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OP4177 inputs crosscoupling when on one input is grounded.

Question asked by rgray on Sep 3, 2015
Latest reply on Sep 9, 2015 by harryh

I hope someone can help me with this.

We have a circuit topology that takes in three inputs and outputs the middle value of the three inputs.  Also part of the circuit is a monitoring circuit that determines if one of the three is not near the other two and removes that signal from the function.


There are three op amps at the input of the function.  They are all identical in topology.  On the negative input is a input that goes to a 100K resistor in series to the negative input of the op amp.  Between the 100K resistor and negative input of the op amp is a switch that will turn on to ground if the monitor circuit determines that the signal needs to be taken away from the function.  On the positive input of the op amp is the output of the function (is an output of an op amp).


The trouble starts when the input voltages are around 7 volts and one of the switches turns on.  We find that the other two inputs droop about 1/2 volt.  This affects the output of the function to the point that it stops working.  The 1/2 volt across the 100K seems to indicate a 5uA current going into the op amp input.  But this seems to be caused by grounding a input on another op amp (in the same package).