LPC824 Analog Input reference voltage of recommend from VREFP - VREFN voltage limit.

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by kazu_zamasu on Tue Mar 03 10:43:45 MST 2015
Hello all,

I find to LPC82x.pdf Product data sheet on the page 23.

This is following.(from data sheet copy)

"Remark: For best performance, select VREFP and VREFN at the same voltage levels as VDD and VSS. When selecting VREFP and VREFN different from VDD and VSS, ensure that the voltage midpoints are the same:

If the calculation results VDD is 3.3, the 0.1V is raised VREFN, has to be subtracted from 0.1 VREFP.
The VREFP min voltage is 2.4VDC from data sheet.
Therefor VREFN limit is 0.9V.

I have some questions.

1)Why need calculation result to "(VREFP-VREFN)/2 + VREFN = VDD/2" ?
2) Why have to keep one half VDD?
3) If I will can not keep this result, what happen and impact to performance error?

Please advice to me. I am so troubled this data sheet remark.