There are 3 main parameters to look at for the ADC: noise, linearity and gain stability.
The noise data I showed are for the 1 PLC auto zero case, so the difference between 2 conversions at 1 PLC. At some 1 µV RMS and +-10 V full scale this is about 7 digit resolution. So it if 7 digits at 1 PLC. With averaging (e.g. 100 x) the noise is down in the 8 digit range. The noise level is already lower than some 8.5 digit meters (e.g. Keithley2002, ADT6581, Solartron 8081, Fluke8508).
The noise level depends on the switching frequency used in run-up. Slow modulation gives lowest noise (0.85 µV with the current resistors), but also more INL error from DA. So the speed here is a compromise between INL and noise.
Most of the noise sources are known. A convenient way to note the noise is as a equivalent (series) resistor - 1 nV/Sqrt(Hz) corresponds to the noise of 60 Ohms, going up like the square. So the "resistance" values can be just added for different not correlated sources. Calculated back to a single (no AZ) conversion the noise is equivalent to some 600 KOhms (some 100 nV/sqrt(Hz)). Of this the integrator resistors Johnson noise is at 100 K, the resistor excess noise is at about 100-200K, the critical OP at the integrator about 50 K, the amplification of the references some 20-50 K, the input buffer some 5 K, The final charge reading from the µC internal ADC some 20 K and the switching related effects should correspond to some 200-300K (more or less with a different speed). The main point's for possible improvements are the resistor excess noise and the switching related part.
For the INL I see mainly 3 main sources: the dielectric absorption, thermal effects in the resistors and settling of the integrator. I think I now understand the DA effect and get this smaller than 0.05 ppm of FS with a reasonable modulation speed. For the integrator settling part his should be in a first approximation some slightly different slope for the positive and negative side. I have to do some more test on this (force the error to be large and visible with very short pulses and high frequency). So far this effect does not look to bad - chances are it's negligible. The thermal effect is question of the resistors - so far it looks good with the NOMCA array.
Checking the linearity is rather difficult, so far I am quite confident the INL is < 1 ppmFS and likely better, in the 0.2 ppm range, but I have no way to measure. I can kind of measure the DA effect though and at least the slow part of the thermal effects can also be tested.
With the resistor arrays the gain stability also got quite good, so no real problem there.
So 8.5 digit performance is possible (essentially there
) in this topology.
The critical parts are the switches and resistors at the integrator input. Of cause it also needs a better reference. It may still make sense to have a very good ADC with an LM399 as this could allow for auto cal in a low cost solution.
The ADC in the µC is SAR type, but relatively slow. The amplifier with MCP6002 is also limiting the bandwidth. So the ADC bandwidth is limited does not see a lot of higher frequency noise. When the final charge is measured the output is not changing much (barely visible in consecutive conversions) - the main reason is still the residual DA and not switch leakage or OP bias.
The xx4053 switches are not tested very much at the manufacturer. So the limiting values are high, but the typical leakage is much better. So even with the specs at 1 µA max, I see a net leakage current in the 5-10 pA range for the 3 switches. Even if the leakage current would be high, a constant values is suppressed and would act as part of the reference currents. The main point would be that the DA measurement mode would not work that well anymore. If at all leakage would be a problem for the input path. Here it helps that the switch in using bath halfs of the SPDT switch: one sends the input current to ground and thus keeps the voltage low, the other isolates the integrator.
As a possible alternative there could still be the DG4053 switch - lower leakage specs and not too slow.