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# How to use AD8538 in a thermopile circuit

Question asked by frank_z_z on May 18, 2016
Latest reply on Jun 25, 2016 by iamPhil

My research has pointed me to a link

http://www.analog.com/en/content/using_thermopile_sensor_in_ir_digital_thermometers/fca.html

Unfortuantly that link is now dead but I have found the webarchive version: Using a Thermopile Sensor in IR Digital Thermometers | Analog Devices

But there's no real information on the circuit being shown. Neither on that page, nor inside the AD8538 datasheet.

I have also read through the entire Patent US6320450 and it shows no equations to calculate all of those resistors

I am using a G-TPCO-033 thermopile, which outputs 0mV at 25 degrees C, about 6.5mV at about 100 degrees C. It has a internal NTC thermistor with 100kohm at 25 degrees C and beta of 3955. Also note that the thermopile has an internal resistance of anywhere between 55kohm and 95kohm.

What are the steps to calculate the resistors in the diagrams shown in the old analog devices article? Ideally I would like to power the AD8538 opamp with 5V and have the output be ranged 0V to 4V corresponding to 25 degrees C and 100 degrees C.

Please provide some equations. Thank you. It would be great to have them as a function of the parameters of the thermopile I plan on using.

I am also considering a digital sensor, but for the sake of this discussion, please ignore the fact that digital thermopile solutions exist. (I'm sure you'd love to sell me a few AD8538s instead)

I did search pretty extensively, and the only other mention about that circuit was not adequately answered at all. I have also contemplated running repeated simulations with all possible resistor permutations but the last time I tried this, there's no way my computer can crack these permutations.

Thanks.