We need to lock our ISS to the PD output in real time without worrying about DC fluctuations in the PD response.
I made a dual voltage follower using an OP275 and some capacitors.
The schematic is on the box in picture 2.
I checked both voltage follower input/output pairs with an SR785. We see a nice low pass with cutoff frequency ~0.7 Hz.
I'll install this tomorrow.