I'm curious if it is possible to get decent results when performing a differential measurement using two single-ended o-scope probes. We'll be ordering some differential probes in the near future, but at the moment dealing with what we have. We need to measure the fluctuations on the output of a voltage regulator circuit (provided by a microprocessor chip) and therefore need to do a differential measurement because of the low amplitude. Question is, since one of the probes would be measuring common mode voltage on the ground trace, wouldn't connecting the ground lead to ground just short the probe out and result in zero difference. If so, wouldn't this give the same result as just using one probe channel? Put another way, is it possible to do a differential measurement with two single-ended probes if one of the measurement nodes is ground?

 

I am not quite sure what you´re asking, ground in one circuit is not necesarily the same ground in another circuit, and even less common with wall outlet PE. Maybe post a diagram?

Another important thing is that when you use two probes in subtract mode on a scope, the result will have pretty bad CMRR. Try hooking two probes to a same point and see what you get as their difference.

 

Question is, since one of the points I need to measure IS ground, will I see any signal since the probe is referenced to that same ground? Seems like I would pick up any noise incident upon the ground lead, which is the noise I am looking to cancel. I realize the CMRR will be poor, which is why we are ordering differential probes. I was just curious if doing such a measurement is possible and would yield a better result (cleaner signal) than doing a regular measurement using just one probe.

 

There is no reason to use other probe if you connect the ground leads to the same point as one of the probe tips.
Usually you need diff probes when for example you need to see a voltage referenced to ground and a voltage across an inductor or sense resistor etc. that is not referenced to ground at the same time. If you didn´t need to see the first voltage, then hooking the probe across the inductor is all you need.

 

A differential probe can be useful, even when measuring to ground since it will minimize any high frequency noise due to capacitive ground loops between the UUT and the scope. I've often seen significant noise on an oscilloscope even when I connect the probe directly to the ground clip, when the clip is connected to the ground of a circuit with high frequency currents, such as a switching regulator. Under those conditions a differential probe, with good high frequency common-mode rejection, can reduce the apparent ground noise.

 

I'll give it a try and see what happens. I realize a differential probe is the way to go, but we may not get one for a while.