Error analysis and reduction for a simple sensor-microcontroller interface

Error analysis of a resistive sensor-to-microcontroller interface based on pulse-width modulation and time ratio measurement shows that input and output resistances in digital ports produce zero, gain, and nonlinearity errors. The time-ratio technique cancels these errors when the sensor resistance equals the calibration reference resistor, and reduces errors in the neighborhood of that point. For sensors with wide dynamic range, several calibration resistors selected according to the sensor resistance range will further reduce those errors. Alternatively, two-point calibration and time ratio measurements yield errors that can be smaller than 0.5 Ω for a sensor resistance from about 600 Ω to 3550 Ω