A quick – 9 hour test – of the ADS1211: at 60 Hz data rate, 16x turbo mode. About 2 million samples.
Clearly visible, higher density of codes at the left and right end. Really suspicious – almost certainly, nothing else than a bit of mains feed-through, about 175 µVrms. Seems we get >20 bits, more or less, otherwise we would not be able to see the distribution (note that some counts have a positive deviation – due to non-linearity!).
But all this, with some 60 Hz noise on top…
Assuming that this noise is constant, it can be eliminated either by futher digital filtering, or by averaging/further decimation of the data, which will be done anyway. As a rule of thumb, decimation by half will give an extra 0.5 bit of data, for random noise – and a bit more for constant 60 Hz.
In these diagrams, “full scale” corresponds to 20 V – the current configuration can hande up to 60 V differential signal, at 1 LSB equal 4 µV. So there is still room for improvement of resolution, like 1 LSB equal 2 µV – but this only provides useful data, if we can get noise down well below 2 µV, which will be tough anyway.
Standard deviation, 0.71 LSB equals 0.71 LSB RMS noise, equals 3 µV. Quite reasonable!
Still visible are the two maxima of the distribution, due to the mains noise.
Comparing to a random-noise based decimation-improved resolution (noise free bits), it seems that the converter is yielding about 20 noise free bits, at 60 Hz data rate. Not quite the 22 bits mentioned in the datasheet. This is not surprising, the last 2 bits, at the fast rate, we will only be able to get this with a better, ultra low noise reference, a low-noise bias supply, and low noise analog power supply (currently using the build-in reference, and build-in bias supply, and 5 V supply from USB bus…). But fair enough, about 24 noise free bits (7.5 digits!), at 10 s averaging, and 9 hours zero point drift of less than 0.4 ppm, this might already be good enough.