Design of a High Performance Low-Power ECG Amplifier for Textile Based Un-gelled Electrodes
Citation:Maji, Soumyajyoti, Design of a High Performance Low-Power ECG Amplifier for Textile Based Un-gelled Electrodes, Trinity College Dublin.School of Engineering, 2021
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This thesis presents analytical studies and bench experiments which highlight how electrocardiographic recording equipment which adheres to current international standards can still introduce distortion and affect the recorded signal. Most performance specifications are applicable to standard adhesive electrodes and therefore will not be met in recordings scenarios using un-gelled electrodes. This work was carried out in the light of the current international performance standards IEC 60601 pertaining to electrocardiographic equipment and has led to the identification of shortcomings in this standard. The design criteria for the amplifier input impedance needed for recording diagnostic quality of ECG signals with dry un-gelled electrodes are established and suggestions are made for revision of the standards. However, this would require a knowledge of the electrophysiological properties of the electrodes. The electrical properties of equivalent models for several conductive, textile based electrodes in addition to one conductive rubber electrode and a standard self-adhesive electrode is presented in this work. In ECG recording the skin-electrode-amplifier interface can be modelled as an equivalent electrical circuit having one or two parallel C-R networks and series resistance. The electrical components of these models have been established using a time domain methodology. For the single C-R electrode model values of resistance ranged from 10 kΩ to 28 MΩ while values of capacitance ranged from 0.03 nF to 15 µF. The associated time-constant ranged from 0.1 ms to 5 s. In the case of the double C-R model values of resistance ranged from 1 kΩ to 25 MΩ while values of capacitance ranged from 9 pF to 872 µF. The associated time-constants ranged from 0.05 ms to 10 s. Measurements of noise generated were made using very low noise operational amplifiers and with mains power supply interference eliminated from the recorded signals so that they reflected only genuine noise. Curves of the form y = K/f + C were fitted to the recorded noise spectral density functions. Coefficients obtained for the white noise spectral density ranged from 1.1 to 5.6 nV/Hz and for the flicker (1/f) noise spectral density ranged from 8 to 80 nV√Hz. The work presents a low-power, low-noise ECG amplifier that attains a minimum common-mode input impedance of 10 GΩ within a frequency range of 0.1-150Hz. This boosted input impedance is maintained by reducing input amplifier capacitance through power supply bootstrapping. The amplifier attains a differential gain of 41dB, CMRR was 90dB at 50Hz frequency with the inclusion of right leg drive and the semiconductor noise was measured at 32µV at a frequency of 250Hz. In-vivo measurements were undertaken on two subjects at rest, arm waving and doing the Harvard step test. There was semiconductor noise superimposed on the ECG waveforms but no noticeable distortion was seen.
Author: Maji, Soumyajyoti
Publisher:Trinity College Dublin. School of Engineering. Discipline of Electronic & Elect. Engineering
Type of material:Thesis
Availability:Full text available