Pulling my hair out with this design. I have the following design which was done using analog devices Photodiode Circuit Wizard tool so it should work. The current source in the schemeatic is the photodiode UV-035EQ. the op amps are powered by +-5V.

This setup is for more measuring low level UV light. But as soon as I place the photodiode into darkness I get a horrible waveform with a peak/trough of about +-2V https://drive.google.com/open?id=18lMKfpOSzvvhQ3yqGymRFszVO9QUYHl7

enter image description here

Lookig at the photodiode it looks like its working fine I've tried multiple ones and they all seem the same.


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ What frequency is that? It looks like it might be amplifying 60Hz/50Hz mains hum? \$\endgroup\$ – Hearth May 1 '18 at 16:48
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    \$\begingroup\$ Actually, yes, looking at your scope image that's 50Hz. Somewhere, something in your circuit is picking up mains-frequency noise. It could even be related to the oscilloscope you're measuring it with. \$\endgroup\$ – Hearth May 1 '18 at 16:49
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    \$\begingroup\$ Try aluminium foil over the PD to see what happens. Try earthing the foil. Also try reversing its polarity to see if the problem goes away. What about power rail decoupling - do you have any? Is this amplifier being fed from a sh1tty wall wart? \$\endgroup\$ – Andy aka May 1 '18 at 16:51
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    \$\begingroup\$ What happens if you remove the PD entirely? Is the PD right close to the op-amp or separated? \$\endgroup\$ – Spehro Pefhany May 1 '18 at 16:53
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    \$\begingroup\$ Is this a breadboard design or PCB? It might be 50Hz noise. When there is no light, the PD goes non-conducting and that helps OA4's negative input drift away from ground. The antenna effect will be worse if you're on a breadboard. Also, you might want to probe OA4's output and confirm that it is the source. It could be something weird. \$\endgroup\$ – pscheidler May 1 '18 at 17:00

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