A microcontroller-based interface for active sensors

A low-cost smart interface for active (voltage and current-generating) sensors, based on the use of a microcontroller, is presented. The proposed interface acts as an asynchronous Voltage- and Current-to-Period converter employing a current-to-voltage converter, a voltage-controlled oscillator (VCO) a control logic and a microcontroller. For the VCO a charge-controlled oscillator is applied with a period-modulated output signal. The microcontroller is used to digitize the output signal from the oscillator, to process data, to control the logic circuit and to output the measured results. The interface does not need an accurate A/D converter. The use of the microcontroller and the concept of a three- and four-signal measurement eliminates many undesired nonidealities of the interface. Experimental results show that the interface is able to measure voltage signals in the range of 0 V to 5 V, with a resolution and a nonlinearity of 100 μV and ±320 μV, respectively, and current signals in the range of 0 mA to 20 mA, with a resolution and nonlinearity of 1.2 μA and ±2.9 μA, respectively. The measurement time is less than 100 ms