how close the output can go to 0v (using your supplies) against a 100 kilohm load pulling the output to "mid-supply" i.e. 2.5V.
Without that pull-up, as in your application, the output will go considerably closer to 0V.
However if you need to inject offset; the place to do it is "ref" i.e. pin 6 - whatever voltage is there, is the "0V" reference point for the output.
Note also that the thermocouple will ONLY generate 0V at ambient if it is actually a pair of thermocouples; connected back to back, one measuring the temperature of interest, the other being held at ambient (or 0C or some known point) to act as a "cold reference". (This may be what your "thermocouple" does, but its worth mentioning because the circuit doesn't show it).
Otherwise you have at least three thermocouples in the circuit; two of them being unknown quantities, formed by the connector pins. In which case the actual voltage is anybody's guess.
One further point - I see you connect the thermocouple via a connector. It's worth considering what happens when unplugged : I recommend a high value resistor from
-into+into prevent+infloating. (1 megohm with the 1na bias current implies an offset of 1 mv with no thermocouple, and it will have no measurable effect with the thermocouple present)