A/D converter input buffers

[Lvaz]

Hello!

As an experiment, I am building a simple A to D converter for stereo audio using a cirrus logic chip. I have looked at some designs and noticed all of them use opamp buffers before the differential input to the A/D converting chip.

Could someone explain or shed light on the fact to if its possible to just use a matching transformer instead of the usual opamp buffers? If not, why so or could it be made to work?

I can see that in the usual buffer circuits, they implement low pass filters. I cant really figure out why this would be needed?

Any help would be much appreciated!

Thanks, Lewis

 

[nigelwright7557]

If you allow through higher frequencies than a2d sampling frequency you get all sorts of strange outputs.

n signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. It also often refers to the distortion or artifact that results when a signal reconstructed from samples is different from the original continuous signal. Aliasing can occur in signals sampled in time, for instance digital audio, and is referred to as temporal aliasing. It can also occur in spatially sampled signals (e.g. moiré patterns in digital images); this type of aliasing is called spatial aliasing. Aliasing is generally avoided by applying low pass filters or anti-aliasing filters (AAF) to the input signal before sampling and when converting a signal from a higher to a lower sampling rate. Suitable reconstruction filtering should then be used when restoring the sampled signal to the continuous domain or converting a signal from a lower to a higher sampling rate. For spatial anti-aliasing, the types of anti-aliasing include fast sample anti-aliasing (FSAA), multisample anti-aliasing, and supersampling.