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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    •  Super User 
    on Aug 8, 2013 7:07 AM

         Hello Jason,

         The relatively high resistances of the inverting configuration (Rf = 100K) along with stray capacitance is likely allowing enough phase lag at 250 MHz to make the circuit oscillate.  Unless you have a really fast oscilloscope you may not even see the oscillation, but the unusual offset is a clear symptom.  This also explains why both  the non-inverting circuit (with much lower resistor values) and the AD8610 (with 10x lower BW) do work.

         Best regards,

         Bob

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  • 0
    •  Super User 
    on Aug 8, 2013 7:07 AM

         Hello Jason,

         The relatively high resistances of the inverting configuration (Rf = 100K) along with stray capacitance is likely allowing enough phase lag at 250 MHz to make the circuit oscillate.  Unless you have a really fast oscilloscope you may not even see the oscillation, but the unusual offset is a clear symptom.  This also explains why both  the non-inverting circuit (with much lower resistor values) and the AD8610 (with 10x lower BW) do work.

         Best regards,

         Bob

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