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Unable to change AD620 output voltage when measuring bridge

Question asked by Matt.Duff Employee on Jan 12, 2011
Latest reply on Jan 12, 2011 by Matt.Duff

Originally posted by Dinosaur in EngineerZone feedback community on May 10th, 2010 (before Amplifier community was active)

 

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Hi all

 

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.

 

Any suggestions.

 

Regards

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