The 5V pin on the ADC is connected to the Raspberry Pis 5V GPIO pin so it will pass through the power from the Raspberry Pi. The stability of the voltage is dependant on the power supply you are using for the Raspberry Pi.
Basic switch-mode supplies like those often supplied with Raspberry Pi kits may fluctuate by a few millivolts when the CPU on the Raspberry Pi is under heavy loads so it is best to monitor the 5V pin with a multimeter to see if the voltage does change when you are using the Raspberry Pi.
If your sensor needs a very stable supply voltage then it may be worth using a seperate power supply for the sensors. As long as the ground pins for the sensor power supply and the Raspberry Pi power supply are connected together the ADC Pi should be able to read the voltages coming from a seperate power supply with out any issue.