A VCII-Based Stray Insensitive Analog Interface for Differential Capacitance Sensors

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.

Towards Realization of a Low-Voltage Class-AB VCII with High Current Drive Capability

“…Other requirements are high current drive capability in Y and X ports, low THD for the current transferred from Y to X ports, and high voltage swing at the Z port. In all applications reported in [16][17][18][19][20][21][22][23][24][25][26], both CB and VB sections are not included in the negative feedback loop, and as a result, overall performance is determined by VCII linearity and port impedances. In this Section, design considerations of a high drive class AB VCII under low supply voltage are discussed.…”

“…In [17], a comparison between OA-based circuits and VCII-based circuits was drawn, showing that the problem of the cross-talk effect among inputs in a conventional OA-based non-inverting summing amplifier can be cancelled out using VCII. Literature survey shows the possibility of realizing various VCIIbased analog circuits such as filters, integrators, differentiators, rectifiers, and impedance simulators [16][17][18][19][20][21][22][23][24][25][26][27].…”

“…As VCII design is a new research area, the VCII implementations reported up to this point lack the maximum efficiency. Analyzing the previously reported VCII circuits, it appears that most of them [16][17][18][19][20][21][22][23][24] are designed in class A, which has limited current drive capability at the X port. Due to high power consumption and poor current drive capability of circuits including class A VCIIs, they are unsuitable for low power applications and off-chip loads [28], while the class AB VCII with high current drive capability finds wide application as an integrated circuit front-end block.…”

Current/Voltage Controlled Quadrature Sinusoidal Oscillators for Phase Sensitive Detection Using Commercially Available IC

“…With this feature, it is still cheaper and more convenient compared to the chip fabrication [15]. The synthesis of an analog circuit using commercially available ICs have been continuously proposed [16][17][18][19][20]. Especially the designs of voltage or current controlled quadrature sinusoidal oscillators using commercially available ICs have been found in the open literature [21][22][23][24][25][26][27][28][29][30][31].…”

New simple transistor realizations of second‐ generation voltage conveyor

“…Nevertheless, due to the abovementioned disadvantages of the OPAMP, VCII seems to attract the attention of designers. The application areas of VCII such as voltage integrator, 7,8 transimpedance amplifier, 9 first‐ and second‐order filters, 10–12 differential capacitive sensor, 13 inductor simulator, 14 and full wave rectifier 15 play an active role in the literature.…”