Does a long ground loop make transient response reading worse on scope? - Page 1
Forum section: electronics / general

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: April 09, 2025, 02:14:02 am
content: I know that when I am using a standard passive probe with the several inch long ground clip, I get an inflated ripple reading on the scope compared to using coax with a very short ground loop. But does this also affect the transient response? Or is the transient response of a regulator a relative difference that remains constant regardless of how bad your ground loop is on your probe?Also does this relate to common-mode vs differential mode noise? Is it true to say that a long ground loop increases common-mode noise and that's why the ripple reading is inflated?

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 09, 2025, 11:54:39 am
content: I'm guessing the "loop" you're referring to is the one formed by the scope probe tip, its ground lead, and the path on the circuit under test between the tip and ground lead?That's not what I would call a "ground loop," usually that term is used to describe multiple return paths for signals forming a loop... but even that is quite ill-defined.Anyways, the long ground lead will cause your measurement to contain the signal of interest, plus other undesired signals. For example, spikes from a switching power supply may be magnetically induced in the loop formed by the ground lead, contaminating your measurement of output as additional ripple.This induced "noise" is added to the "real signal". If the signal of interest is a much lower frequency than the induced noise, then you can just filter it out to observe your signal. I'd expect this to apply to the transient response of a DCDC converter (which should be much slower than its switching frequency).However, let's say you want to measure the voltage at the switching node of the DCDC converter which was generating those spikes. In that case the spikes will have similar frequency content to the signal of interest, and they will likely occur right as the signal changes. Therefore it won't be possible to ignore the effect of the spikes on the measured waveform. In such a case you need to eliminate the induced spikes with better probing (like coax, for example).

author: tggzzz
author_rank: Super Contributor
timestamp: April 09, 2025, 04:43:42 pm
content: Quote from: Jim from Chicago on April 09, 2025, 02:14:02 amThe key to understanding is to realise it is a damped resonance, then to understand the physics (what causes the L,C,R) and the maths (centre frequency, duration).After that you can apply the understanding to other circumstances such as the poorly defined...QuoteOnestartingpoint (not a finishing point) for probe resonance ishttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

author: bdunham7
author_rank: Super Contributor
timestamp: April 09, 2025, 05:12:51 pm
content: Quote from: Jim from Chicago on April 09, 2025, 02:14:02 amThere are two effects, one is the noise that is picked up in the area between the ground and signal leads the other is the inductance of that several inches of ground lead. I think the noise picked up is differential noise, not common-mode. As for the transient response, whether the inductance of that section of ground lead causes an issue depends on the timeframe--faster transitions will be distorted and will show ringing.