What is "differential signalling" - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: February 26, 2023, 12:57:29 pm
content: The below article on differential signalling...https://en.wikipedia.org/wiki/Differential_signalling...states that..."Contrary to popular belief, differential signalling does not affect noise cancellation."This is contrary to the following article from Altium..https://resources.altium.com/p/what-are-differential-pairs-and-differential-signalsThe article from Altium appears to be interchanging its reference to "balanced differential signalling", and plain "differential signalling"."Balanced differential signalling" (BDS) appears, in most articles, to be the one that results in most noise cancellation (though one wonders why?).......BDS specifically involves two signals which are opposite in polarity , and equidistant from the zero voltage reference........but in most comms systems...the zero voltage reference is at different potentials at each end of the comms cable.....so what that means for BDS is anyones guess?...presumably its not balanced any more, and you will get a lower SNR?A simple scheme like what i show in the attached LTspice will obviosuly result in a reduction in common mode noise, since obviously, noise of the same polarity on the comparator inputs will be cancelled......so what the name is, of the comms scheme that i show in the attached LTspice, goodness only knows?Searching the web for official articles, reveals a wide variation about what "differential signalling" is, and what it actually does.The method of signalling in CAN (attached) is NOT "differential signalling" according to many articles.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2023, 01:02:27 pm
content: That article seems like full of some autistic masturbation around terminology. Even if the article were correct on some points, it's practically impossible to take any advantage of because of this language gamification, as you can't trust anything if the basics are presented in such misleading way. Typical Wikipedia.Of course noise is cancelled when one couples equal noise signal to both signals of a differential pair. But the article writer just counts this noise cancellation effect in some other mental box, not the "differentialness", for some mystical reason which is totally unpractical.

author: TimFox
author_rank: Super Contributor
timestamp: February 26, 2023, 02:47:28 pm
content: By simple algebra, any two voltagesAandBcan be expressed as---the differential voltageD=B-Aand---the common-mode voltageC= (B+A)/2An ideal differential input should sense the differential voltage (with all its noise), but reject the common-mode voltage (and all its noise).As I posted before (and corrected a typo), one can get the original voltages back by computingB= (D+ 2C)/2 andA= (2C-D)/2

author: Faringdon
author_rank: Super Contributor
timestamp: February 26, 2023, 05:04:50 pm
content: Thanks, i actually think the rejection of common mode noise by a differential receiver, has little to do with the signal on its inputs.........whether the signals are differential, or just a unipolar signal 5V then 0V...and the other signal into the comparator is just say a 2V5 reference....any common mode noise that appears at the comparators inputs and is of the same magnitude on "+" and "-" will be cancelled.As you know, a diff Rx in a comms wire system is just a comparator. For diff signals, each signal goes into one of the comparators inputs.You can also send the signal as a 5V/OV signal wire twisted with the 0V wire.......common mode noise will be cancelled from it just the same...its just that in that case, the comparator's other input will be a ~2V5 reference.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2023, 05:22:06 pm
content: Quote from: Faringdon on February 26, 2023, 05:04:50 pmOf course. Then your options are,* Do what wikipedia author does and claim that differential signalling does not reject common mode signal, but it's the receiver which does.* Accept that your case of a "single-ended signal" plus another "single-ended" 2.5V reference signal actually forms adifferentialsignalling! Some call thispseudo-differential.Terms are flexible. What is important is to understand what is actually going on. Then disagreements about terms do not matter that much. But if your understanding is limited, then the role of terminology becomes bigger than it needs to be.

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 05:24:25 pm
content: FTTS, read Henry Ott electronic noise reduction techniquesSee apps on ethernet cables, PHY, TX, RX RS-485J

author: TimFox
author_rank: Super Contributor
timestamp: February 26, 2023, 05:24:39 pm
content: What did you mean by "i actually think the rejection of common mode noise by a differential receiver, has little to do with the signal on its inputs"?With "proper" differential TX, there is one component of the common-mode voltage that is the bias of the two lines with respect to circuit common at the transmitter (e.g., +2.5 V for +5/0V and 0V/+5 binary levels), and a second component that is the difference between the circuit common at the transmitter and the circuit common at the receiver.A differential receiver can be used with a "single-ended" TX, comparing the single signal voltage to, say, +2.5 V, but you lose the common-mode rejection.

author: Marco
author_rank: Super Contributor
timestamp: February 26, 2023, 06:26:40 pm
content: Quote from: Faringdon on February 26, 2023, 12:57:29 pmIt is though.Quote from: Siwastaja on February 26, 2023, 01:02:27 pmIt's practical and common in audio. Balanced lines, differential receiver, yet signals can be almost completely single ended relative to ground (not completely due to common mode choking in the transmission line).

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 08:00:55 pm
content: of course useless to respond to FTTS questions, but for the benefit of others a few notes....Balanced cables and differential T, RX offer CM noise rejection especially useful with long cable runs in noisy environmentsBack to 1920s, Bell Téléphone Laboratoires pioneered balanced voice and then balanced carrier lines, using transformers to reject CM noise.please see the excellent articles and app notes on audio cables, noise, at SYNAUD CONhttps://www.prosoundtraining.com/why-synaudcon/Ranehttps://www.ranecommercial.com/kb_article.php?article=2107Similar techniques apply for wideband and digital signals,CAN bus, AES EBU, Ethernetsee standards and papers.EnjoyJon

author: magic
author_rank: Super Contributor
timestamp: February 26, 2023, 08:55:12 pm
content: Quote from: Siwastaja on February 26, 2023, 01:02:27 pmI believe the article's point is that a "balanced line", defined as two identical conductors with equal loading to ground, is necessary to ensure that noise couples equally into both conductors and can be rejected differentially. Of course this point is not spelled out explicitly, because WikipediaThe process requires something most people would probably agree to call a "differential receiver", but the article obsessively insists that "differential" somehow means two signals symmetric around zero.

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 09:27:29 pm
content: indeed every paired system can be analyzed by the impedance between the pairs DM and common impedance to earth CM.the currents in those Z cause the CM and DM noise.j

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2023, 02:45:28 am
content: Quote from: TimFox on February 26, 2023, 05:24:39 pmIt is more common in audio, and sometimes precision analog, but a differential receiver may be used with a single ended transmitter with the "reference" being a second line coming from the transmitter. So the differential receiver is subtracting the difference between the transmitter reference and the receiver reference from the signal; it really has three inputs and one output.An alternative that I have sometimes used is to transmit a current instead, and then use a resistor at the receiver to "ground" to recover the original ground referenced signal. The result is the same.With a balanced differential receiver, the third input is the local ground which becomes the reference for the output.For audio, a differential receiver can be used to separate the grounds to prevent ground loops even with a single ended output. That is not balanced, but still has common mode rejection to reject the difference between the two grounds.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 28, 2023, 05:15:28 pm
content: You have issues with autism, masturbation or both?We all gotta be free bro.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 03, 2023, 12:52:28 pm
content: Its what your Mum uses to tell you off!