Calibration‐less direct capacitor‐to‐microcontroller interface

Abstract

A new technique to measure a capacitor or a capacitive sensor by means of a direct sensor-to-microcontroller interface circuit that does not need a calibration capacitor is proposed. Basically, the measurement process consists of three consecutive steps of charge, discharge and charge of the capacitor under test. A non-linear equation is obtained and solved that is dependent only on known circuit parameters. Experimental results show that it is possible to measure a wide range of capacitor values with a maximum deviation of 2 % from the reference value, and that temperature changes from 18 °C to 70 °C yield relative errors below 0.1 %. For the lowest measured capacitor range (33 pF to 4.7 nF) the uncertainty holds below 1 pF which enables measurement of commercially available capacitive sensors. The main advantage of the proposed technique is cost and space reduction of the final design.Introduction: Capacitive sensors are used in a growing number of applications. In order to reduce the cost, complexity, space and power consumption of its interface circuits, direct capacitive sensor-tomicrocontroller interfaces have been proposed. There, the sensor is connected directly to the microcontroller (µC) without using either a signal conditioner or an analogue-to-digital converter. These circuits are an evolution of the conditioning circuits found in digital capacitance meters based on measuring the charging/discharging time of an RC circuit that were proposed before µCs became available [1][2] and that needed several digital and analogue discrete components to measure the desired time interval. Fig. 1a shows the simplest direct capacitor-to-microcontroller interface [3]. In this circuit, the unknown capacitor Cm is charged and discharged consecutively by changing the settings of two digital ports, PA and PB. The first stage ensures that Cm is fully charged by setting PA and PB as outputs. PA outputs a high level (VOH, "1") while PB outputs in low level (VOL, "0".) Afterwards, Cm is discharged towards VOL through R by setting PA as an input hence behaving as a Schmitt trigger in high impedance state (HZ) while PB remains the same. Simultaneously, the μC embedded timer starts counting until VPA reaches VTL, the port's lower threshold voltage. At this point, PA is triggered and the timer stops yielding Tm ≈ NmTCLK where TCLK is the clock period and Nm is the timer register count. Fig. 1b shows the waveform at PA that follows the voltage drop across Cm. Then, Cm can be estimated by

Simplifying Capacitive Sensor Readout Using a New Direct Interface Circuit

“…To this must be added the memory resources occupied by the look-up table. [34] and for the calibration-less method of [35]. (b) Voltage across C X during charging and discharging stages in [35].…”

“…[34] and for the calibration-less method of [35]. (b) Voltage across C X during charging and discharging stages in [35].…”

“…For a broader range of capacitances without the need for calibration processes, [35] proposes the same simple DIC as Fig. 3 (a).…”

IDC Sensor for Low-Cost Water Quality Monitoring Applications

“…Some include external circuitry, e.g., an external voltage comparator, to measure the charging time of a capacitor [16], but in despite of its simplicity, the utilization of external components increases the power consumption of the final solution. The work in [17] proposes the measurement of several charging cycles in the capacitor to obtain a capacitance value that is independent of the voltage applied, and only a function of the measured times, and the resistor value attached to the charging circuit. Although the measure of the capacitance obtained 1pF of confidence, the authors report a dynamic range of 33 pF up to 4.7 nF.…”

“…Similar work has proposed more accurate mechanisms that require the integration of external expensive operational amplifiers [19]. In other cases, the capacitance value is acquired by the usage of nonlinear equations, which increases the internal complexity of the system [20].…”