AD8638 weird behaviour

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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  • Hi Kaung

    I really meant the ADA4622.

    Thanks for the hint with the LT6015. For my application it is not really necessary that the op amp withstands such high voltages as the inputs are externally protected. The input is clamped to the supply rail with diodes.

    But as both amps are in the same package and have the same pinout, I will test both of them and see which one gives the best results.

    Thanks for your help

    Regard,

    Patrick

Reply
  • Hi Kaung

    I really meant the ADA4622.

    Thanks for the hint with the LT6015. For my application it is not really necessary that the op amp withstands such high voltages as the inputs are externally protected. The input is clamped to the supply rail with diodes.

    But as both amps are in the same package and have the same pinout, I will test both of them and see which one gives the best results.

    Thanks for your help

    Regard,

    Patrick

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