What does "Common Mode" mean? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: January 27, 2024, 02:28:14 pm
content: Hi,This was an interview question.I gave answer by saying eg a diff amp rejects common mode noise at the opamp inputs since the noise is the same at each input, so wont get amplified.But that was saying what "common mode noise" meant.Their question was , "What does 'Common Mode' mean"?So , may you answer?

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2024, 02:51:34 pm
content: * OP identity notice *

author: TimFox
author_rank: Super Contributor
timestamp: January 27, 2024, 03:34:28 pm
content: If you have two variables, such as two voltages V1 and V2, you can always calculate the two signalsVdiff = V1 - V2. andVcm = (V1 + V2) / 2(so long as the two variables have the same units such as voltage)Similarly, if you have the differential and common-mode signals, you can calculate the separate signals.

author: tggzzz
author_rank: Super Contributor
timestamp: January 27, 2024, 03:35:11 pm
content: Quote from: Faringdon on January 27, 2024, 02:28:14 pmYes, we may answer.(1) how many decades experience do you have as an electronics engineer?(2) Gyro is right(3)google is your friend.(4) Failing that, textbooks have good descriptions. Don't ask us to waste our time poorly duplicatng those!Apart from that,before bothering to respond, be aware of the OP's reputation. Have a look at his other first posts, and see the responses.Then make your own choice.These are typical:https://www.eevblog.com/forum/chat/a-new-type-of-electronics-degree-course/msg5250390/#msg5250390https://www.eevblog.com/forum/manufacture/prototype-board-with-16msop/msg5238015/#msg5238015https://www.eevblog.com/forum/renewable-energy/sync-buck-driver-has-problem/msg5238129/#msg5238129https://www.eevblog.com/forum/manufacture/test-enamel-insulation-on-torroid-inductor/msg5232909/#msg5232909https://www.eevblog.com/forum/chat/keeping-hold-of-electronics-engineers/msg5042023/#msg5042023

author: Jwillis
author_rank: Super Contributor
timestamp: January 27, 2024, 08:57:21 pm
content: Quote from: Faringdon on January 27, 2024, 02:28:14 pmProbably the easiest explanation is from TDK. " The common mode refers to signals or noise that flow in the same direction in a pair of lines. The differential (normal) mode refers to signals or noise that flow in opposite directions in a pair of lines."Difference between Common Mode and Differential Mode.

author: magic
author_rank: Super Contributor
timestamp: January 27, 2024, 09:07:33 pm
content: Is a differential signal flowing in the direction from point A to point B through a twisted pair simultaneously a common mode signal?Asking for a friend working in a telecoms company outsourcing to the Far East

author: BillyO
author_rank: Super Contributor
timestamp: January 27, 2024, 09:35:31 pm
content: Quote from: magic on January 27, 2024, 09:07:33 pmNo.

author: tggzzz
author_rank: Super Contributor
timestamp: January 27, 2024, 10:38:15 pm
content: Quote from: BillyO on January 27, 2024, 09:35:31 pmI wonder whether the "frend working in a telecoms company outsourcing to the Far East" understands that

author: Faringdon
author_rank: Super Contributor
timestamp: January 28, 2024, 12:41:58 pm
content: QuoteThanks, this depicts the poorness of the term "common mode" as it is used in all the cases...because in all its different uses, it refers to something thats different.For example, the above definition speaks of flowing current, but when we think of common mode signals at the input of an opamp, there is no flowing current,as the opamp inputs are high Z.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 01:44:07 pm
content: Using "Common mode" in isolationhasno meaning. It is a qualifier for other terms.

author: Faringdon
author_rank: Super Contributor
timestamp: January 28, 2024, 06:31:16 pm
content: QuoteThanks, thats what i thought, and that it was a trick question effectively.Military electronics place in West London asked it a year or so back at interview.Or maybe the right answer is that its the opposite of "differential mode"

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 07:00:43 pm
content: Not a trick question, just one probing your understanding, or lack thereof.

author: coppice
author_rank: Super Contributor
timestamp: January 28, 2024, 07:09:30 pm
content: Quote from: Gyro on January 28, 2024, 01:44:07 pmThis is sort of true, but you wouldn't hire an engineer who couldn't articulate the basic difference between something being common mode and something being differential mode.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 07:33:26 pm
content: Also true.

author: tggzzz
author_rank: Super Contributor
timestamp: January 28, 2024, 07:39:21 pm
content: Quote from: Gyro on January 28, 2024, 07:00:43 pmPrecisely.In an interview, agoodanswer often starts with "It depends on what you mean by X; if ... then ..." .It rapidly separates candidates having 10 years experience from those having 1 year repeated 10 times.With that perspective, the OP's bewilderment is not so surprising.

