A Time-Based Electronic Front-End for a Capacitive Particle Matter Detector

Abstract

This paper introduces the electronic interface for a capacitive airborne particle matter detector. The proposed circuit relies on two matched ring oscillators and a mixer to detect the frequency difference induced by the deposition of a particle onto an interdigitated capacitor, which constitutes the load of one of the oscillators. The output of the mixer is digitized through a simple counter. In order to compensate the oscillation frequency offset of the two ring oscillators due to process and mismatch variations, a capacitive trimming circuit has been implemented. The sensor is connected to host through an I2C interface for communication and configuration. The sensor has been implemented using a standard 130-nm CMOS technology by STMicroelectronics and occupies 0.12-mm2 die area. Experimental measurements using talcum powder show a sensitivity of 60 kHz/fF and a 3σ resolution equal to 165 aF.

An Automatic Offset Calibration Method for Differential Charge-Based Capacitance Measurement

“…The absolute value of the reference and sensing capacitor plays a fundamental role in the circuit performance, but it cannot be freely set by the designer since it is related to the specific application. In our case, the circuit in Figure 1 is designed for a particulate matter detector where the value of C s and C r depends on the physical dimension of the capacitive electrode while the capacitance variation is in the order of tens of attofarads [21,22]. Figure 2 shows the differential output voltage, V out+ − V out− , when a capacitance difference ∆C equal to 10 aF is applied at 150 µs.…”

“…The variation of the current mirror gain is therefore more pronounced for high values of ∆C, thus justifying the simulated behavior in Figure 3. Note, however, that the non-linearity of the transcharacteristic in Figure 3 does not represent an issue in applications requiring the "activation" of a capacitive electrodes, like, as an example, airborne particle counters [10][11][12]21,22]. Nonetheless, the linearity of the circuit can be increased by adopting cascode current mirrors.…”

A New Fully Closed-Loop, High-Precision, Class-AB CCII for Differential Capacitive Sensor Interfaces

“…The presence of the R sense resistor allows the open-loop impedance at node Z to be enhanced, but, unfortunately, it also increases the open-loop impedance at node X; if a given closed-loop R X is specified, the gain of the voltage feedback loop and the value of R sense have to be jointly designed in order obtain the required R X value according to Equation (7). Then, the gain of both feedback loops (in terms of A V , A DDA , A Vbuf and 1/g m ) must be designed to be sufficiently high and, at the same time, to permit the desired R Z /R X ratio (>1000 in most practical cases).…”

“…Nowadays, capacitive sensors can be considered a leading technology since they show inherent benefits for different kinds of measurements [1,2]. They are extremely versatile and can be realized in different ways, allowing their use in many applications [3][4][5][6][7]. In fact, capacitance variation can be obtained either by changing the capacitor geometry acting on the surface of the plates, or by shifting them, changing the overlapping area, or modifying the plates' distance.…”

Characteristics of Resistive PM Sensors for Onboard Diagnostics of Diesel Particulate Filter Failure

“…Such OBD PM sensors can be divided into two groups based on the underlying measurement principle: the ones in which a signal is analyzed for a certain time period (accumulation type) and the ones in which measurement is performed in real time (real-time type). The optical [ 11 , 12 , 13 ] and radio frequency [ 14 ] method and induced charge method [ 15 , 16 , 17 , 18 ] are representative examples of real-time measurement methods. However, many difficulties are encountered in the practical application of these methods onboard a vehicle, such as device complexity and harsh installation environments in terms of vibration and temperature, and these technologies involve optical access, high impedance, low current, or high time-resolution, which drives increased complexity and presents new durability challenges [ 19 ].…”