Originally Posted by arnyk
A piece of equipment with an unbalanced output can be beneficially connected to a balanced input using a cable that is wired like this:
The above is most often true, but there are conditions that render it ineffective. Thankfully, in those situations, the shield allows it to be at least as good as a normal unbalanced connection.
In order for the receiver circuit to effectively subtract-out the common-mode noise, that noise needs to be equal on both the positive and negative legs (pins 2 & 3). That requires that the source impedance be equal on both legs at the source, otherwise the source will attenuate the noise on one leg more than the other (making the "mode" less "common). If the source was floating with respect to the receiver, (as your diagram implies) than it typically works as a balanced driver.
However, if one was to connect the shield of that cable to the chassis of the source, then you would greatly attenuate the noise on the black wire. The noise would no longer be common to both legs, and only the remaining
noise on the black wire would be subtracted from the red wire. But as I said, it would still work fine as an unbalanced line, since it is shielded.
The risk of having the source floating is that it could "float" outside of the receiver's "common-mode range". That's not an issue if the receiver is a transformer, as a transformer's common-mode range is almost unlimited. An op-amp based receiver (or an instrumentation-amp, as I typically use), would need some protection to insure against damage.
That brings up the main difference between balanced and unbalanced transmission lines: A unbalanced system depends on shielding to prevent noise from entering the system, while a balanced system subtracts-out
whatever noise does
enter the system. That's why some balanced systems aren't shielded at all.