From my understanding of mathematics, the mode is the value that appears the most in a set of data. As such, how does this translate to "common mode" voltage?

I understand how the cancelling out works, for example in instrumentation amplifiers (at least as far as the math is concerned), but what I am confused on is what is meant by MODE?

Why would cancelling the commonly most appearing voltage have an effect on something? Does voltage interfere, and thus this is the only way to cancel any voltage whatsoever? I am lacking basic concepts it may seem, so pardon my bewilderment if it is unwarranted.

  • \$\begingroup\$ An alternate use of the word mode in electromagnetics that has nothing to do with the statistics usage is "self-consistent field configuration". Unfortunately I can't tie this meaning historically to the usage in lumped circuit theory well enough to make an answer out of it. \$\endgroup\$
    – The Photon
    Feb 3 '17 at 2:11
  • \$\begingroup\$ Thanks for this, I realized how little I know about voltage at all..figured it was just the same as a meter with a little bit tacked on to it..I will try and tie that into my eventual understanding of both waves and electromagnetics as I see that term used fairly frequently. \$\endgroup\$ Feb 3 '17 at 2:19
  • \$\begingroup\$ Why do instrumentation/differential amplifiers want to reject 100% of the common mode voltage in the first place? What does achieving this accomplish? Is "cross-talk"? included in that at all? \$\endgroup\$ Feb 3 '17 at 2:36
  • \$\begingroup\$ When dealing with DC, the CM must be within the acceptable bias range. When it is AC, it is normally interference and then you need balanced input impedance to null the voltage or balanced internal biasing to reject CM signals. CMRR is tied to AC gain and reduces with rising f, thus broadband CM filters are used or Baluns or Ferrite CM chokes to raise the CM impedance and thus reduce the CM current. \$\endgroup\$ Feb 3 '17 at 2:40
  • \$\begingroup\$ Does this then mean that CM can be applied to things other than voltage? I understand how DC and the CM interrelate, limited by the "rails" of the amp. Though, it's worth noting from what I learned that there exists exceptions. I assumed this meant to be with AC, as it seems to be from what I can nominally gather from your post..but, that said, I'm not sure if there are exceptions to this rule when pertaining to DC. Given that you said there is a range, could you perhaps clarify? Sorry if what I said is inconsistent. \$\endgroup\$ Feb 3 '17 at 3:20

"Common-mode" has absolutely nothing to do with mean, median, mode, or statistics in general. Instead, it means that a common signal is present in a differential circuit. And "differential" has nothing to do with Calculus, it just means a circuit where we are interested in the difference between two signals.

Example: two inputs, one at 5V, and one at 6V. We could say that these have a common-mode signal of 5.5V, and a differential signal of 1V. If the inputs are at -3V and -1V, the common-mode signal could be -2V and the differential signal is still 1V.

Common-mode signals are often seen as interfering signals on a long twisted-pair cable. The interference couples equally to both wires. We use a differential input circuit to reject the common-mode interference.

  • \$\begingroup\$ I'm glad that is the case as I wasn't looking too forward to learning statistics..when I fully wrap my head around every thing that you outlined, I will ask any questions I have here, if you would be able to answer them for me. Much appreciated \$\endgroup\$ Feb 3 '17 at 2:22
  • \$\begingroup\$ CM is the average value! and is often used to drive the shields of differential uV electrode signals to reduce the capacitance. Also Right Leg amplifier signal is a standard method that uses the sum each differential input/2 to drive this CM signal to become a virtual ground reference and thus reduce CM noise without requiring a low impedance body ground to earth that may be a safety risk. CM chokes are also standard for Line filters with small shunt caps to earth. Both forward and return current are in the same direction for paired coils to do this, by reversing one winding. \$\endgroup\$ Feb 3 '17 at 3:20
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    \$\begingroup\$ To clarify, common-mode isn't necessarily the average of the two differential signals. In my previous example: "two inputs, one at 5V, and one at 6V. We could say that these have a common-mode signal of 5.5V, and a differential signal of 1V. ", it might also be the case that the common-mode signal is -1V, the two differential signals were originally +6 and +7V, still with a 1V difference. Common-mode often occurs in cables, where it is electromagnetically coupled, but it can also occur where there is DC current running through a ground return, or any number of other cases. \$\endgroup\$ Feb 3 '17 at 17:26
  • \$\begingroup\$ Your clarification above is actually closer to reality than the answer itself. Also "Common-mode signals are often seen as interfering signals on a long twisted-pair cable" is not technically correct. The changes in common mode would be an interference, and differential signaling is exactly the way to deal with these. The presence of constant common mode is often required by the protocol (see ISO 11898-2 for example), and sometimes used as indication of active bus state \$\endgroup\$
    – Maple
    Oct 14 at 21:11

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