IIT Bombay 04/Apr/2014 < pcpandey@ee.iitb.ac.in > http://www.ee.iitb.ac.in/~pcpandey

1. International Conference on Circuits, Systems, Communication & Information Technology Applications, Mumbai, India April 4-5, 2014 (CSCITA-2014) A Versatile Noninvasive Bioimpedance Sensor Using Digital Synchronous Demodulation P. C. Pandey IIT Bombay 04/Apr/2014 <pcpandey@ee.iitb.ac.in> http://www.ee.iitb.ac.in/~pcpandey

2. International Conference on Circuits, Systems, Communication & Information Technology Applications, Mumbai, India April 4-5, 2014 (CSCITA-2014) P. C. Pandey: A versatile noninvasive bioimpedance sensor using digital synchronous demodulation (invited talk) Abstract A noninvasive monitoring of bioimpedance has the potential of serving as a low-cost diagnostic tool and monitoring device in several medical applications, e.g. impedance cardiography (sensing of variation in thoracic impedance to estimate cardiac output and some other hemodynamic parameters), pneumography (sensing of respiratory parameters), plethysmography (sensing of peripheral blood circulation), glottography (sensing of movement of vocal chords), etc. The instrument passes a high-frequency low-amplitude current through an appropriately placed electrode pair, an amplifier to sense the resulting amplitude-modulated voltage across the same or another appropriately placed electrode pair, a demodulator to detect the impedance signal, and signal processing for obtaining the desired parameters. In our work, an embedded system design approach has been used to develop a versatile noninvasive bioimpedance sensor for sensing the basal value of the impedance and the time-varying component of the impedance waveform. A microcontroller and an impedance converter chip are used for providing a stable sinusoidal source with programmable frequency and digital synchronous demodulation for very low noise and demodulation related distortions. A voltage-to-current converter with balanced outputs is designed using two operational trans-conductance amplifiers for current excitation. The sensed voltage is added with a sinusoidal voltage obtained from the excitation source and with digitally controlled amplitude and polarity to increase its modulation index before digital synchronous demodulation and for baseline correction of the sensed impedance signal. Two digital potentiometers are used to provide independent control over current excitation and baseline correction. Synchronous digital demodulation in the impedance converter chip gives real and imaginary part of the impedance. An isolated RS232 interface is provided to set the parameters and to acquire the sensed impedance signal. The design can be used for realizing a body-worn device for monitoring the clinically important hemodynamic parameters during critical care, for ambulatory recording for early diagnosis of cardiovascular disorders and for post-operative care, for monitoring of physiological parameters for use in sports medicine, and as a low-cost diagnostic aid. Dr. P. C. Pandey, Professor, Electrical Engineering, IIT Bombay EE Dept, IIT Bombay, Powai Mumbai 400076, India. <pcpandey@ee.iitb.ac.in> http://www.ee.iitb.ac.in/~pcpandey

3. Outline Introduction Hardware & Software Test Results Summary Reference HitendraSahu: “Sensing of impedance cardiogram using synchronous demodulation”, M. Tech. dissertation, Biomedical Engineering, Indian Institute of Technology Bombay, June 2013.

4. 1. INTRODUCTION • Noninvasive Bioimpedance Monitoring • Low-cost diagnostic tool • Monitoring device • Applications • Impedance cardiography: Sensing of variation in thoracic impedance to estimate cardiac output & some other hemodynamic parameters • Pneumography: Sensing of respiratory parameters • Plethysmography: Sensing of peripheral blood circulation • Glottography: Sensing movement of vocal chords during speech production

5. Instrumentation for Bioimpedance Sensing • Excitation current of high frequency and low amplitude through an appropriately placed pair of electrodes • Amplifier to sense the resulting amplitude modulated voltage across the same or another pair of appropriately placed electrodes • Demodulator to detect the impedance signal • Signal processing for obtaining the desired parameters

6. Example: Impedance Cardiograph • ICG blocks • AC excitation current • Voltage sensing amp. • Demodulator • Baseline correction • ECGextractor • Operation • Excitation current: 20 - 100 kHz, < 5 mA • Amplitude demodulation of the sensed voltage: Z(t) with basal impedance (20 − 200 Ω)& time-varying component (< 0.2 Ω) • ICG: − dZ/dt, processed with ECG as the reference.

7. Objective • To develop a body-worn bioimpedance sensing device for • Monitoring the clinically important physiological parameters during critical care (multi-channel signal acquisition & processing) • Ambulatory recording for early diagnosis of cardiovascular disorders and for post-operative care (recording in the presence of motion artifacts) • Monitoring of physiological parameters for use in sports medicine (recording in the presence of external interference, strong respiratory and motion artifacts) • Low-cost diagnosis (low distortion & high sensitivity)

8. 2. HARDWARE & SOFTWARE • Design Approach • Digital synchronous demodulation for noise and interference rejection • Circuit for increasing the modulation index of the waveform to increase the sensitivity and dynamic range Basic Blocks • Microcontroller “Microchip PIC24FJ64GB04” • Impedance converter chip “Analog Devices AD5933” • V/I convertor and amplitude control • Voltage sensing amplifier and baseline correction • PC-based GUI with isolated serial communication for setting parameters and data acquisition

