Other Parts Discussed in Thread: OPA170,
It seems as if the pinout may be wrong. Our input to the + on pin 1 is being crushed (to 0V) when we enable the Op Amp (/SHDN goes low, which should be off, but their naming is wrong and low is 'on').
So we generate a PWM signal, R/C filter it and present to the + input on Pin 1. Looks fine. Then we enable the Op Amp and the input to Pin 1 is crushed to 0. But nothing else connects to pin 1. This is set up to be a follower (Op Amp output connected to - input). So what is there to crush the input?
We notice that their similar part, OPA170, has a pinout for the "5 pin SOT" (which appears to actually be a SC-70), which is our same pinout (1 +, 2 V-, 3 -, 4 out, 5 V+) but their SOT 23 pinout is different (1 out, 2 V-, 3 +, 4 -, 5 V+). We're wondering if the OPA990S input pins might be 3 and 4 and not 1 and 3.