Measuring output voltage (Single ended probe vs differential ended probe)

Thread Starter

Electronic_Maniac

Joined Oct 26, 2017
248
Hi all,

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?
 

Lo_volt

Joined Apr 3, 2014
82
To clear things up, can you post a schematic of your circuit showing the point at which you are measuring the voltage as well as the points at which you are grounding your probe?
 

crutschow

Joined Mar 14, 2008
23,527
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.
If you have both a ground from the probe and a separate oscilloscope ground then you have a ground loop, which can generate ground currents through the oscilloscope and thus measurable noise in the oscilloscope ground resistance.

Try with only the probe ground.connected to the ground point of the output capacitor.
 

SteveSh

Joined Nov 5, 2019
99
You should be able to get a good measurement with the single ended probe. Placement of the gnd lead of the probe is everything, as crutschow alluded to.

How much noise are your seeing?
 
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