9. ImpedanceConverterAD5933 Features • Excitation voltage generator & digital synch. demodulator • Programmable voltage with a settable frequency up to 100 kHz • Impedance measurement range from 1 kΩto 10 MΩ • Internal system clock • DC rejection, error averaging, phase measurement • Accuracy: ± 0.5%. • I2C interface with a data rate of 100 kHz Adaptations needed for bioimpedance sensing • Measurement using current excitation • Time-varying measurement • Dynamic range extension and sensitivity selection

10. Functional block diagram of AD5933

11. Design using the impedance converter chip • with on-chip sinusoidal source & DFT for synchronous digital demodulation

12. ImpedanceConverterCircuit

13. Digital pot. AD8400 (U3, U7) used for controlling the amplitudes of the excitation current & baseline correction voltage • Resistance: 1 K, 8-bit resolution • Wiper position control: SPI interface • Supply: 2.6 – 5.5 V

14. V/IConverter Voltage generated from AD5933 converted to a current using OTA-based V/I converter OTA “TI OPA861” • gm: 90 mA/V (quiescent current = 4.7 mA) • +5 V single supply operation • Bandwidth: 80 MHz

15. V/IConverterwithBalancedCurrent Outputs • Complimentarybalancedcurrentoutputs, usingtwoOTAs • Quiescent current set by R1and R3 • Output current controlled by changingR2 or quiescent current

16. VoltageSensing Amplifier • Instr. amp. INA155 for amplifying the sensed voltage • Bandwidth: 5.5 MHz • Gain: 10 – 50 • Slew rate: 6.5 V/µs • Supply: 2.6 – 5.5 V • High-pass filter cut-off: 16 kHz

17. where where K = ),β, and αare potentiometer ratios from U3 and U7 respectively • Baseline Correction • Subtractinga sinusoidal reference voltage from the sensed voltage • Amplitude and polarity of the correction voltage digitally controlled by varying digital pot (U7) ratio between 0.25 to 0.75 • Baseline correction output tracked by microcontroller using ADC. • Potentiometer ratio controlled digitally via SPI interface

18. Demodulation • Sensed signal sampled at 1.04 MHz • 1024-point DFT calculated at the excitation frequency with real & imaginary parts • Complex impedance values transferred to microcontroller over an I2C & then to PC over UART or USB

19. Microcontroller • 44-pin PIC24F64GB004 • Supply : 3.0 –3.6 V • 16 MHz clock • 64 KB program memory • 8 KB RAM • Single channel 10-bit ADC • UART module • USB module • SPI module • I2C module

20. Power Supply • Separate analog & digital 3.3 V & 5 V • Analog reference of 1.6 V generated by MCP6021 • LDO MCP1802 used as voltage regulator IC • Input to the LDO from a DC-DC converter LM2622 • Input to the DC-DC converter: 3.6-5.5 V • Li-ion charge control IC MCP73833 used for battery charging. • Total current consumption: ~60 mA • Low battery indication • Provision for powering through USB

21. Power Supply Circuit

22. AssemblyTwo-layer PCB (102 mm x 64 mm) with SMD components

23. Signal Acquisition Interface PC based signal acquisition using RS232

24. 3. Test Results • Exc.: 65.5 kHz, 0.9 mA • Lin. range: up to 400 Ω A) Voltage sensing amplifier: output linearity B) Interference Significant only over a bandwidth of 3 kHz

25. C) Automatic Sensitivity Adjustment Voltage sensing amplifier output vs test resistances for excitation current of 0.6 − 1.5 mA, set by varying β

26. D) Validation using thoracic impedance simulator Excitation: 0.6 mA, 65.56 kHz Simulator settings: R = 49 Ω, ∆R = 0.5Ω , f = 1 Hz Sampling freq.: 200 Hz Excitation: 0.6 mA, 65.56 kHz Simulator settings: R = 20 Ω, ∆R = 0.8 Ω, f = 0.1 Hz Sampling freq.: 10 Hz

27. Excitation: 0.6 mA, 65.56 kHz Simulator settings: R = 30 Ω, ∆R = 0.8 Ω, f = 0.1 Hz Sampling freq. : 200 Hz Excitation: 0.6 mA, 65.56 kHz Simulator settings : R = 19 Ω, ∆R = 0.5 Ω , f = 5 Hz Sampling freq.: 200 Hz

28. 4. Summary Developed A bioimpedance sensor using an impedance converter chip using digital synchronous demodulation Further work • Median filtering for further carrier ripple rejection without smearing transitions • Adaptation for specific applications • Integration with the signal processing software • Field testing

29. References • [1] R. P. Patterson, "Fundamentals of impedance cardiography," IEEE Eng. Med. Biol. Mag., vol. 8, no. 1, pp. 35-38, 1989. • [2] L. E. Baker, "Applications of impedance technique to the respiratory system," IEEE Eng. Med. Biol. Mag., vol. 8, no. 1, pp. 50–52, 1989. • [3] L. E. Baker, "Principles of impedance technique," IEEE Eng. Med. Biol. Mag., vol. 8, no. 1, pp. 11–15, 1989. • [4] H. H. Woltjer, H. J. Bogaard, and P. M. J. M. de Vries, “The technique of impedance cardiography,” Euro. Heart J., vol. 18, no. 9, pp. 1396–1403, 1997. • [5] M. D. Desai, “Development of an impedance cardiograph,” M. Tech. dissertation, Biomedical Engineering,, IIT Bombay, 2012. • [6] H. Sahu: “Sensing of impedance cardiogram using synchronous demodulation,” M. Tech. dissertation, Biomedical Engineering, IIT Bombay, June 2013.

30. Thank You