What does "Common Mode" mean?

[Faringdon]

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?

[Gyro]

* OP identity notice *

[TimFox]

If you have two variables, such as two voltages V1 and V2, you can always calculate the two signals
Vdiff = V1 - V2. and
Vcm = (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.

[tggzzz]

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?

Yes, 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/#msg5250390
https://www.eevblog.com/forum/manufacture/prototype-board-with-16msop/msg5238015/#msg5238015
https://www.eevblog.com/forum/renewable-energy/sync-buck-driver-has-problem/msg5238129/#msg5238129
https://www.eevblog.com/forum/manufacture/test-enamel-insulation-on-torroid-inductor/msg5232909/#msg5232909
https://www.eevblog.com/forum/chat/keeping-hold-of-electronics-engineers/msg5042023/#msg5042023

[Jwillis]

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?

Probably 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.

[magic]

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

[BillyO]

Is a differential signal flowing in the direction from point A to point B through a twisted pair simultaneously a common mode signal?

No.

[tggzzz]

Is a differential signal flowing in the direction from point A to point B through a twisted pair simultaneously a common mode signal?

No.

I wonder whether the "frend working in a telecoms company outsourcing to the Far East" understands that

[Faringdon]

Probably 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.

Thanks, 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.

[Gyro]

Using "Common mode" in isolation has no meaning. It is a qualifier for other terms.

[Faringdon]

Using "Common mode" in isolation has no meaning. It is a qualifier for other terms.

Thanks, 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"

[Gyro]

Not a trick question, just one probing your understanding, or lack thereof.

[coppice]

Using "Common mode" in isolation has no meaning. It is a qualifier for other terms.

This 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.

[Gyro]

Also true.

[tggzzz]

Not a trick question, just one probing your understanding, or lack thereof.

Precisely.

In an interview, a good answer 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.

[Zero999]

[MrAl]

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?

Hello,

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.

[Faringdon]

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.

[MrAl]

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.

The "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.

[tggzzz]

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.

The "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.

Yup.

Now you may becoming aware why early responses warned this is a Faringdon post

[Andy Chee]

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.

[Wallace Gasiewicz]

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

[Zero999]

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.

What 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.

[iMo]

It depends on whether the "military electronics place in West London" messes with RF, or with opamps
A "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 

[Faringdon]

What are you talking about?

Sorry 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.