Jerry,

If you read the datasheet, all of the tests were done at Vref=2.5V. It is at this voltage that you are guaranteed 22 bits resolution. If you scale the Vref down to +/-20mV, then of course the resolution will not be 22 bits, but something less. This is because of quantitization noise,etc. If you had an infinite ground plane surrounding the HI7190, you might be able to achieve the full 22 bits. But alas, there is no such thing in the real world. 16 bit resolution is great in this case, although I doubt that it will be repeatable in production or in the long term.

Jacob