Thermopile Detector Datasheet question
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, &amp; R3 form a divider. Using R1 &amp; 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.