I'm a pilot, not an engineer, so please pardon the layman's language -
AHRS (Attitude & Heading Reference Systems) came into popular use in the 1970's. Initially, they were essentially a multi-use gyroscopic platform which could replace multiple gyros thus saving weight and maintenance.
As their use proliferated, and as laser technology developed, many of the heavy maintenance intensive gyros were replaced by ring-laser platforms. The initial ring lasers had three axes (looked like a triangle). As the aircraft moved about its three axes, phase shifting of the laser beams could be measured, then translated into attitude and heading information to the pilots, in a conventional sense - electro-mechanical instruments: the attitude indicator and the heading indicator. However, for navigation purposes, the INS still needed acceleration information, and to the best of my knowledge still required moving-parts accelerometers.
In the later manifestations of IRS (Inertial Reference System), the ring laser gyro has been modified somewhat so that the strapdown platform of lasers can provide not only Att & Hdg info from the measurable phase shifts, but also acceleration.
The other significant improvement that took place over the years in conjunction with the basic hardware mods was the replacement of electro-mechanical instrumentation in the cockpit by EFIS (Electronic Flight Instrument Displays).
Once the full advantages of EFIS technology was realized in the early 1980's, other systems were plugged into computers that would provide information to the pilots. Eventually, the requirement for the Flight Engineer was removed.
With regards your specific question about ADC inputs into "wide body" aircraft systems, with this system, size really doesn't matter! A B747-400 may have the same AHRS system as a Dash 8!
All AHRS units have ADC input, and a multitude of other inputs. Your local aviation bookstore should have a section on aircraft avionics that will give you more info in this regard.
If I've over-answered your question, my apologies. Otherwise, hope the above is of use.