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IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS 1

An Amplitude-to-Time Conversion Technique Suitable for Multichannel Data Acquisition and Bioimpedance Imaging
Jong Cheol Baeg, Student Member, IEEE, Hun Wi, Student Member, IEEE, Tong In Oh,Member, IEEE, Alistair Lee McEwan, Senior Member, IEEE, and Eung Je Woo, Senior Member, IEEE

Abstract—In this paper we exploit the high timing resolution offered by microprocessors to develop an amplitude measurement approach that is convenient for high channel count portable sinusoidal recording systems such as the bioimpedance measurements used in impedance imaging. This approach reduces thenumber of components required per channel, reducing cost, size and power consumption compared to the traditional approaches. The setup uses two high performance comparators to convert amplitude difference to a timing difference. This is captured by a high speed microprocessor. A straightforward algorithm removes DC and timing offsets. We suggest three modes of operation: fast: less than one period ofthe input, normal: exactly one input period and high precision: multiple input periods. The mean signal-to-noise ratio was 40, 81, and 112.4 dB in fast, normal, and high precision mode respectively for a range of resistive loads. Index Terms—Amplitude-to-time conversion, analog-to-digital converter (ADC), capture unit, bioimpedance imaging, microprocessors, multichannel data acquisition.

I.INTRODUCTION

HE electrical bioimpedance spectrum is an inherent characteristic of biological tissue related to the intracellular and extracellular volume and cell membranes. These change due to the physiological and pathological status of tissues so bioimpednace may be used as a powerful diagnostic tool. Bioimpedance is widely used in applications ranging from cell monitoring [1]–[7] andphysiology [8]–[10] as it is sensitive to changes as small as the ion channels of the cell membrane to the large, temporary change in tissue structure during respiration or blood circulation.
Manuscript received March 24, 2012; revised June 13, 2012; accepted July 19, 2012. This work was supported by a grant from Kyung Hee University in 2012 (KHU-20120821). This paper was recommended by Associate EditorS. Leonhardt. J. C. Baeg, H. Wi, and E. J. Woo are with the Department of Biomedical Engineering, College of Electronics and Information, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Korea. A. L. McEwan is with Department of Biomedical Engineering, College of Electronics and Information, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Korea, and also with the School of Electricaland Information Engineering, The University of Sydney, NSW 2006, Australia. T. I. Oh is with Impedance Imaging Research Center and the Department of Biomedical Engineering, College of Electronics and Information, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Korea (e-mail: tioh@khu.ac.kr). Color versions of one or more of the figures in this paper are available online athttp://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBCAS.2012.2212437

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In bioimpedance, a safe sinusoidal current at controllable frequencies is applied to a pair of electrodes and the resulting voltage changes are measured at additional electrodes [11]. Noise reduction may be greater with bioimpedance than straightforward biopotential measurement as we can demodulate at the known applied frequencies.In this paper we chose a 500 Hz frequency source to test the system as it is above the Hz) and low enough bandwidth of physiological signals ( not to be adversely effected by reactance ( MHz) [12]–[14]. Imaging systems based on multichannel bioimpedance, such as electrical impedance tomography (EIT) will benefit from high channel counts but this increases the cost, size and power consumption...