Why do differential signals need an inverse wave?

Question

At the moment, I'm working on converting a single ended audio signal to a differential one (source is a CS42L55, need a differential input for a CSR8670. The latter's datasheet is indecisive about how to approach this issue). I have a few options, and I'm wondering about the validity of each.

1. Tie the negative differential input to analog ground
2. Use an op-amp to create an inverse signal
3. Use a transformer (I've only just read about this, not sure how it works yet)

This got me thinking - why does a differential signal need an inverse input, if the noise on both cables is the same?

For example (all waves start from 0v):

                   /\
Positive input:   /  \

Negative input (neutral) : ----

If the same waves had noise, like this for example:

                 __
Positive input: /  \
                          _  _  
Negative input (neutral):  \/

Therefore, pos - neg = pos without noise right? (the sum being the function of the converter?) Is there a benefit to using an inverse wave instead of ground (op-amp > just tying to gnd?)

I guess I am missing something here, and I'd like to know what.

Answer 1

why does a differential signal need an inverse input

They don't. What makes "differential" signals work is that the impedance of both wires is equal (balanced) which is necessary for noise to be picked up as common mode which can then be rejected. If the impedance of both wires is different, then some of the noise will be picked up as differential signal.

For example if you use a current sense resistor you'll have better noise rejection by routing the sense lines as a differential pair. In this case there is no inverted signal, but routing as a differential (and balanced) pair ensures the noise picked up by both lines is common mode and rejected by the diff amp.

Using a positive and inverted signal just doubles the available signal amplitude, which gives you a bit of extra signal to noise ratio or headroom. It also minimizes some distortion mechanisms in the receiver (ie, odd order distortion).

Answer 2

The idea behind differential lines is to avoid common mode noise.

When using a single ended line the signal line will have a difference impedance to ground than the ground reference line. Therefore, environment noise will generate a different potential on the line than on the ground line, hence the receiver will see the noise.

in the case of a proper differential (balanced line), you have 2 equal lines, each with an opposite signal, when substracting the signals, since they are opposite you will get a signal twice the amplitude, however since the lines are equal, the noise component in both will be equal, substracting two equals = 0, hence you eliminate the noise that way.

You can see in a typical xlr connector that you have 3 pins, where 1 is shield, which acts as ground reference and provides shielding for the balanced line 1-2.