What is something a differential probe can NOT do that a single ended probe can? - Page 1
Forum section: electronics / testgear

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: November 17, 2021, 05:00:21 pm
content: Dear all,I have read quite a bit about low voltage active oscilloscope probes (1GHz bandwidth is enough for me) and while there are a lot of guides on the applicability of active probes in general (e.g. they put a lighter load on the circuit at high frequencies) and the differences between single-ended and differential probes (the latter especially for differential signals and non-ground referenced measurement points), I have not yet found an answer to one question:Why are there single-ended active probes at all? What are some things that can NOT be achieved with a differential probe of similar specifications (or provide inferior results)?Maybe I am tackling this question from the wrong direction, maybe I overlooked something. Please enlighten me :-) If this is a dumb question, please point me to the correct post.Also welcome: teaching videos on the general nature of active probes and how to use them, projects involving extensive use of active probes etc. Everything that helps me getting better at probing techniques at high frequencies. I have seen the videos of Dave on the subject and some webinars of R&S and Keysight.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 17, 2021, 05:18:23 pm
content: Diff probes have generally higher noise, higher input capacitance and bad high frequency response besides more complex and more expensive comparing similar specs.So single ended active probes are *very* useful for low voltage signals and high frequency fidelity.Even when there is common mode (high speed common mode and/or high voltage common mode) a differential probe usually has bad CMRR at high frequencies which translates into distortion, whereas a single endedisolatedprobe will do a much better job at rejecting the common mode.So single ended inputs are better and simpler structures to measure signals overall.

author: David Hess
author_rank: Super Contributor
timestamp: November 17, 2021, 05:38:38 pm
content: Differential probes have have at least 3dB higher noise. Differential probes have low common mode rejection compared to single ended probes for which common mode rejection has little meaning.Isolated singled ended probes are available with the best features of both if asymmetrical input impedance is acceptable.

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2021, 06:13:47 pm
content: Quote from: David Hess on November 17, 2021, 05:38:38 pmI don't understand this statement. Obviously, CMR is not defined for a single-ended probe, so how can differential probes have lower CMR than single-ended probes?

author: MadTux
author_rank: Frequent Contributor
timestamp: November 17, 2021, 06:26:56 pm
content: Measure voltages at an offset, like floating 12V Mosfet drive signal on 580VDC rail, on inverters, variable frequency drives, welders and such.....

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 17, 2021, 06:43:08 pm
content: By the way, there are two completely different types of differential probes:Active Differential Probe, 6 GHzand70MHz Differential Probe(both specimen pseudo-randomly selected)While the first one has better signal quality for low signal HF frequencies. I don't think it has 3dB higher noise compared to single ended for signals it is targeted for. While the last one is targeted for something completely different.I think some people mix them up here. The OP talks about a low voltage differential probe. So I would think he talks about the first type (maybe not one that costs > $5K).

author: nctnico
author_rank: Super Contributor
timestamp: November 17, 2021, 06:48:55 pm
content: Quote from: MasterTech on November 17, 2021, 05:18:23 pmThat is only true for high voltage differential probes. For low voltage differential probes that isn't true. You can buy these with sub-pf input capacitance and several GHz of bandwidth.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 17, 2021, 07:01:50 pm
content: Quote from: nctnico on November 17, 2021, 06:48:55 pmThe pF inputs may be true, but if there is no common mode, the HF response of single ended probes will be better as differential probes always have ringing. This will be higher the longer the test leads.Even a 1GHz probe has 250MHz usable bw with not so long leads

