Hello and thanks beforehand.
I'm designing a circuit to measure 4 wheatstone bridges using 4 Analog Devices AD8422 in-amps. Now, i'm feeding the AD8422 output to a 3.3V ADC, so i'm offsetting the output 1.65V in order to be able to measure the full scale output of the bridge. However i'm aware that tiny differences in the bridges, opamps and resistors will require slightly different offsets for every bridge (i'm expecting something like +-0.05V would do).
The AD8422 requires the reference to be fed from a low impedance source
Now, to create the 1.65V reference, i'm using a 3.3V source, a resistive voltage divider, and a Texas instrument's OPA333 as a voltage follower. However, to individually adjust every AD8422, I would need 4 of those circuits with potentiometers in the voltage divider, each one feeding into one AD8422. The boards that have the AD8422 are very small and integrating another opamp (even a SOT23 one like the OPA333) is pretty difficult (and expensive if we want to expand the system), so, here comes the question:
How would you apply small variations in a reference voltage without altering the low-impedance nature of the output?
I'm aware that the potentiometer aproach would not work, as you would need precise knowledge of the intensity going into the REF pin and pretty high resistances (2k+) which would be like a voltage divider.
Thanks for your help!
simulate this circuit – Schematic created using CircuitLab
Best Answer
I wouldn't bother trying to do this in hardware; I'd just compensate the ADC reading digitally to overcome the slight discrepancies in offsets. I'd produce one solid 1.65 volt reference voltage and feed this to all AD8422 InAmps.
After all, if there are resistor variations that produce voltage offsets, those variations will also produce gain variations that are likely to need compensating in software.