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I am measuring huge 50Hz oscillations on my oscilloscope, but I am not entirely sure the probe is generating those by behaving like an antenna (an ADC at the same point gives stable values but it's not sampling fast enough to give a definitive answer).

Would adding a capacitor in parallel to the probe (tip and gnd) filter out that noise (22nF in parallel to 1Mohm in my case)?

I will answer that question as soon as I test it (for the sake of others too, it appears there are not a lot of answers to that question online, and no good one), but I would like an intuitive or step-by-step description of what is or would be happening.

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    \$\begingroup\$ It would be helpful to say where you attached your scope ground clip and to draw the schematic. \$\endgroup\$ – Martin Mar 6 '14 at 16:43
  • \$\begingroup\$ Thanks for your comment. I intend it to be more of a general question, but in my particular case it's at the output of an inverting amplifier with symmetrical power supply basically. The input is a signal from LDRs exposed to LED light. \$\endgroup\$ – Mister Mystère Mar 6 '14 at 16:50
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    \$\begingroup\$ Also, the magnitude of the 50Hz oscillations might give some clue to the cause. \$\endgroup\$ – Phil Frost Mar 6 '14 at 19:17
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This smells strongly of not having the probe ground connected, or perhaps there is a break in that wire or connection somewhere. Try putting the probe on the ground of the circuit that contains this A/D. If you still get a lot of noise, then the probe ground is not connected.

To see if something is broken inside the probe, hook the probe onto the ground clip of the same probe, then touch that connection to the ground of the circuit. If the probe ground connection is working, then you should seen nothing or a few mV of noise maybe.

Depending on what you find, diagnose from there.

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  • \$\begingroup\$ Thanks for your answer, it will certainly help out someone in the future as it is indeed many times the issue - though in my case, I have checked that the ground of the oscilloscope was connected properly. \$\endgroup\$ – Mister Mystère Mar 6 '14 at 17:43
  • \$\begingroup\$ Are you using the shortest ground lead possible, ie a ground spring? edn.com/design/test-and-measurement/4312794/… \$\endgroup\$ – dext0rb Mar 6 '14 at 19:04
  • \$\begingroup\$ Only a crocodile clip that comes with the probe. But I'm starting to think it comes from the design... Here is my schematic: dropbox.com/s/c3x7kmti30boemq/circuit.png - when I add the resistor highlighted to inject current and compensate for a bias current from the detector, the output falls as expected but also injects a lot of 50Hz noise that I was able to witness using the ADC by decreasing the resolution. Why? \$\endgroup\$ – Mister Mystère Mar 6 '14 at 19:32
  • \$\begingroup\$ What kind of light is falling on the PSD? \$\endgroup\$ – Spehro Pefhany Mar 6 '14 at 19:48
  • \$\begingroup\$ A bit of LCD from my screen but mainly white LEDs (I've switched all the room's lights off). What I can't explain is why it does that only when I plug in that 10Mohm resistor. \$\endgroup\$ – Mister Mystère Mar 6 '14 at 20:51
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You are using a Hamamatsu S2044 to measure the position of "something". Have you considered that under luminescent lighting (less under normal light bulbs) the signal you may be seeing is due to the lamps. Also, adding the resistor to -5V may make part of the photodetector imbalanced. I'm no expert on these things and it's still more likely the ground break on the scope that Olin mentioned but try turning the lights off and see what happens.

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  • \$\begingroup\$ Thanks for the answer. I should probably have made 2 topics, one general about that capacitor thing and the other for my problem. As long as the general one eventually is answered I don't see why I couldn't benefit from your help. The lighting is not luminescent, only LED. In fact it's interesting to notice even under neon light the signal is really clean. It's only when I add this resistor that it gets messy. What do you mean by "imbalanced"? Do you see another simple way to inject bias currents? \$\endgroup\$ – Mister Mystère Mar 6 '14 at 20:53
  • \$\begingroup\$ Do the lights have any effect with the resistor to -5v dude? \$\endgroup\$ – Andy aka Mar 6 '14 at 20:56
  • \$\begingroup\$ With the resistor on, it's behaving as expected with a correct offset, but there is a lot of oscillation around the correct value. \$\endgroup\$ – Mister Mystère Mar 6 '14 at 21:23
  • \$\begingroup\$ I'm trying to understand if the lights cause it to become worse with the 50Hz pickup with or without the resistor added \$\endgroup\$ – Andy aka Mar 6 '14 at 22:00
  • \$\begingroup\$ Without the resistor, everything is nice and clean but I have ~2.5V of offset when the detector is obscured (only with that pad). After the resistor is added, I have -0.2V of offset (sort of what I want) and the same rate of change with incident light (I think), but also 50Hz noise which gets worse and worse as the detector is lit, starting at 0.2Vpp though to 2 to 4Vpp! \$\endgroup\$ – Mister Mystère Mar 6 '14 at 22:16

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