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: November 17, 2021, 09:19:08 pm
content: Quote from: David Hess on November 17, 2021, 05:38:38 pmI do have an isolated Fluke Scopemeter for lower frequencies (<100MHz), but I am interested in my options for measuring signals >200MHz.Quote from: Twoflower on November 17, 2021, 06:43:08 pmThanks, I am indeed referring only to low voltage, high frequency differential probes and how they compare to low voltage, high frequency single ended probes.I am kind of limited to probes that are either battery powered or made by Siglent, Tektronix (Tekprobe Adapter) and Lecroy (ProBus Adapter). The Siglent SDS5000X provides great value and a not-so-great selection of active probes.Tekprobe is cheap on ebay though, which is nice.Quote from: MasterTech on November 17, 2021, 07:01:50 pmHow does ringing relate to a probe being differential? I thought that ringing is a result of the complex input impedance, which is also present in a single-ended probe?I don't understand the second argument - you take a 1GHz differential probe, attach long leads to it (which limits the frequency range) and argue that this is a problem of differential probes. Shouldn't the same thing happen if you were to attach long leads to a single-ended probe?Thanks for all the replies so far

author: tautech
author_rank: Super Contributor
timestamp: November 17, 2021, 09:23:16 pm
content: Quote from: FlexibleMammoth on November 17, 2021, 09:19:08 pmHave you seen the 2 new active probes ?https://siglentna.com/products/accessories/probes/active-probes/

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: November 17, 2021, 09:25:59 pm
content: Quote from: tautech on November 17, 2021, 09:23:16 pmYes, unfortunately they are way out of my price range (for now). Unfortunately, the used market for Siglent gear is non-existent in Germany (aside from entry-level devices) so all Siglent active probes really expensive for me.

author: FlexibleMammoth
author_rank: Regular Contributor
timestamp: November 18, 2021, 02:40:02 pm
content: As the replies have slowed down, I'd like to summarize and bump one last time in the hope that some of the remaining questions can be answered.All statements refer to low voltage, high frequency probes.1) In general, differential probes of similar specs are more complex and more expensive than single ended probes.2) There is some discussion that single-ended probes have lower noise and capacitance (without proof).3) Single-ended, isolated probes combine characteristics of both. They can be used to measure differentially with better CMRR than differential probes (without proof). They suffer from asymmetric input impedance.4) I don't really understand the link between excessive ringing and whether a probe is differential or not.I am especially interested in the physics behind the statements made so far.Thanks again!

author: edigi
author_rank: Regular Contributor
timestamp: November 18, 2021, 03:56:39 pm
content: Perhaps you could check this thread:https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/150/Also if you don't need high impedance/BW and to check low amplitude signals take a look to the Keysight 6GHz passive divider probe (54006A), it's maybe not mission impossible to create a similar (but lower spec) single ended DYI probe.Ringing to me indicates insufficient BW of the measurement setup compared to the rise time (BW) of the signal.Check for example:https://www.signalintegrityjournal.com/articles/2175-how-to-avoid-gibbs-ringing-artifacts-in-measurements

author: penfold
author_rank: Frequent Contributor
timestamp: November 18, 2021, 04:50:08 pm
content: Whilst it may not directly address the points in that summary (nicely observed and stated, I enjoyed seeing that) I just thought I'd throw this out hypothetically,The single-ended and differential active probes can lend themselves to different geometries of probing, where a single-ended probe might allow you to preserve some coaxial-ness to the signal and ground connection, a differential one naturally tends to have two discrete probing points with a bit more loop area (or a slightly harder to keep consistent loop area). In either case, you'd need to be wary of how well damped the resulting LC circuit is, it just may be easier to control in the single-ended case.CMRR, there's quite a lot going on there with either case, only the diff mode has any real DC CMR, but HF CM noise is applicable to either. Single-ended can put you at the mercy of probing 'arrangement' and earth inductance in particular, where the diff probes may put you at the mercy of how well matched the two inputs are (and how consistant that stays with frequency) to how well they can subtract the CM from one another. You could surely design or employ either variety to give you bad results, but the single-ended sounds easier to 'control'.An isolated active single-ended probe only (ok, not "only" but surely "mostly" at least) has a common mode return path via the parasitic capacitance of the isolator, which for the ground adds only its impedance, but the signal will include its input impedance, that will be rather high.

