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

Abstract

This paper presents the analysis and design of an integrated interface for grounded capacitive sensors. To reduce the effects of parasitic cable capacitances, a feedforward technique has been applied. In combination with the use of a special front-end amplifier this yields high immunity for a parasitic cable capacitance. The major nonidealities of the interface circuit have been analyzed. The complete interface has been designed and implemented as an integrated circuit, using standard 0.7 μm CMOS technology. Experimental results, which are in good agreement with theoretical analysis and simulation, show that for sensor capacitance down to 10 pF, shielded connection cables up to 30 m can be handled with an absolute error of less than 0.3 pF. The measured nonlinearity of the interface amounts to about 3 × 10 −4 for 30 m of the cable. For 40 ms measurement time, the resolution amounts to about 16 bits.

Design trade‐offs of a capacitance‐to‐voltage converter with a zoom‐in technique for grounded capacitive sensors

“…Depending on how the capacitive sensor is connected to the electronic interface circuit, the interface principles can be categorized as (1) capacitive sensors with a grounded target electrode [9][10][11][12][13] ; (2) capacitive sensors with an active target electrode [14][15][16][17][18][19][20][21][22] ; and (3) capacitive sensors with a floating target electrode. 23,24 It should be mentioned that in all categories, to reduce external interference, the sensor plates need to be connected to the interface via a shielded cable.…”

“…11,12 Several solutions for interfaces with a grounded capacitive sensor have been proposed. [9][10][11][12][13] In the papers referenced, it has been shown that the use of a switched-capacitor-based approach improves the stability of the active guarding, significantly. However, not much research has been conducted yet on the effect of switched-capacitor active guarding on systematic errors and absolute accuracy with grounded capacitive sensor interfaces.…”

“…During time interval T 1 , S 2 is off and S 1 is on. [10][11][12] Since the final voltages of nodes X and Y are equal (V DD , V DD /2, 0), the capacitor C P1 will not be charged and discharged and therefore has no effect on the output of the CVC circuit. At the same time, the top electrode of the sensor capacitance C X is connected to V DD via S 4 .…”

“…[10][11][12] The limited gain is also resolved with proper amplifier design. The offset is removed by applying a chopper.…”

“…10,12 Moreover, switch S 2 also cannot create any charge injection error, because when it is turned off, the output will be connected to virtual ground via S 1 . 10,12 Moreover, switch S 2 also cannot create any charge injection error, because when it is turned off, the output will be connected to virtual ground via S 1 .…”