5V Square wave but not when polarity reversed

Thread Starter


Joined Jul 30, 2015
So I'm generating a 5V square wave at 1000Hz through a 400ish ohm resistor. When I put the probe on, it showed up as a -5V square wave at 1000Hz. Ok, so swap the leads on the probe. Except when I do that, it has a terrible overshoot at the start of the cycle and a terribly slow decay for the second half of the cycle. I feel like this is not expected behavior.

Why would a polarity change across a non-polar, linear element look like a completely different signal on my oscope? I included captures of the two signals. Thanks!




Joined Feb 5, 2010
If your powersupply and your Oscope share a common AC source voltage you might be grounding the signal output when you reverse the scope connections.
Description of your power connections would let us know if this is happening.


Joined Jan 15, 2015
Are you using an attenuator probe as in 1X or 10X selectable or a 10X probe. Does the probe allow for a compensation adjustment? Sounds to me and looks like a probe compensation issue.

Normal Compensation:

Normal Comp.png

High Frequency Accentuation:

Over Comp.png

High Frequency Attenuation:

Under Comp.png

While the above images do not quite duplicate what you have you may want to check your probe compensation.



Joined Mar 14, 2008
It's likely that the oscilloscope and the circuit have some sort of ground connection in common through the power line safety ground.
Normally you do not connect the ground lead of a 'scope to the signal lead of a circuit.