Electronic – How the low inductance of short ground clip probes prevents interference

One simple example

Let's say you have two scope probes. Probe A is correctly grounded to the circuit you are measuring but probe B doesn't have a ground connection. Let's also say that probe A is measuring a 1Vp-p squarewave on the circuit and probe B is looking at a DC level.

Despite scopes having a 1M\$\Omega\$ input impedance, there will be currents that flow through the probe due to the edges of the square wave - the scope input impedance is also likely to comprise 20pF capacitance (smaller for x10 probes usually).

This capacitance "takes" a pulse of current down the probe lead every time the square wave alternates causing a small volt-drop down the lead. This won't matter for probe A but, this will appear as a small "spike" on channel B's display because the glitch "becomes" in series with the probe B signal due to it being ungrounded.

If probe B were correctly grounded at the circuit this "glitch" won't be present on the scope B waveform.

What point/frequency does this become significant? I would answer that by saying just don't take short cuts. If you are probing a circuit for whatever reason, use best practise and ground both probe ends at the circuit you are measuring or expect anomalies.

If probe A is looking at 1Vp-p squarewave and probe B is looking at a 5mV analogue signal you'll have troubles on channel B.

Depending upon your scope probe maker it may be possible to order replacement parts from the manufacturer. This usually gets you the short GND lead with the probe GND lug on oneside and the alligator clip on the other end.

When you receive the spare parts you cut off the alligator clip end and solder the wire to your new custom end fixing.

Advantages of using the original probe manufactures GND lead assembly include that lug end is known to be correct for attachment to the probe. I have seen variation from manufacturer to manifacturer and there is little certainty that Mfg A's parts will work in Mfg B's probes. Another factor is that decent GND probe leads are made of very flexible stranded wire that may be difficult to source without purchasing a whole roll. Regular wire you find in a lab is typically a bad choice for these GND clips because it can break with a small amount of usage.