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

Parents
  • Harry, I can also add what I'm trying to work toward.  The final configuration should be a gain of 10-20x up to 400 kHz.  The first stage will likely have to be ~10x due to GBP limitations.  The noise limits will be the source impedance (in the range of 1k-10kOhm real, 100-300pF imag).  I'm considering multiple configurations, including a charge amp.  Noise current density is the one of the most important noise contributors at these source impedances.

Reply
  • Harry, I can also add what I'm trying to work toward.  The final configuration should be a gain of 10-20x up to 400 kHz.  The first stage will likely have to be ~10x due to GBP limitations.  The noise limits will be the source impedance (in the range of 1k-10kOhm real, 100-300pF imag).  I'm considering multiple configurations, including a charge amp.  Noise current density is the one of the most important noise contributors at these source impedances.

Children
No Data