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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  • Simon,

    The current bias on these amplifiers was indeed causing the large voltage offset at the output.  My source impedance is too high to use with the ADA4897; however, some of the amplifiers suggested by harryh proved to be better suited anyway due to the lower current noise spectral density (<1pA/rtHz at high frequencies).

    Thanks for all of your comments,

    Jason

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  • Simon,

    The current bias on these amplifiers was indeed causing the large voltage offset at the output.  My source impedance is too high to use with the ADA4897; however, some of the amplifiers suggested by harryh proved to be better suited anyway due to the lower current noise spectral density (<1pA/rtHz at high frequencies).

    Thanks for all of your comments,

    Jason

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