author: MrAl
author_rank: Super Contributor
timestamp: January 28, 2024, 09:06:02 pm
content: Quote from: Faringdon on January 27, 2024, 02:28:14 pmHello,The phrase "common mode" is a phrase that is similar in usage to a word like "symmetry", in that it refers to two things at the same time and usually their relationship to each other. First note that "symmetry" can be used in a lot of different ways to refer to the relationship between two things and that is because there are a lot of ways of showing that there is some sort of regular relationship between two (or more) things."common mode" just refers to two (or possibly more) things and their relationship to each other, and in electronics that often refers to two signals or two measurements on different nodes or though different branches or some similar set of things. So we can have common mode voltage, common mode current, and even common mode resistance, common mode inductance, etc.Common mode voltage simply means that we look at two nodes and try to understand what happens when the signals are the same on both nodes.Common mode current simply means we look at two branches and try to understand what happens when the currents through both branches are the same.A common use is for the common mode rejection ratio of an op amp. That's when the two inputs have the same voltage applied to them and how the op amp behaves during that time. If the amplifier does not amplify either signal in that condition, the rejection ratio is said to be high. That's very desirable because we want the op amp to only amplify the difference between the two signals, and if they are the same we don't want any output.

author: Faringdon
author_rank: Super Contributor
timestamp: January 28, 2024, 09:17:49 pm
content: Thanks, the diff amp kindly sent by Zero999 is interesting because the common mode voltage is 2v.However , with different values, both the opamp inputs could be outside the common mode input range of the opamp, but VCM could be within the common mode range, but it would still be a violation of common mode input range....even though VCM was within the opamp's input common mode range.IMHO I suspect that the true meaning of "common mode" is w.r.t the common mode gain of an opamp, (ie, a single input voltage being "common" to both inputs) and it being the opposite of the diff mode gain.IMHO i suspect that all other uses of "common mode" are a bit kind of "contrived" from this true meaning, and are not really useful uses of the term "common mode", its just that we dont have any more exact jargon.As we know "common" is an old timers term in electronics....if you "common" some things, you "connect them together".Its like grounds are connected together...."common" also having once been a common term for "ground"...a voltage that is "common" to all components/systems.

author: MrAl
author_rank: Super Contributor
timestamp: January 29, 2024, 10:59:43 am
content: Quote from: Faringdon on January 28, 2024, 09:17:49 pmThe "true meaning" has already been explained. The phrase "common mode" is used for a lot of things and is equally important for each use. There is no more important use for one over another it depends what you need to know about the device. If you need to know the common mode range that's one thing, if you need to know the common mode rejection ratio, that's another thing, if you need to know the common mode resistance that's yet another thing. The only time one would become more important than the other is when you need to know that one specific thing in order to meet some design criterion. The others may becomes secondary then.

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2024, 11:03:28 am
content: Quote from: MrAl on January 29, 2024, 10:59:43 amYup.Now you may becoming aware why early responses warned this is a Faringdon post

author: Andy Chee
author_rank: Super Contributor
timestamp: January 29, 2024, 12:21:59 pm
content: One example of common mode current that immediately comes to mind is a dipole antenna connected to coaxial cable.I can imagine some in the oxygen free monster speaker cable manufacturing space, might use the term "common mode" as a marketing ploy. But that doesn't mean the term "common mode" is contrived.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 29, 2024, 04:41:57 pm
content: Andy, that is probably the best start for understanding what is meant by "common mode"And probably the most "common" meaning.For those interested in more complete coverage of this subject here is a link:https://catalogimages.wiley.com/images/db/pdf/9781119237617.excerpt.pdf

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2024, 05:11:25 pm
content: Quote from: Faringdon on January 28, 2024, 09:17:49 pmWhat are you talking about? Are you referring to the fact the input voltages can fall outside of the common mode range of the op-amp? Look at the effect of the resistor feedback network and apply Ohm's law. Start with the +input and then the -input, with the knowledge the op-amp adjusts its output to keep both of its inputs at the same voltage.

author: iMo
author_rank: Super Contributor
timestamp: January 29, 2024, 05:59:30 pm
content: It depends on whether the "military electronics place in West London" messes with RF, or with opampsA "common mode" AC or DC voltage or current is simply superimposed onto any other voltages or currents the same way - with the same phase and amplitude (the same "mode" is then superimposed in a "common" way on all of them).You may have 1456 wires with any kind of AC/DC signals running through them back and forth, and all may have a certain common mode voltage superimposed on them.Thus when somebody is putting a single "common mode choke"on those signal wires, and omits some of the returning signal's paths pass through the CM choke, those signals affected get choked

author: Faringdon
author_rank: Super Contributor
timestamp: January 29, 2024, 06:49:24 pm
content: QuoteSorry i wasnt referring to your kindly sent example...i was going on about the "VCM"...its the kind of midpoint of the opamp inputs.I often hear of this...but what does it actually mean?...it doesnt connect to the opamp....Its the "midpoint" of the input voltages yes...but why is it called "VCM".Ok, each signal has a kind of amount of VCM within it....but why does that matter?So why do the opamp datasheeters speak of this "VCM".I mean the common mode range refers to the actual voltage level at the actual inputs....not some contrived voltage away from them.

