RYO-san, Generally you want to bring up VDD before applying voltage to the analog inputs. The Absolute Maximum Ratings table (on page 5 of the datasheet) shows that the analog input voltage should not go above VDD+0.3V. In this device (like most devices) there are reverse-biased diodes from AINx to VDD for ESD protection. You can see this in Figure 8 of the datasheet on page 11 in the diagram for the input multiplexer. Joseph Wu
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