Twinning supply voltage cables?

Discussion in 'General Electronics Chat' started by Pyrdon, Apr 12, 2010.

  1. Pyrdon

    Thread Starter Member

    Mar 17, 2010

    I am quite a novice to electronics but are very interested to learn more.

    I know that if your signals are sensitive to noise you should take extra care twinning them with ground or to themselves (if they are differential) to make sure the same noise is induced in both of them.

    Do you normally twin supply voltage wires too (vdd/gnd) to make sure that the voltage across your circuit is as constant as possible or this just stupid?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    If, by "twinning", you mean using twisted pairs, it is a good technique to minimize noise pickup. Twisting the supply line with ground will not do anything to stabilize circuit voltage, though.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    As far as signals are concerned, it all depends whether you are using "balanced" or "single ended" connections.

    Most domestic audio equipment is single ended: the ground wire is the signal return wire. The signal is "referenced" to ground. The signal input connector only has two connections: signal and ground. Best to use co-axial screened cable. And keep it short because it is quite likely to pick up interference.

    Most professional audio equipment uses balanced connections. Here, a twisted pair is used. Usually screened as well. The connector often has 3 pins. The important difference is that each wire of the pair carries the opposite (inverted) polarity signal. When one wire goes positive, the other goes the same amount negative. This only works if the device at the end of the cable is only sensitive to the DIFFERENCE between the two wires, not the actual voltage of the wires compared to ground.

    There are two advantages to this:-

    • External interference usually affects both wires the same. But this will not affect the voltage difference between the wires. Therefore the signal is not affected.
    • If the pair is tightly twisted, the voltage and magnetic fields around the wire created by the signal are mainly cancelled out. Therefore the signal will not cause interference to others.
    The easiest way to convert a balanced signal to single-ended is to use a transformer. Electronic methods can also be used (differencing amplifier). The important factor is the Common Mode Rejection Ratio (CMRR) - how insensitive the input is to signals that are equal on both wires.

    USB and Ethernet wiring also uses twisted balanced pairs for the same reason.

    As far as your power wires are concerned, it is a good idea to twist them or keep then close together. This reduces interference picked up or radiated by magnetic fields. Especially important when high frequency high power devices like switch-mode power supplies are used.