On Feb 25, 2012, at 3:21 PM, N0body H0me wrote:
Very simple:
AN unbalanced cable is one where one conductor
is grounded. Most coaxial cables are un balanced: the outer
shield is grounded, the center conductor carries the signal.
For balanced lines, neither conductor is at ground potential;
neither line is grounded. A good example of this is the
pairs used on differential SCSI interface (as well as SMD and
possibly others). Relative to ground, both wires carry signal.
The additional benefit of balanced conductors is that they have
a much higher immunity to common-mode noise (i.e. noise that is
identical on both signals) because the actual signal is the
*difference* between the two legs. Thus, if your actual signal
is A-B, and your noise makes it (A+n)-(B+n), the noise will be
cancelled out.
That's the theory, anyway; noise is seldom 100% common mode,
and even when it is, most real-world differential amps will
let through a little common-mode noise (and if there's enough
to overdrive the inputs, you get other interesting distortion
effects).
Note that a balanced signal doesn't necessarily need to be
entirely free of ground; LVDS, for example, has very defined
common-mode voltage offsets so that systems can be DC-coupled
instead of sticking a capacitor inline to block a DC offset
that's not what your input amp is designed for.
- Dave