1. National Conference on Virtual and Intelligent Instrumentation (NCVII -09),
BITS Pilani, Nov. 2009
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A High Sensitivity Bioimpedance Detector B. B. Patil P. C. Pandey V. K. Pandey S. M. M. Naidu
IIT Bombay

2. Presentation Outline Introduction Bioimpedance Detector Circuit
Test Results
Conclusion

3. Introduction
Bioimpedance Detector Circuit
Test Results
Conclusion

4. Sensing of the Variation in the Bioimpedance
Introduction (1/4)
Sensing of the Variation in the Bioimpedance
Noninvasive technique for monitoring
♦ changes in the fluid volume
♦ underlying physiological events
Impedance Cardiography A noninvasive technique for monitoring stroke volume and obtaining diagnostic information on cardiovascular functioning by sensing the variation in the thoracic impedance during the cardiac cycle.
Sensing of the Thoracic Impedance A current ( 20 kHz – 1MHz, <5mA) passed through a pair of surface electrodes and the resulting amplitude modulated voltage sensed using the same or another pair of electrodes.

5. Introduction (2/4)
ICG Instrumentation

6. Introduction (3/4)
Bioimpedance Detection ♦ Detection of extremely low modulation index ( 0.2 – 2 %) ♦ External noise suppression ♦ Carrier ripple rejection AM Detector Ckts ♦ Peak detector ♦ Precision rectifier det. ♦ Synchronous det. ♦ Slicing amplifier det. (Fourcin, 1979): high sensit., increased ripple ♦ Synchronous S/H at carrier peak: very low ripple

7. Introduction (4/4)
Proposed Technique Features ♦ High sensitivity ♦ Carrier ripple suppression without filtering ♦ Noise reduction Realization ♦ Slicing amplifier with sampling at the peaks of the sinusoidal excitation : high sensitivity, low ripple ♦ Summation of the signals obtained by sampling the +ve & -ve peaks : external noise reduction

8. Bioimpedance Detector Circuit
Introduction
Bioimpedance Detector Circuit
Test Results
Conclusion

9. Bioimpedance Detector Ckt (1/6)
Demodulation
♦ Two channels of slicing amplifier with synch. S/H at the +ve and –ve peaks of the excitation
♦ Addition of the two outputs: suppression of noise & low freq. drift
Slicing Amplifier
♦ Realized using voltage clamp amplifier IC AD8037 (Greater of the V+ & VL inputs connected as the non-inverting input)
♦ Ckt config. and resistors selection:
♦ V+ > VL : Output diff. i/p ♦ V+ < VL : Zero output
Sample-and-hold (IC HA5351)
Sampled near the excitation peak & held for ripple suppression

10. Demodulator using Slicing Amplifier & S/H
Bioimpedance Detector Ckt (2/6)
Demodulator using Slicing Amplifier & S/H
IC3 : AD8037 voltage clamp amp., IC4: HA5351 S/H

11. Slicing Amplifier Waveforms Bioimpedance Detector Ckt (3/6)
Vo1: slicing amp o/p
Vo2: S/H o/p

12. Bioimpedance Detector
Bioimpedance Detector Ckt (4/6)
Bioimpedance Detector
AM demod. of the sensed voltage using two channels of slicing amplifier with sync. S/H

13. Sinusoidal Excitation & S/H Pulse Generation
Bioimpedance Detector Ckt (5/6)
Sinusoidal Excitation & S/H Pulse Generation
Two direct digital synthesizer (DDS) chips (AD 9834)
♦ DDS-1: Sinusoidal o/p for current excitation
♦ DDS-2: Square o/p with settable phase shift for S/H pulses
Circuit Features
Microcontroller based digital control of
♦ Excitation current level using a digital pot.
♦ Excitation frequency
♦ Slicing amplifier ref. level, using a digital pot.
♦ Phase shift between the two DDS outputs for precise alignment of hold edge of S/H pulses to the +ve and –ve peaks of the excitation

14. Detector Ckt Waveforms
Bioimpedance Detector Ckt (6/6)
Detector Ckt Waveforms
Vs: DDS-1 o/p (exc.)
VΦ: DDS-2 o/p (phase shifted w.r.t. Vs)
V3 & V4: slicing amp. Outputs
V5 & V6: S/H outputs
VSH1 & VSH2: sampling pulses

