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I have a pressure sensor that outputs 0.5V with a +/- 20 mV noise. I am trying to identify the root cause of the noise using an oscilloscope. The noise occurs in waves of twice the mains frequency, and each noise wave pulse is made of smaller pulses of much higher fequency.

The noise used to be much higher when I was using a "wall wart" 12V power supply to the sensor than when I used the bench top power supply or a proper 12 V dedicated power supply.

Weird clue 1) although the noise was partially related to the power supply used, the power output of the wall wart supply didnt show any noise.EDIT: the noise is there, but catching a 50mV noise on a 12V signal was just difficult on my oscilloscope.

Weird clue 2) when I touch with my finger the conductive probes (either positive or negative) of the oscilloscope, while measuring the sensor signal, the noise decreases

I got minor improvement in the noise by putting capacitors between the signal and the ground, but I have a feeling my main issue is elsewhere. Can you please clarify the root cause of the noise ?

Edit: after suggestions, please find the oscillo screen using different power suppliesenter image description here

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    \$\begingroup\$ It's "correlated noise" if you can identify a specific frequency. In this case, it's correlated to your mains frequency (probably doubled because of a bridge rectifier.) It's also likely that the non-linear behavior of these diodes in the bridge rectifier, as they very rapidly flood with current during a short period each half-cycle, is a source of the higher frequency pulses you also observe. There may be other sources, but those are the first two I'd concentrate on. Wall warts are known to be horrible on these aspects, too. So no surprise there. \$\endgroup\$ – jonk Mar 31 at 19:59
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    \$\begingroup\$ Try to use other 12V supply such as batteries which gives clean DC and then observe. \$\endgroup\$ – Unknown123 Mar 31 at 20:12
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    \$\begingroup\$ Your high impedance amplifier is unbalanced somewhere and is picking up stray E-fields from somewhere, show your work \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 31 at 21:31
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    \$\begingroup\$ Your power supplies have primary-secondary transformers. The diode rectifier fast-edges couple from secondary to primary, and then to every power outlet, and to Ground, of your entire lab and environment. All possible paths back to the diodes is taken by the electrons, including thru you. \$\endgroup\$ – analogsystemsrf Mar 31 at 21:35
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    \$\begingroup\$ You seems to be confused. Seriously, the easiest way to figure it first is using clean DC source. So you can have a certainty that the AC-DC SMPS supply is the main culprit or not and thus may also factor it out or in. You don't tell us your current demands, so I could say that you could put 8 AAA batteries in series or using one 12V lead-acid battery. \$\endgroup\$ – Unknown123 Apr 1 at 0:29
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A capacitance from the DC output to the table kills all the 50hz noise

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