Reference Voltage (Vref)
The fixed voltage against which an analogue-to-digital converter compares its measurement; the digital value read out is the ratio of the input voltage to this reference. If the reference drifts, the result changes even though the measured voltage has not. That is why the supply line of the potentiometer and the reference of the converter are taken from the same source.
A converter does not measure absolute voltage, it measures a ratio. A 10-bit converter working with a 5 V reference returns the number 512 when it sees 2.5 V at the input. If the reference falls to 4.8 V, that same 2.5 V now reads roughly 533; nothing has changed, yet the speed reference appears to have increased by 4%.
- Ratiometric measurement: If the top end of the pot and the converter's reference are both tied to the same 5 V rail, then when the rail moves both sides move together and the ratio stays constant. This arrangement compensates for the drift by itself.
- Separate reference: Where absolute accuracy is required, such as current measurement, a separate reference source that does not drift with temperature is used.
- Noise: Ripple on the reference line couples directly into the measurement; the line must be strongly bypassed with a capacitor.
If you are getting complaints that the speed setting drifts when the battery voltage drops, checking whether the reference line is regulated is the right place to start.
Where this term is used: DC Motor Driver Selection Guide