author: Smokey
author_rank: Super Contributor
timestamp: January 29, 2024, 07:45:44 pm
content: Common mode is obviously your regular, ordinary, everyday, run-of-the-mill mode. Nothing special here. Totally common.Differential mode is the mode that lets your tires spin at different speeds as you go around a corner. This mode comes in open, limited slip, and locking types.

author: coppice
author_rank: Super Contributor
timestamp: January 29, 2024, 07:53:21 pm
content: Quote from: Smokey on January 29, 2024, 07:45:44 pmThat won't get you through the interview. You haven't identified if this is the pleb-to-pleb interaction mode, or a mode used by the superior among us to interact with the plebs.

author: MrAl
author_rank: Super Contributor
timestamp: January 29, 2024, 09:54:48 pm
content: Quote from: tggzzz on January 29, 2024, 11:03:28 amHa ha, yes thanks for that.I just love it when newbies come in and ask experts in the field about something, then when they hear the correct answer they declare that it must be something else and proceed to render the right answer themselves, even though they have no prior knowledge about it. Makes me laugh every time.I think it may be a natural thing though, when they hear something that they don't understand it is natural to think that is wrong or just not right, and that's because they still do not understand it. If they understood it fully they would recognize the answer was right all along.There is a name for this condition and the basic cause is because the newbie does not have much knowledge to begin with so it is hard for them to recognize the right answer when they hear it. They still don't understand the subject well enough so they can not properly judge right from wrong at least in that one area. If their knowledge is limited then they only have that to go by up to that point. Anything outside of that may seem automatically wrong.

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2024, 10:22:23 pm
content: Quote from: MrAl on January 29, 2024, 09:54:48 pmFaringdon, previously known as ocset and treez, is not a newbie. Far from it.If you believe his story, he has decades of experience designing SMPSs. I suspectthat partis true.

author: Manul
author_rank: Super Contributor
timestamp: January 29, 2024, 11:51:45 pm
content: When we have a signal, we need a reference for it, only then it has a meaning and is measurable. So it's a pair. Common mode (signal) is a signal of that pair referenced to yet another reference. In fact, concepts of "common mode" and "differential" can go in endless layers given that you add more and more new references.

author: Smokey
author_rank: Super Contributor
timestamp: January 30, 2024, 12:40:57 am
content: Quote from: tggzzz on January 29, 2024, 10:22:23 pmLet this be a lesson to all of us! Too much SMPS design make you a little crazy.

author: Faringdon
author_rank: Super Contributor
timestamp: January 30, 2024, 07:08:50 am
content: .....mind you, its like when they put "common mode chokes" in signal lines where essentially no current flows....to filter out the common mode currents.(!)Then again, presumably they refer to noise currents?Also, a mains common mode choke is often called a "current sucking transformer" instead....because it plays out the N1.I1 = N2.I2 thing.Sometimes it seems a more appropriate term.Specially when you need common mode chokes at the input of SMPS that have no earth connection. Just line and neutral.And some say its called a "common" mode choke becuase it filters that noise on both line and neutral, which is respective to earth (earth is AKA "common").But then other times we say its a "common mode choke" because it filters out noise which is common to both line and neutral....and dont speak of earth.

author: MrAl
author_rank: Super Contributor
timestamp: January 30, 2024, 10:41:42 am
content: A common mode choke is just a name for a particular kind of transformer with leakage inductance.A better name might just be something like a "Line Choke" or anything you like.With that in mind, for a so-called Line Choke there are at least two ways to analyze it:1. Common mode2. Differential modeThis would be with respect to the direction of current in each lead.Common mode analysis would be an analysis of what happens when the two currents both flow into the choke in the same direction.Differential mode analysis would be an analysis of what happens when the two currents flow in opposite directions, one into and one out of the choke.Here we can notice that it depends how we analyze the device and that is so we can understand how it works better.I suppose it would be somewhat simple enough to use a regular transformer model with some leakage inductance to do a theoretical analysis in order to discover what the basic features are in both types of analysis. The models do get a little complicated though so there would be more to it than just a resistor network for example.All that aside, from what I understand there are a lot of different uses for this kind of choke so we might hear of different descriptions of how they work. The kind I am most familiar with are the kind used for the input to other devices such as power converters. In that case, the Line Choke helps to keep harmonics from the power converter from getting into the input line. The converters generate harmonics internally by the way the power converter draws current and that can cause problems external to the converter, and this problem gets worse when there are a lot of converters hooked up to the same line. Back in the 1980's Sandia Labs did an extensive study on this.

author: newbrain
author_rank: Super Contributor
timestamp: January 30, 2024, 01:21:10 pm
content: Quote from: Smokey on January 30, 2024, 12:40:57 amThis must be true.In my first job, the PSU guy was absolutely a wild card. Maybe rivaled only by the antenna designers.

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2024, 01:33:34 pm
content: Quote from: newbrain on January 30, 2024, 01:21:10 pmDid his PSUs work reliably in the field? No, I'm not going to explain that question