Self-Calibrated AC Zero Potential Circuit for Two-Dimensional Impedimetric Sensor Matrices

Abstract

Zero potential circuits (ZPC) can be used for reducing cross-talk effects within the 2-Dmatrices. ZPC using AC excitation signals (AC-ZPC) with different frequencies in every row can address all the sensors in a matrix simultaneously. The main challenge thereby is the remaining deviations. To analyze the reasons for this, an analytical model of the column-side output voltages is elaborated, which takes into account the non-ideal characteristics of the operational amplifiers, including the input impedance, finite open-loop gain factor, leakage current, output impedance, and load impedance. Based on this model, a novel method is proposed to realize a higher AC measurement accuracy by suppressing the matrix row side impedance through voltage followers and compensating the matrix column side non-ideal amplifiers through reference units. For compensation, one row and one column of given impedimetric units are included in the target matrix as reference elements. The measurement deviations are reduced by comparing the output differences between the targeted sensor and the reference elements. In a simulation for a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 4$ </tex-math></inline-formula> impedimetric sensor matrix composed of RC elements from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9\,k\Omega \vert \vert 361.7\,pF$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100.4\,k\Omega \vert \vert 7.3\,pF$ </tex-math></inline-formula> , this new method achieves measurement deviations below 0.1%, which is a significant improvement over previous works that reached deviations above 1%.