Can anyone recommend a ADC?

Okay, so I put the multimeter right on the power lines before anything.

This is the results I get:

4.991 steady - Just running

4.996 steady - Running with Serial Monitor.

5.029 steady - Running with Serial Monitor, and change LCD color - dark blue

5.014 seedy - Running with Serial Monitor, and change LCD color - light blue)

5.029 steady Running with Serial Monitor, and change LCD color - dark blue

As you can see the voltages change a bit, but once they change it's steady. So this tells me because I'm doing different things on the circuit it is causing the voltage to change. Normally this wouldn't be a problem because it's so small, but in my case it's causing my calculations to be off whenever I try and change something electronically.

My questions are:

  1. Do you agree that it is the power supply USB that is causing this problem?
  2. Is there a way I can stop the power supply from doing this? Like what if I build my own powersupply with a battery and an LM7805?
  3. Shouldn't the ADC change it's range based on the power?