An Analog to Digital converter is what takes the actual information that has come from the microphones (and preamp) and converts it from a sound/grouping of frequencies into little 1's and 0's which are eventually placed onto a DAT. The better the conversion (measured in bits) the more "complex" and complete sound that you are receiving. If anyone has ever looked at a .wav file, it is very precise with waves along the wave. So with a better A/D converter comes a more complete and a more authentic recreation of the actual show. This is also the process in the taping scheme that will choose the frequency at which something is recorded, 48kHz or 44.1 kHz. If there is no converter involved in the sequence, then it is the deck that is used that will do the A/D conversion.
Here's a quick look at the most common A/D converters:

Apogee: The AD1000, as mentioned in the preamps is not only a preamp, near the top of the market. It's 24 bit conversion provides the best re-creation of the actual events into a digital format. It's predecessor, the AD500e is 20-bit, but still an excellent converter, and a major step up in terms of sonic quality from the SBM-1.

Graham Patton: There are 2 models that GP makes, the DMIC-20 and the ADC-20, as you may guess from their names, the both have 20-bit A/D conversion. The only difference between the 2 models is that the DMIC-20 also doubles as a preamp.

Sonic Sense: The new Sonic Sense AD2K+ was just released in September of 2000 and is the hot new item among tapers. It is a 24-bit A/D converter like the AD1000, and is supposed to be a step up from the popular AD1000.

Sony: The Sony SBM-1 has been around for quite sometime and sitll remains popular, because it's affordable and still does a good A/D conversion at 20 bits.

Hopefully this helped you out somewhat. If you find this confusing have any suggestions for me to clarify, e-mail me