I am experimenting with a microcontroller project with a very wide-ranging, noisy DC input voltage at VCC. The controller processes a sensor input to the ADC and outputs a pulsed signal on all pins of a separate port. I need some advice on the best way to configure the system and microcontroller (ATmega168) supply (VCC/AVCC/AREF) to make the ADC and sensor as immune to all of the switching noise as possible. I’ve drawn up 3 possibilities of the top of my head below, but not sure which, if any, is optimal for separating digital and analog noise as much as possible:
Currently I’m testing with both Vcc and AVcc powered from a linear bench supply (output 5.3V), and using a TL431 as 5V reference at Aref (similar to possibility 3). A potentiometer is standing in for the sensor. Despite large capacitors between +5.3V and COM (4400uF total), the multiplexing currently causes a lot of noise on the reference output. Below is a scope screenshot of the reference output, and this is without the switcher:
What is the best basic topology to supply this circuit? Any comments, recommendations or better ideas than any of these would be greatly appreciated.