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AD8629 slow response

When using an AD8629 in a circuit (see attachment) for in loop capacitive load compensation, the devices shows a very slow response to get within a few uV of the final value.  On the order of seconds.

The values for the circuit would be:

CL = 0.4uF

Rf=Rg=4k

Rx=200

Rl ~ 10k

Cf = 1200pF

Vin @ Rg is the output of a DAC typically between 0 and 400mV

Vin @ IN+ is a 200mV dc level.

Power supplies are +/- 2.5 volts.

I've run a quick simulation at my end, and I'm getting the proper response within a few milliseconds, which is on the order of what I would expect.  I also realize that the 1200pF is probably too small for the proper compensation.

Is there anything else going on inside these auto zero amplifiers that would present difficulties in a circuit such as this?

Thanks,

Tim

  • Hi Tim,

    Autozero amps typically takes a longer time to settle because its autozeroing circuitry takes time to settle.

    I ran some simulations with the current PSPICE model on the web, and it shows that the part takes about 1.5-2ms to settle.

    So, I have some further questions:

    What is the exact input waveform to the amplifier (from the DAC)? Any dc offset, etc?

    You mentioned the customer says his “system” takes seconds to settle. Settle to what accuracy? How is he measuring the settling time at the output? Using a scope? Is he overloading the scope?

    Do you have a schematic from the customer that shows what other components he has around the amplifier?

    This discussion is also taken offline to further understand the application.

  • Your schematic has the inverting and non-inverting inputs swapped (assuming you are designing a closed loop amplifier). Is that how you've siimulated it?

  • That is just a typo in the diagram.  I am connecting the device correctly.  As I understand, there's nothing in the input structure of the device that could cause this type of behavior, so I'm looking elsewhere.  Thanks for the help.

  • This question has been closed by the EZ team and is assumed answered.