I have a SMPS buck 3.3V output. I am trying to measure the output voltage at the output capcacitor using Oscilloscope.

I am using a single ended probe with spring ground tip to measure the output voltage.

The oscilloscope ground is connected to the module ground. (The module ground is not the same as board ground)

When I measure the output voltage at the output capacitor (with single ended probe with ground tip) , I find the output voltage to be very noisy.

But when I measure the output voltage using differential probe, I am not able to find the noise?

Can someone tell me where the noise is coming from?

Is this due to different grounds or some common mode current?

Can someone explain me a little clearly with basic terms?


2 Answers 2


That noise is going to be of the common mode type. Meaning it is on both parts of your scope probe (the tip and the ground pin).

The differential probes work by subtracting the two signals that it sees on each probe tip. So any noise that is present on both inputs will be subtracted from each other and goes away, leaving only differential signals.


One potential problem with a single-ended 'scope probe is the loop area formed by the probe plus six-inch ground clip. Any high-frequency alternating magnetic field entering this loop area generates a voltage fed back to the 'scope:
noise entering probe loop area.
When you're probing a switch-mode power source, this loop area may be very near an inductor whose chopping frequency is not well-contained...noise!

Some good probes include a small springy fixture that attaches to the probe's tip that reduces the loop area greatly...the ground connection is annoyingly short and requires some dexterity to get probe tip and short ground both connected:springy probe tip ground

There are other ground-loop scenarios too, involving your scope (which is earth-grounded for safety) and the device that you're probing which is also earth-grounded...that's a huge loop that may add much 50/60 Hz noise to your measurement.


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