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Unexpected gain rolloff with AD8031A

Question asked by RayZ on Feb 23, 2018
Latest reply on Mar 3, 2018 by RayZ

I have a using the AD8031 op amp shown below. Note the TLV271 op amp was replaced by the AD8031.

AD8031, gain=7

The input signal connected to R10 is from a sine wave from a DDS; unipolar, about 0.5Vp-p with about 0.3V offset. The output is connected to a power amp (Apex MP38) with a 10E9 input impedance.

The circuit was originally designed to operate at 10KHz and worked well. When a new application required 200KHz, I swapped the TLV271 for a AD8031.

I set the DDS to output a 50KHz sine and adjust R1 to center the output on 0V, then adjust R2 for an output of 3.75Vp-p. But as I increase the frequency, the output voltage starts to fall. At 100KHz it's down to 3.0Vp-p, at 150KHz it drops to 2.5V and at 200KHz it mysteriously stays at 2.5V. It may drop slightly from 150KHz to 200KHz; there is about 0.25V of noise from the DDS 25MHz crystal riding the sine because of poor board layout.

I did not expect this behavior, as the AD8031 has 80MHz bandwidth (G=1) and 30V/us slew rate. Can anyone help me understand what's going on?