Question: What will be the effect of fluctuation in supply voltage on CSD(CapSense Sigma Delta)counts?
Response: Counts of CSD depends on difference between supply voltage and reference voltage. If reference is "VBG", there will be noticeable change in counts due to fluctuation in supply voltage.
For application where supply voltage is not stable, ASE11 reference is recommended:
When the reference voltage is set by ASE11, the threshold voltage is a constant percentage of Vdd. This effect helps to regulate the level of Raw Counts with changes in Vdd.
Please find the below count variation for CY8C21434, with ASE11 reference:
1. 2.7V nominal: +5.2% for variation in Vdd=-0.2V
-3.1% for variation in Vdd=+0.25V
2. 3.3V nominal: +2.7% for variation in Vdd=-0.2V
-2.6% for variation in Vdd=+0.25V
If variation in supply voltage is high, CY8C20X66 device should be used which has internal regulator for reference voltage.