vivkr
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Hi Yawei,
You are right. The configuration used by Sumanen et al. does cause a limiting of the reference. Moreover, the limiting is dependent on process, temperature etc. I imagine that for the author, it was not very critical to have these limitations.
For most applications, you would need to guarantee that the absolute reference levels are fairly stable. Hence, my comment on the desirability of using an external filter to condition a well-defined buffered DC-stable reference level. The alternative is of course to burn a lot of power in a high-speed buffer without filters.
However, if you are not worried about the absolute value of the reference so long as it does not vary too much, and is reasonably large, then you could always try to increase the reference path gain, by scaling up the capacitors that feed in the reference charge. This would need some extra caps which are charged only to reference, and would slow down your MDAC a bit due to the reduced feedback factor. However, depending on your overall ADC, it might provide an option. You could easily scale up the effective value of the reference by 2x without excessive degradation in performance, and that ought to be enough.
Regards Vivek
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