A Closed-Loop Capacitance-to-Frequency Converter for Single-Element and Differential Capacitive Sensors

Abstract

A novel closed-loop switched-capacitor (SC) capacitance-to-frequency converter (CFC) is presented in this paper. The proposed CFC is capable of measuring from either a single-element or a differential capacitive sensor, providing ratio and ratio-metric outputs, respectively. Most of the existing autobalancing schemes for capacitive sensors use the closed-loop approach but require precise sinusoidal AC excitation and provide an analog output that is sensitive to parasitic capacitances. Also, the use of voltage-controlled resistors (VCR) in many of these schemes limits the linearity and accuracy of their output. The SC-CFC presented in this paper, employs a simple DC reference for excitation, and gives a digital output that is insensitive to parasitic capacitances, by virtue of design. Additionally, the output is linear, irrespective of the sensor characteristic, and independent of the nominal value of the sensor. This feature, along with its compatibility with singleelement and differential capacitive sensors, facilitates its ease of integration with a wide range of capacitive sensors. The CFC has a one-time correction mechanism that significantly reduces the impact of component mismatch. The prototype of the proposed scheme exhibits a maximum non-linearity error (NLE) of 0.24%, a resolution of 12.59 effective number of bits (ENOB), and a rise time of 6 ms. In addition, the proffered design is fit for integrated circuit (IC) fabrication as it employs a switched-capacitor approach.

A Tutorial on Mechanical Sensors in the 70th Anniversary of the Piezoresistive Effect

“…In terms of the number of sensing elements, capacitive sensors can be classified in two subtypes: single-element and differential [ 23 ]. A single-element capacitive sensor (SCS) just requires a couple of electrodes (A and B), thus resulting in a single capacitive sensing element.…”

“…According to (15), the relation between the capacitance and the relative variation in distance (i.e., Δ d / d 0 ) is not linear here but hyperbolic. Since both Δ S / S 0 and Δ d / d 0 normally change in a linear relation with the mechanical input (from now on, x ), expressions (13) and (15) can be rewritten, respectively, as [ 23 ]: where k is a proportionality constant.…”

Simplifying Capacitive Sensor Readout Using a New Direct Interface Circuit

“…In [23], with the same switched capacitor integrating technique but replacing OAs with a successive approximation register (SAR), capacitive sensors in the range of 1 to 3.6 pF are measured with a sampling clock of 18.51 kHz. In [24], a novel closed-loop switchedcapacitor circuit for capacitance-to-frequency converter (CFC) is presented with a maximum nonlinearity error of 0.24%.…”

A Supply-Voltage Driving Scheme for Grounded Capacitive Sensor Front-Ends

“…The auto-balancing bridge (ABB), also known as an ac bridge with current detection, is a usual and effective method to measure floating capacitors [18], [19]. output voltage v O , which is a sinusoidal signal whose rms 153 value V O is given by ( 1).…”

“…As explained in Section V-B, the linearity of the system is needed to easily correct the gain error by a simple OPC, and it is usually assessed by the nonlinearity error (NLE) [8], [17], [19]. It was computed as the difference between each showed an experimental gain error very similar to the error predicted by (11).…”