Instrumentation For The Recording And Digital Processing Of ... - EDGE

Instrumentation for the recording and digital processing of multichannel ECG data.' CA. Grimbergen, A.C. MettingVanRijn, A.P. Kuiper, A.C. Linnenbank, A. Peper Medical Physics & Informatics Dept., University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands

Abstract- A 64 channel isolated front-end with small dimensions is described. A nearly perfect isolation is obtained by using an optical fiber link and battery power which makes a high quality recording possible with very little susceptibility to interference. The signals are An, converted in the front-end and easy interfacing has been developed to standard microcomputers using commercially available DMA-boards.

MAINS POWER

I

I

11'

C'"'T

instrumentation amplifier

isolation VI

I. INTRODUCTION Multichannel ECG measurements are used to get more information of the electrical activity of the heart than the standard 12 lead ECG. For the presentation of these measurements so-called body surface maps are generated. For multichannel ECG recordings (32 - 256 channels) a considerable amount of hardware is needed. One complicating factor is the large number of electrodes which must be applied. Several electrode strap solutions have been developed to make the application of that many electrodes practical. A second complication is the need for a large number of physiological amplifiers which tends to make the set-up rather bulky, susceptible for interference and impractical for clinical use. A number of workers has been successful in developing compact hardware for this purpose often mounted on a trolley, that can be wheeled to the bedside of the patient. There is a growing need for the application of multichannel ECG measurements in interference rich locations like the catheterization room [l].In addition, there is a tendency to strive for a higher resolution and lower interference level in the data [2]. It has been shown that this calls for a well isolated measurement set-up and an amplifier system with small dimensions [3]. The specifications of the 64 channel measurement system that will be described in this paper, are: a bandwidth of 100 Hz per channel, a dynamic range of more than 70 dB and a virtually ideal isolation of the front-end. 11. HARDWARE DESCRIPTION

In an isolated ECG measurement with right leg electrode,

I Cbody==

1

AMPLIFIERCOMMON

14

Vim

Ciao

J

==

1 used to reduce the common mode voltage [2], there are two interference currents which contribute to the common mode voltage (fig. 1). This is the interference current (il) caused by the capacitive coupling(Cp) of the patient with the interference source and the interference current (i2) caused by the capacitive coupling (Csup)of the amplifier common with the interference source. Both interference currents are partly flowing through Z , causing the common mode voltage. These currents through Z,, can be reduced by using batteries for the power of the preamplifiers instead of DCDC converters or other magnetically coupled power sources and by miniaturizing the preamplifiers making the capacitance of the preamplifier common with the interference source (Csup)and with ground (Ciso) small. However, a high isolation may result in a high isolation voltage (Vim).The isolation mode rejection ratio (IMRR) becomes an important parameter in that case and a signal transfer through a low capacitance path and a large IMRR is needed [4]. We applied optical fiber technology for the signal transfer which results in minimal isolation capacitances and an optimal IMRR using a digital

This work was supported by the Technology Foundation STW under grant No. 991667

0-7803-0785-2/92$03.OO OEEE 726

transmission format (IMRR > 180dB). The amplifier box developed contains 64 amplifiers, measures 27~10x5cm and is manufactured with surface mount technology (fig. 2). In this way a very low isolation capacitance is obtained if compared to the capacitance of the patients body to ground (q, e < h). This decreases the common mode voltage. The common mode voltage is further reduced by a driven right leg circuit which drives the common with the average electrode potential [2], improving the common mode rejection by approx. 50 dB. The digitalisation is chosen to be performed in the isolated front-end and a low power 14 bits Analog to Digital converter was applied (P = 150 mw). The signal transfer through the optical fiber therefore has a digital format. Assuming the electrode noise to be the limitation of the

TABLE I Specifications

Eq. input noise voltage Eq. input noise current

0.5

Bandwidth per channel Diff. AC input range Dynamic range Common mode rejection ratio Isolation mode rejection ratio Power consumption front-end

5 0.16 - 100 12 72 119 > 166 210

PVms P4UIS

Hz mvpk-pk

dB dB dB mW

A special synchronization technique was developed for the synchronization of the front-end and receiving hardware, in view of the one-way connection of the fiber optic link [4].

IV. CONCLUSIONS The developed front-end makes it possible to record multichannel ECG-data of high quality in a very practical way. The specifications of the 64 channel front-end are given in table I. The nearly ideal isolation and the small dimensions of the preamplifier box are beneficial for interference reduction. The optical fiber link and the small dimensions of the front-end enable the cardiologist to have the preamplifiers very near to the patient reducing cable length, while the instrumentation for presentation and analysis can be positioned at will. Common mode voltage reduction and the use of shielded electrode cables with guarding make high quality multilead ECG recordings feasible in interference rich environments like the catheterization room. REFJ~RENCES Fig. 2. Front-end for a 64 channel ECG measurement.

measurement and estimating this noise to be in order of 1 pVtt,with 14 bits of resolution a total dynamic range of 12 mV range can be covered without loss of information. A sampling frequency of 1 kHz is used to keep the filters in the preamplifiers simple. A fast infrared laser which can operate economically with a very low duty cycle (