Ratiometric Potentiometer Input (0–3.3 V)
A ratiometric input is an analogue input that evaluates the voltage produced by the potentiometer not as an absolute value but as a ratio to its own supply voltage. Because the supply shifts along with it, the ratio does not change and the reading is unaffected. On the GS740-3P the per-channel command signal is read this way over the 0–3.3 V range.
A potentiometer is a voltage divider: a reference voltage is applied to its ends and an intermediate value corresponding to the position is taken from the centre terminal. Because the controller sees both the reference and the centre terminal from the same source, the ratio stays constant even if the reference drifts by a few percent. That is a decisive advantage in a vehicle environment where battery voltage wanders between 12 V and 14.4 V.
- Requirement: both end connections of the potentiometer must come from the control board. Connecting an outer terminal to chassis or to another source breaks the ratiometric arrangement.
- On the GS740-3P the green terminal is the potentiometer GND and the yellow terminal is the centre-terminal signal; the shield of the shielded cable is also connected to the green terminal.
As the cable gets longer, the voltage drop on the line and the effect of noise increase; that is why signal cables are run separately from power lines and shielded. Connecting the shield at a single point prevents a ground loop.
Context in which this term is used: Electronic speed and throttle control on diesel engines