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AD620 for signal conditioning

Question asked by PNee on Mar 18, 2013
Latest reply on Mar 20, 2013 by PNee

I am using an AD620 to convert a floating voltage sensor (Mettler Toledo Pt4805-DPAS-SC-K8S) to a referenced voltage (ground). The signal will subsequently be amplified and shifted using op-amp circuits (yes, I realize the AD620 could amplify, but the other parts of the circuit were pre-existing).

 

This circuit was working, but has ceased working. I am trying to debug it. I have purchased new AD620s (two separate models), so I don't believe the chips are bad.

 

I have set up a test circuit, shown in the attached image. The sensor reads 320mV when open using a volt meter. The circuit reads 0V when I tied pin 2 and pin 3 together (as I expect).

 

However, when I plug the sensor directly into pins 2 and 3, the sensor reads only 80mV, and the output of the chip is about 20 mV (not unity gain).

 

What might be causing the chip to affect the voltage on the sensor? This was not occurring when I first built the circuit.

 

I have hit a brick wall in debugging this.

 

Thanks in advance.

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