The input ADC pin is floating, causing the VREF to be pulled low

Louis1916 · ‎11-15-2023

Customer feedback that after the MCU SPC5744PFK1AMLQ9 pin62, pin64, and pin67 are floating, the VREF is pulled to 3.8V, and the voltage will automatically return to the normal value of 4.1V after a while. If these signals are pulled up or down, there will be no VREF dropping， Why is it floating causing the VREF voltage to be pulled down？ thanks！

davidtosenovjan · ‎11-17-2023

It is pretty normal. Floating pad, configured as input, is high impedance pad. It means you read random value coming from inducted noise, typically slowly floating up or down.
As the impedance is high, converted value is mainly affected by events on adjacent channels. If input impedance is not adequately low, it does not make sense to deal with conversion accuracy, customer can measure basically anything.

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davidtosenovjan · ‎11-17-2023

It is pretty normal. Floating pad, configured as input, is high impedance pad. It means you read random value coming from inducted noise, typically slowly floating up or down.
As the impedance is high, converted value is mainly affected by events on adjacent channels. If input impedance is not adequately low, it does not make sense to deal with conversion accuracy, customer can measure basically anything.