The measured value is proportional to the excitation voltage. An example of "ratiometric" measurement would be a position sensing potentiometer, use as a 3 wire device. If it was sensing a position of 50% stroke, with an excitation voltage of 5 volts, the sensor's output signal would be 50% of that, or 2.5 volts. With an excitation voltage of 8 volts, the signal would be 4 volts. Bill Gausman System One Control, St. Paul, MN ------------------------------
> what is the meaning of 'ratiometric' with respect to measurement?
In some instrument systems, spectrometry in particular, it is common to measure both the unknown value and a known (or "reference") value. In the above example, a true ratiometric measurement would have two analog inputs; one reading the excitation voltage and the other reading the voltage from the "pot". The measurement "output" would then be the ratio of the "pot" to the excitation. This technique is often used in spectrometers to null out the effects of variations in light source output, variations of transmission/absorbtion with wavelength, etc. In general, ratiometric methods work well in measurement systems where a reliable "standard" or "reference" is available and variations in the measurement device are difficult to control.