Differential probe, curiosity - Page 1
Forum section: electronics / beginners

author: DisasterNow
author_rank: Regular Contributor
timestamp: April 02, 2024, 02:49:57 pm
content: The differential probe, by its name, measures a signal between two points at different potentials. Unlike normal probes, the black reference may also have a voltage (and not necessarily be the ground).But my curiosity concerns when I measure an alternating signal (let's take the example of the mains voltage), I saw that it is enough to connect one of the two probes and on the display I have the perfect signal (equal to if I had connected a probe to the line and one to the neutral ). In the differential probe, no probe is referenced to earth. Why is it that a probe is enough to have the exact wave of, for example, an alternating voltage? Thank you

author: golden_labels
author_rank: Super Contributor
timestamp: April 02, 2024, 03:53:52 pm
content: Both inputs of a differential probe are referenced to ground. An amplifier inside the probe boosts the difference between inputs, rejecting the common part. If one of the inputs isn’t connected, it is effectively bound to ground.The difference from plain probes is that the connection to ground is relatively high impedance. So it doesn’t affect the tested circuit or signals.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 02, 2024, 04:14:01 pm
content: If you look at the front end design for any of these differential probes you'll see both inputs terminate to ground/output common, there isn't any true isolation, just high enough resistance (several megohm of HV resistors) to be safe. You can measure this yourself with a DMM.

author: tggzzz
author_rank: Super Contributor
timestamp: April 02, 2024, 08:04:02 pm
content: In almost all oscilloscopes, the probe shields are connected to the scope case, and for safety that casemustbe connected to protective mains earth. If it isn't, it can be lethally dangerous!For a lot of careful explanations of the different types of probes, where they are applicable, and how they work, see the references listed athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/There's no point in us poorly duplicating those.explanations!

author: DisasterNow
author_rank: Regular Contributor
timestamp: April 03, 2024, 10:44:33 pm
content: Quote from: golden_labels on April 02, 2024, 03:53:52 pmQuote from: golden_labels on April 02, 2024, 03:53:52 pmIn fact I measured the resistance of the normal probe and that of the differential: a few ohms versus 8 MohmsQuote from: tggzzz on April 02, 2024, 08:04:02 pmThanks for the address, I downloaded some content.------However, I have seen that with the differential probe, only with the alternating signal of the mains voltage I can see the wave using only one probe; with a continuous signal, or a square wave, both probes are needed.

author: tggzzz
author_rank: Super Contributor
timestamp: April 03, 2024, 10:50:43 pm
content: Quote from: DisasterNow on April 03, 2024, 10:44:33 pmSomething is very wrong, either with your measurement, your description of your measurement, or the scope+probe.QuoteGlad it was helpful.You need to understand EMI and coupling through "stray" inductances and capacitances.

author: DisasterNow
author_rank: Regular Contributor
timestamp: April 03, 2024, 11:14:11 pm
content: Quote from: tggzzz on April 03, 2024, 10:50:43 pmI just downloaded the contents and read them calmly. Thanks again.In what sense is there a mistake? What should I have expected when measuring the resistance of a normal probe and a differential probe? Thank you

author: tggzzz
author_rank: Super Contributor
timestamp: April 03, 2024, 11:29:50 pm
content: Quote from: DisasterNow on April 03, 2024, 11:14:11 pmYou need to specify exactly what you are measuring, and how.In order that the circuit is not disturbed by the measuring tool (scope or otherwise), measuring voltage requires high impedance, and measuring current requires low impedance.

author: DisasterNow
author_rank: Regular Contributor
timestamp: April 04, 2024, 01:27:13 am
content: Quote from: tggzzz on April 03, 2024, 11:29:50 pmI simply measured the mains voltage (230 v) with the differential probe, using only one of the two probes: the wave on the display was perfect. Then I instead measured a DC voltage (9v battery) and had to use both test leads. Finally I measured the calibration square wave of the oscilloscope, and even in this case I had to use both probes to get the wave. In essence, only with the AC mains voltage do I see the wave on the oscilloscope using only one probe (which of the two is the same). Thank you