In the english manual for AN84783 here: www.cypress.com/ you find on page 10 a note concerning noise. Did you check and measure all of that discussed major noise sources and verified that each of them (and the sum) is smaller than your wanted noise level. Remember that 20 bit resolution @ Vref = 1.024V is equivalent to ca. 1µV/Lsb. Checking noise and ripple is done best using an oscilloscope, AC nd high gain and look for the level of the overshoots.
Thank you for reply. Though it is normal distribution with a resolution of 20bit, it is not a normal distribution it's a resolution of 18bit. By not become a normal distribution even when the resolution of 20bit if there is noise in the input? I think noise is the lowest input of the ADC because it short.
...and the shorting wire is not able to catch up some noise? How did you wire the cap for bypassing the Vref??
We are talking about µV, this is quite less than what you get when you near your hand to a high impendance input. You shorted the inpiut pins. Shorted to what? Ground?? Shorted, but left floating??? The application note clearly states that even the noise margin of a resistor would be too much and that you need a precision voltage source to be able to measure the performance of the ADC correctly.
And precisely: 1.024V / 218 ~ 4µV/lsb
Can you post you measured histogram plots for all three ranges ?
Attached a guide to meauring for the case input shorted. A scope irrelevant,
and in fact its noise path typically 8 bit digitizer, so even with averaging not
useful in accurate 20 bit measurments, let alone 16, for accurate signal path
noise measurements pre ADC inputs for these kinds of resolution.
Assumed that you setup as follows(per ap note, note DC source not ground referenced) -
Of course you can make the measurements with 0 Vdc (differential in) by shorting
inputs, but use ohmmeter tool to establish short path R, which in turn adds thermal
noise at the input. Lastly note that when making rail measurements, you must have
the inpuit buffer of ADC bypassed in its properties, otherwise you are outside its CM