I'm hoping Bruno can shed a little light on this.
Firstly I know that ideally an audio ADC should be dithered, and this should ideally be with something like TPDF noise.
However if we're talking about a 24 bit converter (non oversampling), with its tiny quantization steps then I can't see how you could actually generate that TPDF noise without also generating far more gaussian noise at the same time.
With sigma delta converters the quantization steps are larger, so it seems more doable, though I can't remember seeing any mention of analog dither on converter datasheets (may have missed it, wasn't looking for it at the time), and it would have to be either part of the IC or an input pin, since it would have to go between the integrator and the quantizer within the sigma-delta modulator.
Of course when we're looking at 24 bit audio converters the input signal is invariably going to have several bits worth of gaussian noise on it, making it large and chaotic with respect to a single quantization interval, thus resulting in uncorrelated errors, i.e. noise like in character, though this is not the ideal input noise distribution.
So my question is, are any of these converters dithering at the initial quantizer stage (as opposed to the decimation filters), and if so, how? And if not, how much of a problem is it considering the expected characteristics of the input?