Other Parts Discussed in Thread: OPA392
We are measuring faint photodiode output with a transimpedance amplifier (uses AD8605) AC coupled into a PGA (uses PGA113) followed by a low-pass filter (uses LTC1563-2).
We modified a working design by replacing the LTC6910 family PGA with the PGA113 for higher gain and SPI programming. We added Vref per Figure 72 of the datasheet (the LTC6910 had an internal half-supply reference). Unfortunately, the output drifts away from Vref. See the images of the amplifier circuit, the reference voltage circuit (called Vdc in the schematic), and a video showing behavior when the PGA gain is reduced from a value that is saturated. At first the output shrinks in half, giving a good signal, but then it drifts up to the supply voltage and saturates.
Any advice on what is going wrong? Should we add a resistor to ground somewhere to counteract the drift?