Hi,
you did not specify the MCU, lets assume S32K144.
ADC input should be between VREFL and VREFH. Recommended condition is to have VDD=VDDA=VREFH. Anyway VDDA and VDD must be shorted on PCB, but VREFH can be connected to another source. And there is a restriction, VREFH should be equal or less than VDDA + 0.1V and VDD + 0.1V.
BR, Petr
