ADS1118-Q1: How to deal with unused ADC channels

Hello Hangjie,

Realistically there are 2 categories of options: Connect to a known, nonzero voltage (VDD or VDD/2 preferred) or connect to GND.

Connecting to GND will result in nano-amps of current leakage out of the inputs because of the way the MUX is designed.

Connecting to VDD or VDD/2 results in no leakage but can be troublesome for layout. There is no need for pullup resistors but some customers decide to add resistors to limit input current if something happens to the VDD rail the resistors are connected to. So pull up resistors are for extra protection but not necessary. 

Best,

-Cole