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Can someone explain what "common-mode" noise is, and how it can be problematic?

I understand "noise" on a signal in general. If I have a "noisy" +5V rail on a circuit board, I'm not going to be getting a constant value of +5, it will be bouncing around above and below that nominal value... ... but still relative to circuit COM.

My very vague understanding of "common-mode" noise is that it is where both sides are varying equally together. (This is where my understanding breaks down) That is, the pair is bouncing around with respect to ... ... to what? Earth ground?

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What is common-mode noise?

Practically all integrated circuits (and circuits in general) have a pin named "ground" or "GND", or the datasheet says things like "connect VSS to ground".

When transmitting data "a long distance", the wires act as antennas and can easily pick up a few volts of noise, and also radiate noise. So, for example, an output pin on a chip in one box may transmit a "0" as about 0.5 V and transmit a "1" bit as about 2.5 volt, measured relative to the ground pin of that same "line driver" chip.

At a distant location, the other end of the wire is often connected to a pin on a "line receiver" chip. Because of noise, the voltage on that input pin, measured relative to the ground pin of that same line receiver, might often be anywhere in the range -1.5 V to +2.5 V when the transmitter is trying to send a "0", and anywhere in the range 0.5 V to 4.5 V when the transmitter is trying to send a "1".

So how can the receiver possibly know whether the transmitter is trying to send a 1 or a 0, when it gets a voltage like 0.9 or 2.2 ?

Because of this, data transmitted over long distances is often sent using differential signaling over a balanced pair, often a twisted pair. In particular, USB, CANbus, and MIDI cables include a single twisted pair for data; "2-line" telephones and FireWire use two twisted pairs; CAT5e Ethernet cables include four twisted pairs; other systems use even more pairs. Often (but not always), there is some other "ground wire" in the same bundle of cables.

We label one of these wires "plus" or "positive" or "+" or "p", and the other wire "minus" or "-" or "negative" or "n". So when I want to transmit a "CLK" and a "MOSI" signal from one place to another, my cable has 4 wires labeled pCLK, nCLK, pMOSI, nMOSI.

The common mode voltage of CLK is the average of the two CLK wires, (pCLK + nCLK)/2, measured at the receiver -- relative to the GND pin of that receiver.

The common mode voltage of MOSI is the average of the two MOSI wires, (pMOSI + nMOSI)/2, measured at the receiver -- relative to the GND pin of that receiver.

People who design line drivers try to make them pull the "p" line up just as much and at the same time as the "n" line goes down, and vice versa, so the average voltage (measured at the driver) is constant -- in this example, the average at the driver is a constant 1.5 V. (Alas, they are never completely successful).

If there were no noise, then the common mode voltage would also be the same constant value -- but alas, it is not.

Whenever data is transmitted with differential signaling, the difference between the noise-free common mode voltage and the actual common mode voltage is entirely caused by noise. That difference is called common-mode noise.

There are 3 main causes of common-mode noise:

  • Many differential pairs are driven in ways that don't switch the "+" and "-" wires at exactly the same time, or by exactly the same voltage, or perhaps small amounts of noise on the line driver's power rail leaks onto only the "+" wire and not the "-" wire, causing some common-mode noise. (A ferrite choke on the "driver" end of the cable is commonly used to reduce common-mode noise from this source).
  • Other wires in the cable bundle can leak more energy into one wire of the pair than the other -- typically through capacitive coupling. (Twisting each pair a different number of twists per length is commonly used to reduce common-mode noise from this source).
  • Outside interference -- often through inductive coupling.

how can common-mode noise be problematic?

People try to design line receivers to reject common-mode noise. (Alas, they are never completely successful). But even in a system that uses differential signaling with such line receivers, common-mode noise can still be problematic:

  • Long communication wires act as antennas. If the line driver sends too much common-mode noise down the wires, it causes radio-frequency interference with other devices, and causes the system to fail FCC testing or CE testing or both, for electromagnetic compatibility (EMC).

  • Some of the common-mode noise leaks through the line receiver -- the common-mode rejection ratio is not infinite. This is a big problem with analog signals; usually not a problem with digital ones and zeros.

  • Most integrated circuits don't work right when any pin is forced too high or two low -- voltage lower than 0.6 V below the GND pin and higher than 0.6 V above the power pin usually causes problems. Since common-mode noise can easily push the "+" or the "-" signal, or both, outside that range, line receiver circuits must either connect the wires to special integrated circuits (such as "Extended Common-Mode RS-485 Transceivers") that can handle such excursions; or connect the wires to some non-integrated circuit component that protects the ICs from such excursions -- such as the opto-isolators used in MIDI or the transformers used in Ethernet.

