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.