Features and Design Constraints for an Optimized SC Front-End Circuit for Capacitive Sensors With a Wide Dynamic Range

Abstract

Abstract-This paper presents optimization criteria for an integrated switched-capacitor front-end circuit for capacitive sensors with a wide dynamic range. The principle of the interface is based on the use of a relaxation oscillator. A negative-feedback circuit controls the charge-transfer speed to prevent the overload of the input amplifier for large input signals which thus enables a wide dynamic range of capacitor values. Moreover, it has been shown that the use of negative feedback can also result in much better noise performance. However, for the interface to function properly, there is a serious limitation for the value of a specific parasitic capacitance. Therefore, a method which extends the acceptable range of this parasitic capacitance is proposed. A novel method of linearity measurement which takes the influence of PCB parasitic capacitances into account, is also presented. The circuit has been designed and implemented in 0.7 m standard CMOS technology. The supply voltage is 5 V and the measured value for the supply current is about 1.4 mA. Experimental results show that for the capacitor range of 1 pF to 300 pF, application of negative feedback yields a linearity of about 50 10 6 (14 bits) with a 16-bit resolution for a measurement time of 100 ms. Tests have been performed over the temperature range from 55 C to +125 C.

An integrated interface circuit with a capacitance-to-voltage converter as front-end for grounded capacitive sensors

“…This result is about one bit better than that reported in [6] for comparable conditions. We also measured the nonlinearity according to the method presented in [11]. In order to be independent of the absolute component accuracy, four different measurements are performed for C ref1 , C ref2 , C ref1 + C ref3 and C ref2 + C ref3 , respectively.…”

Biomedical Signal Acquisition Circuits

“…Some previously reported results provide excellent solutions for capacitive sensor signal acquisition, which convert the capacitance into voltage, frequency, or pulse width modulation for further analysis and processing [4,[31][32][33][34][35]. These traditional capacitive sensor interfaces are usually composed of a switch matrix, a sigma-delta (Σ-Δ) converter, the standard data interface, and other modules.…”

“…Another type, named quasi-digital sensor interface, transforms the sensor variations into quasi-digital forms, including frequency, time period, duty cycle, etc. [33][34][35]. The quasi-digital type usually has wide input dynamic range (up to 300 pF), but suffers from low update rate (usually 10 or 100 ms for one channel), high power consumption, and low process consistency, which is not suitable for multisensors applications.…”

A readout circuit with wide dynamic range for differential capacitive sensing applications

“…The readout circuits are often implemented by realizing either a capacitance to frequency or capacitance to voltage/current conversion. However, most of such interface electronics are designed for and work for a given application [4], [3]. The proposed circuit design in this paper is rather flexible and may be employed in numerous applications.…”