What is "common-mode" noise?

Question

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?

Answer 1

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.

Answer 2

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!!!

Answer 3

"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.

Answer 4

Motivations

"Common-mode noise" is a fundamental concept in electronics. Yet, because it appears in a lot of related but different contexts - amplifiers and signaling, 120 V or 230 V power supply, or EMI/EMC - just to name a few, it has many seemingly contradictory definitions. This situation creates so much confusion among beginners.

For example, EMC engineers may tell you: "Your system's radiation is coming from the 5-volt DC cable due to common-mode noise." And you don't understand what they're talking about.

• So you look it up and found the definition: "A differential-mode voltage exists between the inverting and non-inverting input of a differential amplifier, a common-mode voltage exists between the inputs and ground." You are confused. Amplifier? What amplifier? So you keep looking.

• Then you stamped upon another definition: The noise between Live and Neutral conductor in the AC mains is differential-mode noise, and it can be suppressed by a Class-X capacitor, the noise between Live/Neural and Earth conductor is common-mode noise, and it's suppressed by a Class-Y capacitor or a common-mode choke. You are even more confused. AC mains? Neural? But it's a 5-volt power supply. Earth? But it's battery powered.

• Finally, you found yet another definition before giving up: "If we have two conductors, the current that flows on both conductors in the same direction is the common-mode current. The current that flows on both conductors in the opposite direction is the differential-mode current." At this point, you finally gave up. I have +5 VDC and GND here. How can a current flows in the same direction? Does it even make sense?

In my opinion, none of the existing answer is satisfactory because they don't explain this problem. My answer is an attempt to clarify this confusion in the context of EMI/EMC.

A Current Definition of Common-Mode Noise

When we are speaking of analog or digital signaling with a differential amplifier, the voltage definition is often used - it's the noise across a signal and a common ground reference. However, when we are talking about it in a more general sense, it's often unclear where the "common ground" is. Thus, it's more useful to use a current definition of common-mode noise.

To put it in plain but sloppy language, in many practical applications, a common-mode current is often the unwanted current that interferes with the circuitry, while a differential-mode current is often the useful current necessary for the operation of the circuitry (but there can be differential-mode noise too). Ideally, the common-mode current in cables should be zero, in other words, it should not even exist.

To understand what I meant, consider the previous 5-volt DC power supply example. We have an electronic device powered by a 5-volt DC supply. The power is delivered by a DC cable with two wires: + 5 VDC and GND. We can represent this system in a simplified schematic.

Kirchhoff's Current Law tells us, \$ \sum_{k=1}^n {I}_k = 0 \$. In other words, there should be an equal and opposite current on +VDC and GND. The current flows into the load must equal to the current flows out of the load. For example, if Ivdc = 500 mA, Ignd = -500 mA. The net current in the cable as a whole should be zero.

However, if we perform this experiment with a real switched-mode power supply, and a real circuit board with several digital logic chips on it, you'll see something different: Ivdc may be 500.0025 mA, while Ignd may be -499.9975 mA. To our surprise, at the ground wire, the 5 μA of current is missing, and there's a net current of 0.005 mA (5 μA) flowing into the load. This 5 μA current, is the common-mode current.

Mathematically, we can say that the current in the cables is a superposition of two different currents. There's a differential-mode current that flows toward the opposite directions, and a 2.5 μA current on both VDC and GND wires that flows toward the same direction to the load.

Thus, we have our first definition of current-mode current: If we have two conductors, the current that flows on both conductors in the same direction is the common-mode current.

But KCL tells us that "what goes in, must come out", and the current must return to the source. How can the current travels on both wires on the same direction? What's going on? It's easy to see the answer once we realize that (1) we have AC current, not DC current. Because both the switched-mode power supply and the digital chips on the circuit board work by switching rapidly, the frequency components of the current can be as high as 100 MHz. (2) Parasitic capacitance is everywhere in a real circuit, and at 100 MHz, the impedance of a 5 pF capacitor is just 318j Ω.

Thus, the 5 μA common-mode current simply jumps to a nearby conductor via the parasitic capacitance and returns to the source via an alternative path. The exact path taken by the current is often ill-defined and somewhat unpredictable. Often it's the metal enclosure. For systems without a metal enclosure, It can be whatever conductive object that happens to be here nearby: a metal table, a file cabinet, or literally the ground.

_{Source: Electromagnetic Compatibility Engineering by Henry Ott, fair use.}

Due to the uncontrolled nature of the common-mode current, it usually travels in a large loop, and this loop radiates electromagnetic interference. And actually, the common-mode current is not merely flowing in a conductor according to basic circuit theory, it's the result of electromagnetic field. A wire on over a plane creates a monopole antenna that produces radiated electromagnetic waves.

A special scenario is when the system is a 120 V or 230 V mains power supply. In this case, we indeed have a well-defined reference. The AC mains have three conductors: Live, Neutral, Earth, and the enclosure is grounded to Earth, a current that otherwise should return via Neutral often returns via Earth instead. Thus, this explains another definition of common-mode noise: The noise between Live and Neutral conductor in the AC mains is differential-mode noise, the noise between Live/Neural and Earth conductor is common-mode noise.

Measuring Common-Mode Noise

Common-mode noise current cannot be measured by a multimeter, because they are created by RF current at hundreds of megahertz, with a magnitude of a few microamps. Instead, in EMI/EMC pre-compliance testing, it's measured by a spectrum analyzer with an RF current probe. And the result is a noise voltage, not a current. When the probe is characterized with a transfer impedance curve, the voltage can be converted to current.

The RF current probe senses the current via the magnetic field. If we put the current probe across an individual wire, the current on that wire is measured. If we put the current probe across the entire cable, e.g. including both power and ground, the net magnetic flux is zero inside the probe, and we can measure the leakage, or common-mode noise current.

Here's an example of a commercial RF current probe.

_{Source: Review: Tekbox TBCP1 RF current probe, EDN, fair use.}

If one doesn't have a spectrum analyzer, a low-cost solution is using a SDR dongle. A $10 RTL-SDR is enough to show the frequency peaks of these EMI sources. Another option is amplifying the current probe with an RF Low-Noise Amplifier (LNA), then analyzing the result on a digital oscilloscope via FFT is also an option. Both methods are unreliable and unsuitable for quantitative measurements, but good enough to be an educational demonstration.

The path taken by the common-mode current is often ill-defined, and can be whatever conductive object that happens to be here nearby: a metal table, a file cabinet, or literally the ground. Thus, to make a reliable and repeatable measurement in conductive EMI compliance testing, solid metal plates are grounded to Earth and placed on the horizontal and vertical directions near the device-under-test, so that it creates a predictable path for common-mode current.

_{Source: Electromagnetic Compatibility Engineering by Henry Ott, fair use.}

For a simple pre-compliance testing at the workbench, Ott suggested that one can use a DIY lab cart in the following arrangement to approximate the test environment.

_{Source: Electromagnetic Compatibility Engineering by Henry Ott, fair use.}

Answer 5

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.

Answer 6

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