Abstract
In this paper, a novel approach to implement a stray insensitive CMOS interface for differential capacitive sensors is presented. The proposed circuit employs, for the first time, second-generation voltage conveyors (VCIIs) and produces an output voltage proportional to differential capacitor changes. Using VCIIs as active devices inherently allows the circuit to process the signal in the current domain, and hence, to benefit from its intrinsic advantages, such as high speed and simple implementation, while still being able to natively interface with voltage mode signal processing stages at necessity. The insensitiveness to the effects of parasitic capacitances is achieved through a simple feedback loop. In addition, the proposed circuit shows a very simple and switch-free structure (which can be used for both linear and hyperbolic sensors), improving its accuracy. The readout circuit was designed in a standard 0.35 μm CMOS technology under a supply voltage of ±1.65 V. Before the integrated circuit fabrication, to produce tangible proof of the effectiveness of the proposed architecture, a discrete version of the circuit was also prototyped using AD844 and LF411 to implement a discrete VCII. The achieved measurement results are in good agreement with theory and simulations, showing a constant sensitivity up to 412 mV/pF, a maximum linearity error of 1.9%FS, and acknowledging a good behavior with low baseline capacitive sensors (10 pF in the proposed measurements). A final table is also given to summarize the key specs of the proposed work comparing them to the available literature.
Highlights
In recent years, capacitive sensors have found numerous applications in measuring pressure, displacement, velocity, etc. [1,2,3,4,5,6,7,8,9,10]
As the performance of complete smart sensor is highly dependent on the readout circuitry, the development of an interface which can fulfil the above-mentioned requirements has become a hot research topic of our times
Adopting current mode signal processing leads to circuits with reduced complexity, high speed, and low voltage operation [7,8,10,22], when the sensor is excited by a current source, the parasitic capacitances become a limiting factor since it consumes part of the input current
Summary
Capacitive sensors have found numerous applications in measuring pressure, displacement, velocity, etc. [1,2,3,4,5,6,7,8,9,10]. Adopting current mode signal processing leads to circuits with reduced complexity, high speed, and low voltage operation [7,8,10,22], when the sensor is excited by a current source, the parasitic capacitances become a limiting factor since it consumes part of the input current. The interface fulfils many important features namely: Very simple implementation and insensitivity to large sensor parasitic capacitances thanks to an active compensation of the current taken from them. Even when the parasitic capacitance has a large value in the order of the baseline, the proposed circuit exhibits high accuracy and high linearity with high read time, which are comparable to those of previously reported works, while it employs a very simple structure and avoids using any complicated switching circuitry.
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