How to add a balanced input stage in front of a ADC?

Question

I'm building a simple audio device, mostly out of ready-made modules.

I'm using a PCM1808 as an ADC and powering everything from a single +5 V supply. But I want the input accept a line-level signal and work with both balanced and single-ended inputs.

I'm using a CJMCU-1808 module:

I've found some example schematics based on TL072 and OPA1632:

But both call for negative rail. I'm aware that some of the opamps can work from a single rail. But looking at the spec sheet, this seems to severely limit the input range and affect the rest of the device (ground reference?)

I've seen that there are devices called a 'negative voltage generator' but I'm not sure how noisy they are or if they're suitable for this.

Answer 1

Yes, you are on the right track. In fact the example circuits you show are almost correct, however every pro circuit I've seen would include DC blocking caps at the input, and resistors of about 100k (it's not critical) to tie the input before the cap to 0V. That leaves you with something like this:

^{simulate this circuit – Schematic created using CircuitLab}

This will do a pretty decent job. The 10u/10k input comniation sets the LF rollof to somewhere way below 20Hz, while the 330n/10k in the opamp feedback loop rolls off the high end somewhere around 40kHz. This is also advisable for stability, although at this gain with a TL07x that probably isn't an issue.

Yes, this circuit does require a midrail. You could do this with two resistors, a cap and the other op amp section as a unity gain buffer, or a "mid rail IC" such as the TLE2426. Using some kind of switcher to generate a negative rail is NOT advised for audio, unless you also include a linear negative regulator to get rid of switching noise. This link explains the problem and possible solutions quite well.

Note that you will also need to add a DC block capacitor and biassing to make sure that the output of this circuit is correct for the mid point of your ADC input range.

I did not include the extra RC stuff on the inputs of your example circuit. They don't do any harm, but they are not really needed. If you want to condition the input to get rid of HF, there are better ways, but if you are doing that kind of thing you would also want to include protection against overvoltage transients and so on, and that is probably outside the scope of what you are trying to do. If we are going there, using a good quality audio transformer starts to be a very attractive option anyway.