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AD8638 weird behaviour

Question asked by PMatschek on Feb 22, 2018
Latest reply on Feb 26, 2018 by PMatschek

Hello

I'm testing a simple circuit with the AD8638 Op Amp.

The Amplifier is supplied with 12 VDC single supply and works as a kind of voltage limiter. The limiting voltage should be 9.9 V which is derived from a voltage reference.

The auto zero amplifier is there because i dont want a drift in the limit.

If I test the circuit the limiting function works nicely, but for voltages lower than 9.9 VDC at the input i get extremely bad results. If I disconnect the source (VS3) and pull the 5k1 resistor to GND i get around 8.5 VDC at the inverting input of the op amp.

I could see that this behaviour comes from the rectifier at the output.

When there is no feedback, i can also see that the input is pulled up to about 8.5 VDC.

I couldn't see this in the simulation. There everything works fine.

 

Question is: how is it even possible that the high impedance input can be pulled up when the feedback is not closed ?

 

I hope somebody has an explanation for that behaviour.

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