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.
ADA4622 is a JFET input amplifier and not a zero drift amplifier. I'm not sure if you meant ADA4522 since this is a zero drift amplifier and you mentioned you wanted an auto zero amp.
ADA4522 will have similar behavior as the AD8638 but with a different diode voltage of +/-5V. (Refer to Figure 72 of http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4522-1_4522-2_4522-4.pdf). I do not recommend using ADA4522 if you have to use this type of circuitry.
AD4622 should work but take a look at LT6015 for this type of application. See the links below for an application similar to yours. Over the top (OTT) amplifiers are better suited for this type of application if your input signal can use the input of op amp to go beyond the supply rails. Using OTT amplifiers will protect your downstream parts.
The LT6015 is offered in a 5-lead SOT package. The LT6016 dual op amp is available in an 8-lead MSOP package.