Interfacing thermopiles

[phatmonkey]

I should probably read data sheets before buying things, I didn't realise thermopiles produced a voltage! The circuit in this data sheet (http://yeint.ru/suppliers/melexis/pdf/MLX90247_v2.pdf) uses a surface mounted amplifier, which isn't terribly useful.

Does anybody have any suggestions for hooking up these thermopiles to analogue inputs on a little Atmel chip (more specifically the Gumstix Robostix)? I suppose they will need to act as resistive sensors off a 5v supply.

[AntonK]

Not sure about your question here but Ill try to explain the thermopiles as I understand it. The sensors work in pairs, and the Op amp is a differential one, meaning to subracts the voltage difference between the 2 sensors(Left and right) for instance. This voltage difference based on the reference voltage is very small, and needs to be amplified. I believe the op amp gain on a stock FMA sensor is 2000. Im not sure if thats the multiplication of the voltage, but I do know the sensor voltage on its own is too small for a atmel ADC to read. Schematics for the boards are in the paparazzi CVS if you want to see for yourself how the circuit works.

AntonK

[albany tom]

I believe Anton is correct, in that you're not interested in the absolute output of the sensor, but the difference in two sensors looking in opposite directions. The thermal drift of the two devices is probably greater than the range of values you would be measuring. By taking the difference between the two sensors, any drift is effectively cancelled.

To measure the difference, look for circuits called "instrumentation amplifiers". Data sheets for op-amps would be one place to start. Then feed the amplified difference (output of the inst amplifier) to the A/D of the microcontroller. Trying amplify and A/D, or just A/D each signal and then subtract them in software would probably just end up with a measurement of the noise.

Another option would be to buy a single IC instrumentation amplifier, such as the LT1102. A more expensive option than building one out of op amps, but a little simpler.

[phatmonkey]

Yeah, I'm probably just going to use the paparazzi circuits. Something that's tested and proven will probably be best, and it gives me the chance to move paparazzi or borrow snippets of code!