Capacitive Sensor Estimation Based on Self-Configurable Reference Capacitance
N. Petrellis, C. Spathis, K. Georgakopoulou and A. Birbas
Affiliation: Computer Science and Telecommunications Dept., TEI of Larisa, TEI Campus, Larisa 41110, Greece.
A number of capacitance readout systems presented in the literature and covered by recent patents are briefly
described while a biosensor array readout system developed by the authors is presented in this paper. This biosensor array
readout system is based on a charge sensitive amplifier that estimates accurately the difference between a selfconfigurable
reference capacitance and a biosensor. In the specific implementation, the self-configurable reference
capacitance is automatically configured to a value between 0 and 630pF in steps of 10pF. The configuration of the
reference capacitance is controlled by a simple finite state machine that can be implemented using a simple CPLD
(Complex Programmable Logic Device) such as Altera EPM7064SLC44-10. The proposed system can be used to measure
an absolute biosensor capacitance of up to 640pF with a theoretical sensitivity of 2.5fF. The 2.5fF resolution can be
achieved using a low cost/high accuracy 12-bit ADC (Analogue/Digital Converter) like a one that has been described by
the authors in a recent patent instead of a 17-bit ADC that would be required to measure the biosensor capacitance value
with the same accuracy if a constant 320pF reference capacitor had been used. For this reason, the proposed architecture
leads to a lower area/power and higher accuracy implementation compared to a read out circuit that would use a fixed
reference capacitance. Although the design of the proposed system was inspired by the development of a biosensor array
readout system, a similar architecture can be used by any sensor application that should support a broad capacitive input
range with high resolution.
Keywords: Biosensor readout, configurable reference capacitors, analogue-to-digital conversion, charge sensitive amplifier,
switched capacitance interface.
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