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AD8422 inputs held high on power up

Question asked by jg_analog on Feb 2, 2015
Latest reply on Mar 8, 2016 by ScottH

I am using an AD8422 instrumentation amp in a simple single supply configuration.  See Below:

 

ad8422.PNG

The inverting input is on one half of the bridge with a precision thermistor in one leg, the 2.048 V is from the other side of the bridge at the fixed 2.048 V.

 

VS+ = 5 V.

VS- = 0 V.

REF = 2.048 V.

 

 

The problem is that probably 75% of the every time I power it up, the IN+ and IN- pins seems to be pulled up to around 4 V and get stuck there.  The output stays at about VREF (0 output).  This even occurs when the bridge begins completely in the correct common mode range.  Also with no Rg (Gain = 1).

 

I can shake it out of this state by momentarily shorting across R4.

 

Why would this get it out of the "latched" state?  Why is it doing this in the first place?  I must be doing something wrong, but I cannot figure it out.  It works exactly as expected when it is not in this mode.

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