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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

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  • \$\begingroup\$ You recognize that you need to amplify the signal but just need ideas on the -2mV to +4mV? If you are going to amplify it why do you think you will still be in the -2mV to +4mV range? \$\endgroup\$ – Kortuk Mar 18 '12 at 15:33
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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.

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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.

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  • \$\begingroup\$ What's with the downvote to this answer and the one from JRobert? If you think there is something wrong with what we said, you should explain that so it can be judged and peer-reviewed just line our answers. Once again folks, anonymous downvoting just leads to vadalism. Are the SE people ever going to fix this!? \$\endgroup\$ – Olin Lathrop Mar 18 '12 at 16:24
  • \$\begingroup\$ I am actually your upvoter but it sounds like they did not agree with your answer. If you feel your answer is correct there is no need to worry about it. It would be better if they explained so that you can correct their missconception, or they feel that there is a better way to answer the question and just think that your solution is not a good one. Which I could understand feeling that attempting to use a very high resolution ADC instead of amplifying the signal is not a good choice. Being able to disagree without making a vendetta with someone is something I still think is acceptable. \$\endgroup\$ – Kortuk Mar 18 '12 at 17:39
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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:

enter image description here

The voltage gain of this circuit is 1+R2/R1.

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