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AD7730: Can't get 24-bit resolution

I can't 24bit resolution from this part at an output data rate of 50Hz. Why?

 

At an output update rate of 50Hz, you don't have 24bits of resolution
available.
You can use the noise tables on pages 10 and 11 of the datasheet to predict the
resolution for a given Input range and update rate. You can see that for a 50Hz
output update rate ( this also locates the first filter notch at 50Hz and all
subsequent filter notches at multiples of 50Hz) and an input range of 20mV the
maximum resolution is 17bits.  You will get 24 bits out of the part, i.e. the
digital word length is 24bits, but the peak to peak resolution of the
measurement is 17bits or 120,000 counts. At this level you are limited by the
quantisation noise inherent in the conversion process.

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