What is ratiometric instrumentation architecture?

Answer 1

Ratiometric instrumentation architecture is common in the automotive industry, specifically for analog sensors attached to an engine. The idea is to help divorce a measurement system from the influences of circuit aging, temperature, power supply voltage change, etc. In a ratiometric system A to D converters are referenced to a power supply voltage, the same power supply used for sensors (e.g. a Throttle Position Sensor), making the power supply a scalar (reference) for the ADC. The net result is a system that has two inputs, a sensor voltage and a power supply voltage. Mathematically, the system test for ratiometricity is to vary an input voltage and supply voltage by the same factor (percentage), and then verify the result is invariant. For example, a thermistor at 25°C produces, through signal conditioning, a voltage of 2.500V while using a 5.000V supply. That same thermistor at 25°C must produce 2.550V when the supply voltage is raised to 5.100V (+2%) in order for the system to be considered ratiometric.