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2021, 05:06:35 pm
content: Quote from: edigi on November 18, 2021, 03:56:39 pmYes, you can just put a 450R or 950R resistor at the end of a coax going to a 50R input for a 10X or 20X probe. I think Mr. EEVBlog has actually done a video on that. Getting 1GHz+ performance isn't very easy, though. There's reason Tek was getting bucks le'huge for their P6156 stuff.QuoteRinging and Gibbs are two different things. Actual ringing is typically due to inductance in the probe tip loop (the path from the tip back to the shield of the coax through the ground lead) and can be demonstrated on an analog scope that isn't susceptible to Gibbs.

author: David Hess
author_rank: Super Contributor
timestamp: November 18, 2021, 07:17:14 pm
content: Quote from: TimFox on November 17, 2021, 06:13:47 pmCommon mode rejection does not apply to single ended probes because it can be considered infinite, and other limitations come into play. For instance isolated single ended probes have common mode slew rate limitations instead of common mode rejection ratio limitations. However common mode rejection for differential probes, high or low voltage, is a real limitation on performance, and often the most important limitation.Quote from: Twoflower on November 17, 2021, 06:43:08 pmQuote from: nctnico on November 17, 2021, 06:48:55 pmDifferential probes always have at least 3dB higher input referred noise because they have two active signal paths in series, one on the positive input and one on the negative input. The same applies when comparing differential input amplifiers with single ended input amplifiers.High voltage differential probes also increase noise by their attenuation factor, but this is no different than single ended attenuating probes. Note that some "low voltage" active probes have input attenuators, and those attenuators also increase input referred noise.Some oscilloscope input circuits are differential hidden behind a single ended input, and they have 3dB higher input noise also. This is done for either lower input offset voltage drift, or for differential comparator applications where an input offset is applied. The old Tektronix classic vertical amplifiers like the 7A13 and 7A22 illustrate this when only one input is used with a singled ended probe, and the other input is necessarily shorted. The Tektronix 475 portable oscilloscope uses this sort of input structure because the entire input amplifier is implemented as a hybrid. Obviously this compromises input noise so most designs avoid it if they can.

author: edigi
author_rank: Regular Contributor
timestamp: November 18, 2021, 08:55:25 pm
content: Quote from: bdunham7 on November 18, 2021, 05:06:35 pmI just thought that serial inductance of the probe is part of the measurement setup and it adds to the BW limit but perhaps I was wrong.Ringing can be easily demonstrated with any scope with a normal probe. Remove the probe hook and plug it via BNC adaptor to a fast rise time generator then remove the BNC adaptor and use the long ground lead + probe tip and see the result.Quote from: David Hess on November 18, 2021, 07:17:14 pmThe transmission of a really high speed signals must always use differential to avoid EMI issues (DSO internals are no exception to this). If amplification/conversion whatever will be 3dB higher noise or not makes no difference to this. The input referred noise is relevant for high attenuation high voltage probes that the OP stated multiple times not the target here.

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2021, 02:27:54 am
content: Quote from: edigi on November 18, 2021, 08:55:25 pmThe input referred noise only depends on the early amplification stages, and any attenuation. For a high impedance input, the FET source follower is the largest source of this noise and if two are placed in series, like with a differential input, then noise must be 3dB higher. The same applies to the differential pair configuration of transistors.There are all kinds of good reasons to use a differential input, like with a differential probe, but lowest possible noise is not one of them. I love using my Tektronix 7A13 vertical amplifier with only a single "singled ended" probe, but not for low noise.Differential signalling for the analog signal path of an oscilloscope has advantages other than resistance to EMI. Differential stages thermally balance offset voltage which would otherwise ruin settling time. You can see this effect in poorly balanced analog oscilloscopes when you have two traces displayed, and changing the position of one trace shifts the position of the other.A lot of effort and design was put into controlling thermal balance issues. Incidentally this problem cannot be solved using closely coupled monolithic matched transistors because the close coupling required for thermal balancing introduces too much electrical coupling at high frequencies, even 10 MHz. I suspect this fundamentally limits settling time of fast precision operational amplifiers.