Have a peak @ GinLab https://hackaday.io/project/20700-ginlab-20700

All from the browser! Switching relays between AC & DC, single ended & differential inputs. vRef controlled by RGB LED coupled to LDR (light dependant resistor) in dark layers of heat shrink tube controlled by PWM making it a Variable Resistor. Voltmeter is built with another Arduino ADC 0-5V. The RGB LED at the bottom right is mirroring hidden LED (vRef is not calibrated)

On Arduino can also control AREF pin, used by analogRead(), by externally setting vRef and in the sketch setting analog reference to EXTERNAL, if using Arduino ADCs https://www.arduino.cc/en/Reference/AnalogReference

Home made Variable Resistor @ https://hackaday.io/project/21480-ldr-pot-making-matching-plotting