missing code of SAR ADC

[huangjw]

Hi, all,
I encounter a problem on my 14bit ADC, when i test it, the code at 2^10-1, 2^11-1 and 2^12-1,2^12-2 will be missing, and this will degrade the INL and DNL to 3LSB. but when i test the dynamic performence, the THD and SINAD is good.
so i want kwon what can make the word missing? and what is the relation between DNL,INL and THD, SINAD?
thanks in advance.

 

[dick_freebird]

This is the common "major carry" problem and it relates to the match
(in bit-weight*N) of big vs many little sources / switches. Ideally you
would make the big ones out of 2^n little ones and so on, match the
resistors perfectly and not use bare sources / switches at the output
of the DAC or whatever your scheme is. But you probably don't want
the layout penalty in area & performance associated.

 

[huangjw]

hi, dick_freebird, thanks for your reply. can you describe the problem in detail, i can't understanding the problem.
and in the adc, calibration is used, so i think match is not the problem.