C.A. Grimbergen

High-quality recording of bioelectric events. Part 1. Interference reduction, theory and practice.

In the first part of the review the various mechanisms that could be the cause of interference in bioelectric recordings are considered. It is demonstrated that the performance of a good amplifier can be seriously degraded in its functioning if the whole measurement situation is not taken into account. Several techniques used to reduce interference, of which guarding and driven right leg circuits are the most important, are analysed. In the second part of the review some examples of the application of the theory in practical situations are presented. The instrumentation amplifier circuit normally used in bioelectric recordings is improved for measurements under difficult circumstances. Another application is a low-cost 64-channel amplifier for multichannel ECG recordings. The third application is a device that can be added to bioelectric measurement systems and will provide a major reduction in interference.

High-quality recording of bioelectric events

A multichannel instrumentation amplifier, developed to be used in a miniature universal eight-channel amplifier module, is described. After discussing the specific properties of a bioelectric recording, the difficulties of meeting the demanded specifications with a design based on operational amplifiers are reviewed. Because it proved impossible to achieve the demanded combination of low noise and low power consumption using commercially available operational amplifiers, an amplifier equipped with an input stage with discrete transistors was developed. A new design concept was used to expand the design to a multichannel version with an equivalent input noise voltage of 0·35 μV RMS in a bandwidth of 0·1–100 Hz and a power consumption of 0·6 mW per channel. The results of this study are applied to miniature, universal, eight-channel amplifier modules, manufactured with thick-film production techniques. The modules can be coupled to satisfy the demand for a multiple of eight channels. The low power consumption enables the modules to be used in all kinds of portable and telemetry measurement systems and simplifies the power supply in stationary measurement systems.

Investigation into the origin of the noise of surface electrodes

In the recording of biomedical signals, a significant noise component is introduced by the electrode. The magnitude of this noise is considerably higher than the equivalent thermal noise from the electrode impedance. As the noise in surface electrodes limits the resolution of biopotential recordings, it is important to understand its origin. It was found that the noise mainly originates in the electrolyte-skin interface and that it is highly dependent on the electrode gel used and the skin properties of the test subject. Depending on skin treatment, magnitudes between 1 and 20μVrms were measured among subjects. When the metal-electrolyte interface was allowed time to stabilise, electrodes of different metals measured face to face all showed a negligibly small noise magnitude (<1μVrms). In pre-gelled electrodes, where the metal-electrolyte interface has stabilised, no difference in noise properties was found between Ag−AgCl electrodes and other metals when measured on the skin. In subjects at rest, the contribution of EMG signals to the total noise level was shown to be negligibly small compared with the noise contribution of the electrolyte-skin interface. The magnitude of the noise of electrodes appeared to be inversely proportional to the square root of the area of the electrode on the skin.

Low-cost active electrode improves the resolution in biopotential recordings

The accepted models of the origin of interference in body surface potential recordings predict that it would be advantageous to perform signal amplification on the electrode. A suitable circuit for a miniature biopotential amplifier to be manufactured in thick-film technology is described. A prototype series of active electrodes was produced and tested. The high immunity to interference resulted in ECG and EEG recordings of a very high quality.

Amplifiers for bioelectric events: A design with a minimal number of parts

A design for an amplifier for bioelectric events is presented that has fewer parts than conventional designs. The design allows the construction of amplifiers of a high quality in terms of noise and common mode rejection, with reduced dimensions and with a lower power consumption. Gain, bandwidth and number of channels are easily adapted to a wide range of biomedical applications. An application example is given in the form of a multichannel EEG amplifier (gain is 20000), in which each channel consists of three operational amplifiers (one single and one dual), six resistors and two capacitors. The equivalent input noise voltage and current are 0.15 μVrms and 1 pArms, respectively, in a bandwidth of 0.2–40 Hz, and a common mode rejection ratio of 136 dB is achieved without trimming.

Patient isolation in multichannel bioelectric recordings by digital transmission through a single optical fiber

A design for patient isolation in 64-channel electrocardiogram (ECG) recordings is presented. Small dimensions of the isolated section and the use of an optical fiber as the only connection between the isolated section and the grounded section of the measurement system ensured a minimal capacitance between the patient and the environment. The consistent low-power design of the isolated section resulted in a power consumption of 210 mW, which allowed a 10 h continuous operating time of the battery-powered isolated section. The system handles 64 signals with a dynamic range of 75 dB. Analog-to-digital conversion is performed in the isolated section with a sample rate of 1 kHz/channel. The receiver interfaces to a commercially available DMA board for a standard personal computer.<<ETX>>

The isolation mode rejection ratio in bioelectric amplifiers

The necessity of a very high isolation mode rejection ratio-the ability of the amplifier to suppress feedthrough from voltages across the isolation barrier to the output-is argued. Two ways of preventing an interference output signal result from isolation mode voltages are described. One approach is to reduce the actual voltage across the isolation capacitance with an additional circuit. Another approach is to design the amplifier in such a way that the feedthrough of high isolation mode voltage remains negligible. It is shown that an eight-channel isolation amplifier with a very good isolation mode rejection ratio can be built with commercially available components, provided that special attention is paid to the shielding of the amplifier inputs. A design of a multichannel amplifier with an isolation mode rejection ratio of 160 dB is described.<<ETX>>

A method for the measurement of the properties of individual electrode-skin interfaces and the implications of the electrode properties for preamplifier design

A method to measure impedances of individual electrodes has been developed. The DC- and AC-loading conditions of the electrode can be varied in a large range. Impedance values of electrodes on unprepared or alcohol rubbed skin are measured to be very high and vary between electrodes both on the same test person and on different test persons. This leads to important consequences for interference reduction and current noise properties of the bioelectric amplifier.

Instrumentation for the recording and digital processing of multichannel ECG data

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 A/D converted in the front-end and easy interfacing has been developed to standard microcomputers using commercially available DMA-boards.

Continuous localization of ectopic left ventricular activation sites by means of a two-dimensional representation of body surface QRS integral maps

By means of Karhunen-Loeve transformation it is possible to represent electrocardiographic body surface QRS integral maps using only two coefficients while retaining more than 80% of information. These coefficients are computed for a number of mean paced QRS integral maps, which were acquired from well-defined ventricular segments and are contained in a database used for localization of the origin of ventricular tachycardia. It is shown that these two coefficients are sufficient to compute the pacing site in a continuous fashion, without reference to a limited number of segments. These coefficients facilitate and improve the ventricular tachycardia localization procedure based on using the database of paced QRS integral maps, by establishing the relation between QRS integral map and corresponding site of origin.