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The original question was how to decrease power supply noise, but after discussion it seems to be just a measurement problem.

Case 1: When I short the ground lead to the probe, I read about +-2mV rms - acceptable by me. Case 2: When I connect the ground lead and the probe to the 0V of the power supply, I get a lot of noise with spikes up to +-50mV - not acceptable.

Common sens and several people say that in both cases, I should have a reading as in the case 1. What could be wrong with the oscilloscope?

The oscilloscope has an earth ground pin in the AC power cord. The power supply also has the ground pin in its AC power cord. I do not connect power supply's negative negative terminal to the earth ground (the green connector on the front panel). Even if I do, it does not seem to affect the measurement - I still get the same random noise.

Please, help me to find the source of this noise.

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    \$\begingroup\$ Do you have a reliable ground connection while measuring this? Noise can often be introduced through the probe, try to connect the probe ground as short as possible to the LDO ground. And what kind of capacitors did you use (elecotrolytic, ceramic)? \$\endgroup\$ – Arsenal Feb 3 '15 at 19:15
  • \$\begingroup\$ @Arsenal The oscilloscope gnd and probe are connected directly to the LDO pins (as well as caps) \$\endgroup\$ – Nazar Feb 3 '15 at 19:18
  • \$\begingroup\$ Is the noise that big right out of the power supply? If there is not too much current you can add a small resistor (10 ohms or so.) between the regulator and the opamp/ bypass cap. (A little low pass filter.) \$\endgroup\$ – George Herold Feb 3 '15 at 19:25
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    \$\begingroup\$ Make sure that this isn't a measurement problem. Connect the scope ground lead to your circuit as normal, then touch the probe tip to that same ground point. If you still see noise, that means that the scope is picking up noise from somewhere else. Most common culprit is the scope's AC Mains ground pin. \$\endgroup\$ – Dwayne Reid Feb 3 '15 at 19:29
  • \$\begingroup\$ @DwayneReid Could you, please, expend more on this idea? When I short the probe to the ground clamp without connecting it to anything, there no spikes and the noise rms is about +-2.5mV. However, when I connect the clamp to the circuit's ground and probe at that point, the noise comes up. Thus I measure this noise at 0V/GND. \$\endgroup\$ – Nazar Feb 3 '15 at 19:37
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You problem probably is with the earth reference between the scope and your circuit. The oscilloscope ground lead is always referenced to the earth connection of your AC plug (except battery operated oscilloscopes). Are you sure that the DC power supply you are using to power your circuit is well referenced to the same earth connection? That does not only mean that the DC power supply ground is connected to earth pin of your AC plug, it also means there is actually a connection between the DC power supply earth to the oscilloscope earth. If it does not, you will probably get the noise you are experiencing.

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The most likely culprit is your bench supply. First thing to do is look at the regulator input. If you check the data sheet, you'll see that the output noise figure is specified for a bandwidth of 10 Hz to 10 KHz, and the spikes you see on your scope have much higher bandwidth that this. There is no specified response time of the regulator to changes on the input, but it is certain to be much, much slower than those spikes.

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  • \$\begingroup\$ Does this mean that it should not be affected by those spikes? \$\endgroup\$ – Nazar Feb 3 '15 at 19:47
  • \$\begingroup\$ It means that it will not react to those spikes and thus pass them through. \$\endgroup\$ – AaronD Feb 3 '15 at 20:29
  • \$\begingroup\$ Linear regulators are basically automatic resistors. They constantly adjust their value to maintain a certain voltage between the output and reference pins. Therefore, anything that's not reacted to gets passed along. \$\endgroup\$ – AaronD Feb 3 '15 at 20:32
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Use a small capacitor between the output and ground. The cap will discharge during sags and charge otherwise, and shouldn't interrupt your output because it's not in series with it. You would have to calculate or experiment to find the right size cap.

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  • \$\begingroup\$ I use 10uF cap between GND and output, or you meant something else? \$\endgroup\$ – Nazar Feb 3 '15 at 19:19
  • \$\begingroup\$ So you use a cap between GND and output already, and you still have noise? Have you tried increasing the size of the cap? \$\endgroup\$ – MDMoore313 Feb 3 '15 at 19:21
  • \$\begingroup\$ If you add another 10nF-100nF ceramic capacitor or something like that in parallel to the 10µF it might reduce the noise as they work better for higher frequency components of the noise. \$\endgroup\$ – Arsenal Feb 3 '15 at 19:21
  • \$\begingroup\$ Makes sense @Arsenal, a smaller cap takes less time to charge/discharge. \$\endgroup\$ – MDMoore313 Feb 3 '15 at 19:22
  • \$\begingroup\$ The caps do not help. Then I measure the ground, I get similar noise in magnitude (at gnd/0V) \$\endgroup\$ – Nazar Feb 3 '15 at 19:43
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What voltage / current is your power supply putting out to your circuit under test? The quick test is to simply operate your circuit from batteries and check the noise again.

Then connect just the negative lead of the power supply to your circuit (while it operating from batteries) and check again.

You should be able to narrow down the cause of the noise in fairly short order.

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  • \$\begingroup\$ At this point I do not connect the circuit. Simply, ground lead clamp and the probe are both connected to the negative terminal of the power supply. \$\endgroup\$ – Nazar Feb 4 '15 at 21:14

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