Huge offset voltage with ADA4897-1

I have a standard inverting amplifier stage with Rf = 100k and Rg = 10k (A = -10 V/V).  The amp is powered from +/-5V and the positive input terminal is connected to 0V.  Pins 1 and 5 are open.  Pin 8 (nDISABLE) is also left open.  Very straightforward setup.

The problem is that the output has a DC offset of -2.2V, but otherwise behaves as expected for AC signals.  Tying the disable pin high has no effect.  This was repeated on three ADA4897-1 parts and all were identical.  Inserting a bias compensation resistor of 9kOhm between positive input terminal and ground brings the DC offset to about 100mV.

Without the bias compensation resistor, I swapped out the part for an AD8610 and the offset disappears and the circuit behaves normally.  Interestingly, the offset voltage is absent when I use the ADA4897-1 in a basic non-inverting configuration (Rf = 247, Rg = 13, A = 20 V/V).  Did I just receive a batch of bad parts, or is there something I'm missing specific to these components?

Thanks,

Jason

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  • Thanks for responding so quickly.

    Bob, I don't see any signs of instability up to 100MHz, but I can inspect the circuit with a higher frequency scope.  That's a good suggestion.  I initially had a feedback capacitor in place to limit the bandwidth.  This amp is fully compensated (stable gain of 1), so I didn't consider that to be a potential issue.

    Harry, I do understand the noise impact on these component values.  The values in the inverting amplifier configuration is not intended to be anything but a sanity check.  If I can't get it to work in this configuration that would likely mean something else is going wrong (bad layout, bad part, power supply, etc.).

    I will test the inverting amp configuration with lower values (Rg = 10, Rf = 100) and report back here.

    -Jason

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  • Thanks for responding so quickly.

    Bob, I don't see any signs of instability up to 100MHz, but I can inspect the circuit with a higher frequency scope.  That's a good suggestion.  I initially had a feedback capacitor in place to limit the bandwidth.  This amp is fully compensated (stable gain of 1), so I didn't consider that to be a potential issue.

    Harry, I do understand the noise impact on these component values.  The values in the inverting amplifier configuration is not intended to be anything but a sanity check.  If I can't get it to work in this configuration that would likely mean something else is going wrong (bad layout, bad part, power supply, etc.).

    I will test the inverting amp configuration with lower values (Rg = 10, Rf = 100) and report back here.

    -Jason

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