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I am trying to interface a K type thermocouple to the ADC channel of a Microchip part.
I am interested in negative (to -2mV) and Positive ( to + 4mV) voltages but don't know how to feed them into the ADC to get the full 12 bit resolution over this range of input voltage.
I realise I need to amplify the output from the thermocouple and yes, I know all about cold junction compensation etc. I just need some ideas of how to interface the -2mv to +4mV signal into the ADC.
Thanks in advance
You'll need to amplify the signal to the range your A/D can handle - there are existing parts for just this purpose. F/ex, Analog Devices' AD595 thermocouple amplifier. Simplecircuitboards.com sells boards based on it.
For a more comprehensive solution, Maxim MAX6675 is a complete thermocouple conversion device (thermocouple in, digital out). It's currently in "Not Recommended for New Designs" status by the manufacturer and I didn't see a recommended replacement but that may not matter if you're not designing it into a new product. Adafruit Industries sells the part on a breakout board ready to plug into a breadboard.
Another possibility is to use a high resolution delta-sigma A/D converter directly, like the Microchip MCP3550 or various similar ones. These have enough bits so that you can read the -2 to +4 mV signal directly. The input is differential, so you can read positive and negative signals by floating everything near half the supply.
Even at only 18 bits spread over ±3.3 V, you get 25 µV resolution. Or put another way, that's about 8 bits over your temperature range. These devices are available in 20 and 22 bits, so there is quite a lot of resolution available.
Connect your thermocouple to a dedicated thermocouple readout chip, such as the AD8495 "K-type thermocouple amplifier with cold junction compensation". This chip provides an output signal of 5 mV per °C (i.e. about 100 mV at room temperature), which should be more appropriate for your ADC.
At its core, this chip is an instrumentation amplifier, which is the name for a sort of high-gain differential amplifier with high impedance inputs and high common mode rejection.
If you would like additional voltage gain, build a small amplifier to map the voltage range you have to the voltage range you want. This can be done with a simple circuit consisting of a single op-amp and two resistors, such as this non-inverting amplifier circuit:
The voltage gain of this circuit is 1+R2/R1.
