I am debugging a colleagues OPA381 based transimpedance amplifier and am getting this weird 300khz oscillation riding on top of the output. The amp is connected to a photodiode as part of a particle detection sizing instrument. I get decent scatter signals but I also have this
300 khz that I can't nail down to any causes:
- Power Supplies look good, ripple ~2mv
- Tried adding up to 100 pf in parallel with feedback resistor - I can still make it "oscillate" by properly positioning something static (clear acrylic with a scribe line) in the beam path.
- If I increase laser power it may go away or it may stay the same.
- Does not appear to be a microphonic effect.
Questions:
- Are there optical scattering phenomena in in the 300 khz range off of stationary (to the naked macroscopic eye at any rate) that I don't know about (there is much I don't know)
- Would more power supply bypassing help.
Any and all suggestions welcome as always.
Thanks much, Ed V