spice model for computer-aided design of biopotential

BIOMEDiCAL ENGINEERINGAPPLICATIONS, BASIS & COMMUNICATIONS

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SPICE MODEL FOR COMPUTER-AIDED DESIGN OF BIOPOTENTIAL AMPLIFIER Y U N - L I Liu, DONG-LONG L I N , YUE-DER L I N

Department of Automatic Control Engineering, Feng Chia University, Taichung, Taiwan ,y I '.••-!

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Fig 4. Simulation result (a) in time domain, and (b) in Fourier analysis for severer power-line interference environment.

Fig 5. Measurement result (a) in time domain, and (b) in frequency domain for general power-line interference environment.

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BlOMEDtCAL ENGINEERINGAPPLICATIONS, BASIS & COMMUNICATIONS

4. CONCLUSION AND DISCUSSION

Biomed. Eng. Appl. Basis Commun. 2004.16:151-156. Downloaded from www.worldscientific.com by 190.75.68.76 on 06/20/14. For personal use only.

The purpose of this study is to propose a simulation model in Spice for bioelectric signal measurement that will be beneficial for the design of biopotential amplifier. Using the proposed model, the circuit designer can thus estimate the circuit performance and evaluate the effects due to power-line interference in circuit simulation package. Both the development time and cost can be reduced by this approach. To verify the feasibility of the proposed

155 model, we design an ECG amplifier with the aid of this model. After construction of the prototype, the circuit characteristics and practical applications have been conducted for final verification. The results show that there exists a high level of agreement in characteristics between the implemented circuit and the simulated one. Besides, the effect of power-line interference can be reasonably evaluated by computer simulation via the proposed model, where such effect is usually difficult to analyse by hand. It seems reasonable a pure-resistance model can be feasible for skin-electrode impedance. However, the phase contribution from each component in drivenright-leg (DRL) circuit is critical and should be estimated as accurately as possible to attain the circuit stability analysis [8]. Moreover, the complex RC network model can reflect the frequency-dependent property of skin-electrode impedance. It is also critical to evaluate the potential divider effect in biopotential recording, which will convert the common-mode interference potential into differential-mode signal and is primarily due to the impedances imbalance between recording electrodes. Clearly the potential divider effect will also vary with different frequency, and the effect at the power-line frequency is the most important one. Such information will be lost if the pure-resistance model is adopted. For the reasons, the complex RC network model is strongly recommended for skin-electrode impedances. In this study, the model for popular Ag/AgCl electrode is adopted. If dry electrodes are used for signal recording, the equivalent circuit model should be modified. Besides, the parameters in the proposed model depend upon principally the preparation of skin, the contact of electrode with skin, and the interference from mains power supply. Most of the parameter values adopted in the simulation have also appeared in other previously published work. From the comparison between simulation and measurement results, the adopted values seem feasible. There are four modes of configuration for biopotential measurement, they are two- or threeelectrode/isolated or non-isolated configuration. And, there exists different kind of biopotential signals being of clinical importance. To verify the feasibility of CAD methodology for biopotential amplifier, only one type of configuration (three-electrode/non-isolated) and only ECG signal is selected to serve the purpose. The same modelling principle can also be applied to the other three configuration modes and the other kind of biopotential signals.

(b)

Fig 6. Measurement result (a) in time domain, and (b) in frequency domain for severer power-line interference environment.

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ACKNOWLEDGEMENTS

REFERENCE

The authors acknowledge gratefully the financial support from the National Science Council (under the grant NSC 91-2213-E-039-002), and from China Medical College, Taiwan.

1. Park S and Jayaraman S, "Enhancing the quality of life through wearable technology", IEEE Eng. Med. Biol. Mag., 2003; 22: 41-47. 2. Huhta JC and Webster JG, "60-Hz interference in electrocardiography", IEEE Trans. Biomed. Eng., 1973;27:91-101. 3. Winter BB and Webster JG, "Reduction of interference due to common mode voltage in biopotential amplifiers", IEEE Trans. Biomed. Eng., 1983; 30: 58-62. 4. Metting van Rijn AC, Peper A and Grimbergen CA, "High-quality recording of bioelectric events: Part 1. Interference reduction, theory and practice", Med. Biol. Eng. Comput., 1990; 28: 389-397. 5. Wood DE, Ewins DJ and Balachandran W, "Comparative analysis of power-line interference between two- or three-electrode biopotential amplifier", Med. Biol. Eng. Comput., 1995; 33: 6368. 6. Rosell J, Colominas J, Riu P, Pallas-Areny R and Webster JG, "Skin impedance from 1 Hz to 1MHz", IEEE Trans. Biomed. Eng., 1988; 35: 649-651. 7. Neuman MR, "Biopotential amplifier", in Medical Instrumentation — Application and Design, Second Edition, J. G. Webster Ed., Houghton Mifflin, 1992: 227-287. 8. Winter BB and Webster JG, "Driven-right-leg circuit design", IEEE Trans. Biomed. Eng., 1983; 30: 6266. 9. Dobrev D and Daskalov I, "Two-electrode biopotential amplifier with current-driven inputs", Med. Biol. Eng. Comput., 2002; 40: 122-127. 10. Thakor NV and Webster JG, "Ground free ECG recording with two electrodes," IEEE Trans. Biomed. Eng., 1980; 27: 699-704.

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APPENDIX Typical values Typical values for skin-electrode impedance If skin has not been prepared, then Ru^ 120 O, Re= 1 MO, and Ce = 40 nF. If skin has been well prepared, then Ru = 120 Q, Re= 10 KO, and Ce= 10 nF [6]. If the electrodes are well attached to skin, then Rs = 100 n , Rd = 500 O, and Cd=100 nF. For normal condition, if the imbalance of electrode-electrolyte impedance is in the worst case, then the impedances Rd||Cd for two recording electrodes are 2 KO||20 nF and 500 O ||100 nF respectively [9]. Typical values for coupling parameters For general power-line interference environment, I b = 100 nA, and I d = 100 pA [5]. For severer powerline interference environment, 1^500 nA, and Id = 500 pA. C u = 320 pF/m, and Cg = 200 pF [5]. Two values for Cs have been published before, one is 2 pF [10] and the other one is 200 pF [8]. The above values have been reported to agree in both UK (240 V/50 Hz) and US (110 V/60 Hz) power lines [5]. The other parameter Zs=10"n||lpF[5].

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