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I'm working on making a detector using a Thermopile. Here's a link to the page and datasheet. Link

I've looked through and am not sure what the values for Ra, Rb, Rc, and Rd are supposed to be. If any value. Does it depend on the application?

What does Vth stand for? VThevenin?

Thanks, Luke

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Data sheet here

RA RB RC RD form a bias network to set a temperature compensated zero point so that the output reads in eg mV per degree C.

The circuit is probably based on this one from Analog Devices from here

The article has been translated from Japanese or Chinese and while (probably) hard to follow, has some useful information. An English language version seems to have existed at one time but links to it that I found are broken.

Here the device is used to provide a voltage output relative to a reference voltage so that absolute temperature can be determined.

R1, R2 in parallel with R4 and the thermistor, & R3 form a divider. Using R1 & R2 allows the voltage range across Thermistor and R4 to be both limited and "moved up and down" and the thermistor then adds temperature compensation. The result is a temperature compensated "cold point" voltage that the amplified thermopile voltage is added to.

The network and resistors could be replaced by eg a multiturn pot if manual ambient temperature affect adjustment was acceptable.

enter image description here

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  • \$\begingroup\$ Thanks for the input. The link you provided and the description made a lot more sense. One more quick question then, what is Vth? \$\endgroup\$ – Luke Apr 2 '15 at 13:44
  • \$\begingroup\$ @LukePavlic Very likely Vth = Vthermistor - it's the equivalent cold point temperature that the thermopile sits on top of. In the AD cct they don't specifically access it - its at the C7 C8 R5 etc junction above. \$\endgroup\$ – Russell McMahon Apr 2 '15 at 17:15
  • \$\begingroup\$ Excellent! I feel really dumb for not making that connection earlier! \$\endgroup\$ – Luke Apr 2 '15 at 18:15
  • \$\begingroup\$ @LukePavlic Things that are obvious are only obviously obvious after they are obvious :-). It is very very very very easy to overlook what is staring you in the face in all sorts of situations. An exceedingly valuable lesson to learn in life. But about impossible to learn :-). \$\endgroup\$ – Russell McMahon Apr 3 '15 at 15:26

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