When I was reading the manual, I thought they stated that the fast R^2 converter was 24 bits including the sign.
With that 10Gohm input impedance my little cap is not discharging very fast. After 15 hours its down to 7.4 volts. The LSD bins continue to look fairly even.
You would expect the bins to look balanced through a sweep - if not, there is definitely an alignment issue! - this test is really quite good, I'll try it and see what happens here...
I believe the A/D converter works like this: The converter has 5 real bits which are used several times to "nibble away" at the voltage being measured, in 5 repeated measurements, where the remainder between each of the five readings is stored as an analog voltage on a capacitor. This is the "recirculating remainder" that they are talking about.
You would think 5 readings of 5 bits add up to 25 bits, but in fact only the first reading is a 5 bit reading - the next 4 readings all overlap the previous 5 bit "nibble" by 1 bit, so they are effectively 4 bit readings. This is done right at the analog level, by the amplification chosen for the op amps that handle the remainder and feed it back. The reason they do that is to reduce noise, (EDIT:
according to the manual actually I can't remember where I read that, or if I just thought that must be the reason). So, effectively, we get 5 bits plus 4x4 bit readings = 21 "real" bits for a complete cycle.
The sign is handled by a separate circuit that flips the voltage reference to the opposite polarity. I suppose we can call that one extra bit, but the sign is a completely separate business from the recirculating remainder processing. The processing is equally happy working with positive or negative voltages, as long as the reference is flipped!
When you look at how this thing actually works, you can't help being just a little bit awestruck that they actually managed to get this principle to work at all, let alone be accurate, since it is actually all being done by analog electronic circuits that are just "guided" by the CPU!

[Edit:] Yes, the manual does say the result is "assembled into a 24 bit word describing the polarity and magnitude" - but that doesn't actually say how many of the bits are significant!