15. Introduction
Bioimpedance Detector Circuit
Test Results
Conclusion

16. Impedance Detector Performance Parameters
Test results (1/5)
Impedance Detector Performance Parameters
♦ Range of basal resistance
♦ Sensitivity (ΔVo / ΔR)
♦ Frequency response
Thorax Simulator for Testing
the Bioimpedance Detector
♦ Basal resistance (settable: 20   200 )
♦ Periodic resistance variation (settable: 0.1  1.2 %)
♦ µC & digital pot.: settable ΔR, F, waveshape

17. Thorax Simulator Ckt Test results (2/5) I1 & I2: current injection
E1 & E2: voltage sensing
♦ Variation in the thorax impedance
♦ DM & CM voltages (ECG)
♦ Sync. output

18. Microcontroller and Power Supply Ckt of the Thorax Simulator
Test results (3/5)
Microcontroller and Power Supply Ckt
of the Thorax Simulator

19. Testing Using the Thorax Simulator
Test results (4/5)
Testing Using the Thorax Simulator
♦ Excitation: 1 mA rms, 100 kHz
♦ Thorax Simulator
F: Hz
Ro = 196 
ΔR / Ro = 0.1 to 1.2%

20. Sync. & Det. Output Waveforms Test results (5/5) F = 8 Hz, Ro = 196 .
(b)
(c)
(d) Vo
Sync. & Det.
Output
Waveforms
F = 8 Hz, Ro = 196 .
Resistance variation
ΔR / Ro:
1.2 % sinusoidal
0.6 % sinusoidal
1.2 % square
0.6 % square
Time scale: 40 ms/div, Ch1: 5 V/div, Ch2: 500 mV/div.

21. Introduction
Bioimpedance Detector Circuit
Test Results
Conclusion

22. A bioimpedance detector for ICG instrumentation
Conclusion (1/1)
A bioimpedance detector for ICG instrumentation
♦ Slicing amplifier for AM demod. with mod. index < 2%
♦ Sync. sampling for ripple rejection without lowpass filtering
the output
♦ Digital control of
▫ Exc. parameters (Frequency, current level)
▫ Demod. parameters (Slicing amp. ref., Φ-shift for sync. S/H)
Ckt operation verified using a thorax simulator
for detecting ΔR / Ro well below 2%, sinusoidal & square wave variations with freq. of Hz.

23. THANK YOU

24. B. B. Patil, P. C. Pandey, V. K. Pandey, and S. M. M
B. B. Patil, P. C. Pandey, V. K. Pandey, and S. M. M. Naidu, “A high sensitivity bioimpedance detector”, Proc. National Conference on Virtual and Intelligent Instrumentation (NCVII-09), BITS Pilani, Nov Abstract: A bioimpedance detector is developed as part of instrumentation for impedance cardiography. It uses slicing amplifier for increasing the sensitivity for the impedance variation and synchronous sampling for a ripple-free output. The circuit provides digital control of excitation current and frequency used for the measurement. Its operation has been verified using a thorax simulator for detecting the impedance variations well below 2%. Prof. P. C. Pandey Address: EE Dept. / IIT Bombay / Powai Mumbai / India / pcpandey [at] iitb.ac.in

25. References
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W. G. Kubicek, F. J. Kottke, and M. U. Ramos, "The Minnesota impedance cardiograph – theory and applications", Biomed. Eng., vol. 9, pp , 1974.
M. Qu, Y. Zhang, J. G. Webster, and W. J. Tompkins, "Motion artifact from spot and band electrodes during impedance cardiography", IEEE Trans. Biomed. Eng., vol. 33, pp , 1986.
J. Fortin, W. Habenbacher, and A. Heller, "Non-invasive beat-to-beat cardiac output monitoring by an improved method of transthoracic bioimpedance measurement", Comp. Bio. Med., vol. 36, pp , 2006.
J. N. Sarvaiya, P. C. Pandey, and V. K. Pandey, “An impedance detector for glottography”, IETE J. Research, vol 55, no. 3, pp , 2009.
A. J. Fourcin, "Apparatus for speech pattern derivation", U. S. Patent No. 4,139,732, Feb. 13, 1979.
B. B. Patil, "Instrumentation for impedance cardiography", M.Tech. Dissertation, Biomedical Engineering, Indian Institute of Technology Bombay, 2009.
V. K. Pandey, P. C. Pandey, and J. N. Sarvaiya, "Impedance simulator for testing of instruments for bioimpedance sensing", IETE J. Research, vol. 54, no. 3, pp , 2008.
B. B. Patil, V. K. Pandey, and P. C. Pandey, "A microcontroller based thorax simulator for testing and calibration of impedance cardiographs", in Proc. Int. Symp. Emerging Areas in Biotechnology & Bioengineering (ISEABB), Mumbai, India, 2009, pp