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I am performing some measurements on this circuit (simplified).

enter image description here

Some details:

  1. Power supply is linear
  2. Circuit is mounted on a breadboard
  3. Oscilloscope connected using a ground spring
  4. Oscilloscope and DMM connected to two near main sockets
  5. As far as I know, the main sockets should have good ground connection

I noticed that connecting the negative lead of the SDM3055, it injects some 50Hz noise.

This is the output signal when the SDM3055 is not connected:

enter image description here

This is the output signal when the SDM3055 is connected:

enter image description here

I have tried:

  1. to change the DMM probes
  2. twist them together
  3. make them shorter (not to much actually)

but apparently nothing reduces the 50Hz noise.

Moreover, I have tried to disconnect the signal generator but even this doesn't make the 50Hz noise disappear.

Apparently, it is indeed the SDM3055 to inject the noise.

Any explanation or suggestion?

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    \$\begingroup\$ The signal generator and scope are connected to the safety ground. Make sure that the (-) wires from these two devices are connected at the same physical point on your breadboard. (No wire between them). Then try connecting the (-) lead of the voltmeter to different “grounds” on your breadboard and see what happens. \$\endgroup\$
    – user69795
    Commented Jan 7 at 16:55
  • \$\begingroup\$ Signal Generator (-) and DMM (-) are actually connected at the same physical point. I updated the circuit diagram to show that the DMM (-) cannot connected in other places. \$\endgroup\$
    – Fab
    Commented Jan 8 at 8:23
  • \$\begingroup\$ Are you sure the oscilloscope and signal generator don't both have their (-) connected together via mains earth? \$\endgroup\$ Commented Jan 8 at 14:33
  • \$\begingroup\$ Hello, Fab: the scope and signal generator’s negative leads are actually connected through their power plug. This means that your circuit diagram is misleading. For example r4 is shorted out. \$\endgroup\$
    – user69795
    Commented Jan 9 at 21:26
  • \$\begingroup\$ @Simon Fitch: This is a good point. As far as I know the oscilloscope (-) is connected to mains earth but I don't know anything about the DMM and the signal generator. I will try this: 1) replace the signal generator with a battery and 2) replace the SDM3055 with a battery powered multimeter (even though less accurate). Thank you both for your help. \$\endgroup\$
    – Fab
    Commented Jan 10 at 11:45

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