Originally posted by Dinosaur in EngineerZone feedback community on May 10th, 2010 (before Amplifier community was active)
I have a ad620 amplifier board that has the circuit as per attached with a gain of 495
At an input of 0.00 mV I need to pull the output of the ad620 down closer to -5 v
At the moment it sits around -0.3
I have tried what I normally do with other adc's, and that is put a bias resistor between
-5 and one of the +in or -in. (Shown as 510K on the circuit) To do this with the ad620, I measure the output of the
load cell with the In+ and In- disconnected and select a resistor that will get me say -3.00 vdc.
Then when I connect to the ad620, this rises to about -0.7 and no amount of resistors will bring it lower.
The output of the ad620 shows what is expected with that input.
The resistor initially was about 29k. This also introduces a large dead area on the loadcell, where I need
to apply a significant load before it will register.
I have been doing this kind of offset adjustment for more then 20 years with the Ad7701 and others, and have never
struck this kind of problem before.