ATMEGA328P has two supply pins: VCC and AVCC. AVCC pin is supply of ADC converter and generally should be clean supply. So I think to use two 5V regulators. One for VCC and one for AVCC. But this is not enough. Measurements done by switching resistors using digital I/O pins. So actually VCC supply will be part of measurement anyway. May be it will be necessary to add buffer or mux that uses AVCC as supply to solve this problem.