I've been looking at the designs of some popular audio interfaces and I found that none of them contain in-amps (or even diff-amps) for their differential inputs. Instead they separately buffer the plus and minus inputs, route them through some analog switches and gain/attenuation circuitry, then to separate drivers for the differential ADC.
This was not the structure I expected! The ADC (CS4272) likes symmetric signals, so I'd rather have the input stage to immediately convert the diff inputs to single-ended with high CMRR instead of relying on the ADC. Sure, there's not much common-mode DC since the inputs are AC coupled with caps, but what about mains hum and other AC common-mode noise?
Is this for saving cost? Perhaps going diff->SE->diff would require 1-2 more op-amps.
This was not the structure I expected! The ADC (CS4272) likes symmetric signals, so I'd rather have the input stage to immediately convert the diff inputs to single-ended with high CMRR instead of relying on the ADC. Sure, there's not much common-mode DC since the inputs are AC coupled with caps, but what about mains hum and other AC common-mode noise?
Is this for saving cost? Perhaps going diff->SE->diff would require 1-2 more op-amps.
Why would you want to do that? If you want to improve the common mode suppression over that of the ADC, you can make a differential ampifier with common-mode feedback, for example, or two difference amplifiers with swapped inputs.
Maybe I'm underestimating the CMRR of the ADC.
What prompted me to think about this is how single-ended ("unbalanced") inputs would work with this input stage.
What prompted me to think about this is how single-ended ("unbalanced") inputs would work with this input stage.
Differential ADCs are intrinsically 6dB more sensitive to differential signals than single ended, thats 6dB more SNR in the digitization that you would throw away otherwise. The ADC is usually the limiting factor for SNR.