We have the OPA847 configured in the non-inverting configuration with a gain of ~30. To provide some isolation to output capacitive loading, we included a 50-OHM series resistor on the output. The output loading effectively looks like ~1.7K//1.5pF, but our probe can add an additional 9.5pF.
The amplifier behaves very well during our initial testing. No oscillation and no noticeable overshoot observed, even with the additional 9.5pF from probing.
However, we are observing a problem when we use the /DIS pin to save power. The /DIS able is connected to an open-drain FET controlled by a micro-controller. When the micro-controller drives its GPIO high, it would turn on the FET and pull /DIS low to turn off the amplifier to save power. While we can successfully turn OFF the amplifier, we are observing rail-to-rail oscillation on the amplifier output as it comes out of the DISABLED state. There is no oscillation if we simply power cycle the amplifier supply (+/- 4.5V). In addition, we can kill the oscillation if we drive the input with a sufficiently large signal to overwhelm the oscillation -- once we kick it out of the self-oscillating mode, the amplifier operates normally until we toggle /DIS again.
So far we've tried putting a small cap on the /DIS pin in hopes to slow down the ON/OFF transition edge, but that didn't quite work. Any suggestions appreciated. Thanks!
