Electrical – Differential amplifier exercise problem

Since you want to convert the output from an audio mixer, it will be at the line level, which is standardized to be an AC signal with peak values defined by the standard in use. The maximum voltage that you can expect is +-2.192 V, or 4.384 Vpp.

Your ADC takes signals in the range [-0.3 ÷ AVdd+0.3], so you'll need to both amplify your signal and traslate it from centered in 0 to centered in AVdd/2. You'll need to tune your amplifier/shifter (better to use pots) to cover as well as possible the range of the ADC without distorting (consider that amps usually distort before the saturation threshold).

If you look at the datasheet of the ADC, it suggests an input buffer that you can use to condition the signal from the XLR input:

Edited out:

If instead the output is single ended, you can also do it with a normal precision op-amp in level shifter configuration. You can as well use a rail-to-rail op-amp, or use a "normal" one with slightly larger voltage range. But using the same supply of the ADC may result handy. You'll need to tune your amplifier/shifter (better to use pots) to cover as well as possible the range of the ADC without distorting (consider that the op-amp usually distorts near the saturation threshold).

In this post you can find some ideas for level shifters, such as:

Getting the input of a amplifier to work outside its supply voltage range is difficult, but there are possible ways around this:

1. Is the signal AC? If so, capacitively couple it to something that floats around ½ the supply voltage.

2. Is the signal floating (not ground-referenced)? This can be the case when coming directly from some tranducer, for example. In that case, tie the other end to something around ½ the supply voltage.

3. Shift the voltage level. This could be done with a resistor divider to the positive supply, for example. That will also attenuate the signal a little, but that can be made up by increasing the gain a little. Alternatively, the signal can be shifted by using zener diodes or ordinary diodes forward biased.

   Both these schemes will add some offset error. Everything is a tradeoff.

4. make a negative supply. It's not really that hard or expensive, and there are off the shelf chips just for this purpose. For just powering a opamp, this could be a cpacitive charge pump, which will generally be cheaper than a inductive switcher.

   If you already have something that produces a regular clock, a capacitive charge pump can be as easy as a couple of diodes and caps. I have a few times used the clock output of a microcontroller for this purspose, although then I buffered it with two transistors making a push-pull emitter follower.