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Help needed with AD8420 design

Question asked by Dytlyf on Aug 10, 2015
Latest reply on Oct 22, 2015 by Dytlyf

Hello everyone, I have a problem with the AD8420 in-amp in a new electromagnetic flow meter design. Please refer to the attached image of a section of the circuit diagram.

 

The AD8420 is powered from a 3 V supply with the REF pin connected to mid-rail at 1.5 V. The inputs are AC coupled and there are 150k resistors to the 1.5 V supply to provide a path for the input bias currents. The gain is set at 151 with a 150k feedback resistor and a 1k resistor (R17) between the FB pin and the 1.5 V reference voltage. A 1k resistor is placed between 1.5 V and the REF pin. With this configuration one would expect the output to be at 1.5 V, but instead it sits at about 2.8 V. I measure (and calculate) about 8 mV across R17 which implies a corresponding 8 mV differential voltage at the input. However, the inputs are at exactly the same voltage.

 

I have tried to tie the two 'bias' resistors (R25 & R26 in the circuit) to ground; I have tried to remove the RF filter capacitors; I have tried changing the gain resistors; I have even tried cutting the trace between the output of the AD 8420 and the LTC6910. Nothing helps. The output stays near the upper rail. It is not a faulty device - three different prototype boards have the same problem.

 

I am now stuck, and I hope someone can shed some light on this problem.

 

Thanks.

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