A. Peper

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.

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.

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>>

Recording of surface His-Purkinje potentials.

The movement of the activation front in the His-Purkinje system determines the shape of the intracardiac recorded triphasic wave, but has little influence upon the shape of the surface signal. The unfiltered surface signal shows a step at the moment the His-bundle is activated by the av-node. The spike recorded by many investigators is probably the result of high-pass filtening of this step. To lessen the influence of the direction of the activity in the heart upon the shape of the recorded signal, registration of the two dimensional surface field is proposed. The transverse and longitudinal components of the surface field are recorded and used to compute the magnitude of the surface field as a function of time, irrespective of direction. In patients with a short PQ-time, discrimination between the end of the P-wave and the beginning of the HPS-wave can be problematic, especially when these signals overlap. In these cases, use can be made of the method introduced in a previous study, for the separation and suppression of the P-wave in the ECG. An improvement of this method will be discussed.

Separation of his-purkinje potentials from coinciding atrium signals: Removal of the P-wave from the electrocardiogram

A method has been developed for removing the P-wave from the electrocardiogram to facilitate the study of the electrical activity in the atrio-ventricular conduction system and in particular of the activity in the bundle of His. When the PQ-segment in the electrocardiogram of a subject at rest is subtracted from the PQ-segment of the same subject shortly after exercise stress, the resulting signal will, because of the changed PQ-time, be a distored P-wave. Reconstruction of the P-wave can then be accomplished with the help of a special algorithm after which the reconstructed P-wave can be subtracted from the e.c.g.

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.

DC rejection and deblocking in multichannel bioelectric recordings

Multichannel instrumentation for the recording of bioelectric events should have provisions to reject the relatively large DC voltages which are produced by the electrodes involved. Active DC rejection is a suitable high pass filtering technique preserving the common mode rejection ratio of the preamplifiers. High pass filtering may lead to very slow responses of the instrumentation after overload, which can be handled by deblocking. A method to use the DC information of the rejected signal in a multichannel system is presented.