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  • \$\begingroup\$ So, in terms of a digital input to a microprocessor would it be fair to say it's less of a specific problem and more a general matter of magnitude? \$\endgroup\$ – Adam Head Aug 22 '13 at 19:51
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    \$\begingroup\$ Yes. It's not a specific problem that you either have or you don't -- every differential signaling communication system has at least a little common-mode noise. Practically any amount of common-mode noise can be rejected -- giving error-free communication. The magnitude of the noise controls whether you can get away with a simple differential receiver, or must spend a little more for extended common-mode receiver, or are forced to buy something even more expensive to reject that noise. \$\endgroup\$ – davidcary Aug 24 '13 at 4:46
  • \$\begingroup\$ Great answer but in "There are 3 main causes of common-mode noise" the first 2 reasons are not common mode noise in nature but rather forms of differential noise. \$\endgroup\$ – Bhuvanesh Narayanan Feb 20 '17 at 22:57
  • \$\begingroup\$ @BhuvaneshNarayanan: Yes, both of those things cause differential noise. But they also cause common-mode noise, right? \$\endgroup\$ – davidcary Feb 21 '17 at 13:49
  • \$\begingroup\$ @davidcary Yes they could but "driver's power rail leaks onto only the "+" wire and not the "-" wire" and "Other wires in the cable bundle can leak more energy into one wire of the pair than the other" are cases of differential noise. For it to be common mode both the wires have to be equally suceptible to the noise. \$\endgroup\$ – Bhuvanesh Narayanan Feb 22 '17 at 12:43
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"Common mode voltage" is simply the average appearing on both signal pathways. It's easier for me to think of it in the context of two inputs of a differential amplifier, where the common mode voltage is unambiguously defined as \$(V_+ + V_-)/2 \$. Whether this number reflects what some consider noise or what others consider signal is irrelevant with respect to definition.

Now, as for why its problematic, sometimes it is, sometimes it isn't. Usually, my goal is to have all EM noise appear to a good amplifier as common mode, and use twisted pairs to achieve this. By "Good", I mean an amplifier with a high common mode rejection ratio. For such an amplifier, differential signals (\$V_+ - V_-)\$ get amplified and common mode voltages get attenuated (VERY attenuated if you're doing it right). If you don't use twisted pair, each signal path can see a very different pattern of EM noise, so the EM noise is no longer common mode, but differential.

One particular example highlighting the difference is pro audio, which passes signals around using twisted pair cable with XLR connectors, vs consumer audio, which uses single-end signal passing.

Even Common Mode noise is problematic if you don't have a high common mode rejection ratio. For example, if you build a "typical" one op-amp differential amplifier with poorly toleranced (i.e., most) resistors, the common mode rejection ratio will be poor.

So, back to "why is it problematic"? -- its less problematic than differential noise, but not necessarily a magic technique for ridding signals of noise, especially if the hardware isn't built to optimally attenuate common mode signals.

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Correct, they are both bouncing around relative to earth or whatever you call your reference 0V. Imagine it like a battery on a spring - the battery voltage stays constant but the battery itself is flying all over the place. Yeah I know, it's a bad analogy!!!

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  • \$\begingroup\$ I agree with John and appreciate the simplicity \$\endgroup\$ – Adam Head Aug 22 '13 at 18:35
  • \$\begingroup\$ Clearer than you think, good illustration. \$\endgroup\$ – TonyM Feb 20 '17 at 19:48
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Typically, common mode noise refers to both wires of a differential pair bouncing relative to the power supply of the device which is taking input from them. Whether the bounce is measured relative to the negative rail, positive rail, or some point between doesn't generally matter much, because in cases where common-mode noise matters, it's often an order of magnitude more significant than power-supply noise.

If the input to a device has e.g. 0.1 volts of common-mode noise relative to the negative rail, and the device 10mv of noise on its power-supply, then no matter what power supply reference point one picks, the common-mode noise will be somewhere between 0.09 and 0.11 volts. If 0.1 volts of common-mode noise wouldn't be a problem, 0.11 probably won't either; if 0.1 volts would be a problem, 0.09 probably would be as well.

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The common mode noise occurs between three phases or neutral and ground whereas normal mode noise occurs between three phase live conductors. For further details refer to power quality book by Dugan and mark et al.

Dr Nasrullah Khan

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