voltage drift

[E2005]

I am using a strain gauge signal conditioner(120 ohm gauge)with another 120 dummy resistor gauge and rest of the bridge internally balanced by the chip(Analog device 1B31). Ideally the output should be zero if no force is applied on the bridge(balanced bridge) but Every time I start the circuit the output starts at a different value(value offsetted by +/- 0.4 or 0.5V from zero). I have to use a pot to bring it to zero everytime. But after a few minutes sitting idle it starts drifting again. I am using low pass(RC) filters at input and output. Someone suggested its the capacitor causing the drift (as it charges)so I tried different electrolytic capacitor values but hasn’t helped(need cutoff around 30 Hz).I am trying to find what’s causing the drift.

 

[Skogsgurra]

The amplifier shouldn't drift that much if it isn't damaged. If changing the amplifier doesn't help, then check the bridge output voltage with a DMM (microvolt resolution probably needed) or simply wire in a set of resistors to mimic the bridge. That should tell you if it is the amplifier or bridge/wiring that is the culprit.

Also, thermal EMF does just that. Creeping in a few minutes after power-on. Make sure that your wiring doesn't introduce unwanted EMF.

Gunnar Englund

http://www.gke.org