ratiometric measurement
Ratiometric measurement is a clever way to make a reading not care about the exact supply voltage. Normally, if your sensor's output depends on the supply and your supply drifts, your reading drifts too. The ratiometric trick is to power the sensor and the converter's voltage reference from the very same supply. Now if that supply sags or wobbles, the sensor output and the reference both move by the same fraction, and the ratio between them — which is what the converter actually measures — stays rock steady. The drift cancels itself out.
Here is the mechanism. A converter does not really measure volts; it measures the ratio of its input to its reference, reporting input/reference as a number. If a divider, bridge, or potentiometer is fed from the supply Vs, its output is some fraction times Vs. Use the same Vs as the converter reference and the reading becomes (fraction times Vs) / Vs = just the fraction — the Vs cancels entirely. Numbers: a potentiometer at 60 percent on a 5.0 V supply outputs 3.0 V; with a 5.0 V reference the converter reads 0.60 of full scale. Let the supply drop to 4.8 V: the pot now outputs 2.88 V, but the reference is also 4.8 V, so the converter still reads 0.60. The position, not the voltage, comes through clean.
Why this matters: ratiometric measurement is the natural, almost free way to handle resistive sensors — potentiometers, Wheatstone bridges, load cells, RTDs in a bridge. It removes the need for an expensive, ultra-stable supply or reference and rejects supply noise for free. The honest limits: it only works when the sensor's output genuinely scales with the same source as the reference (resistive, ratiometric sensors), and the shared source must truly be shared and low-noise. For an absolute sensor — one whose output is a fixed voltage like a thermocouple or a bandgap-referenced device — ratiometric does not apply, and there you do need a stable, accurate reference.
A load cell rated 2 mV/V is excited from 5 V and a 16-bit converter uses that same 5 V as its reference. If the 5 V supply drifts to 4.9 V, the cell's output shrinks proportionally but so does the reference, so the digital reading does not move at all — the weight stays correct without any precision voltage reference.
Share the supply between sensor and reference, and supply drift cancels.
Ratiometric only works for sensors whose output scales with the same source as the converter's reference (resistive bridges, pots, RTDs). For an absolute sensor like a thermocouple, the trick does nothing and you genuinely need a stable, accurate voltage reference.