VLSI and Embedded Systems Conference, 5-9 Jan 2014, IIT Bombay, India (VLSIDES14)Session: B-2 Embedded Platform, Venue: VMCC-21, Session Time: 4:30 pm to 6:30 pm

An Embedded System Design for a

Synchronous Demodulation Based

Noninvasive Bioimpedance Sensor

P. C. PandeyP. C. Pandey

IIT Bombay

07/Jan/2014

<pcpandey@ee.iitb.ac.in> http://www.ee.iitb.ac.in/~pcpandey

VLSI and Embedded Systems Conference, 5-9 Jan 2014, IIT Bombay, India (VLSIDES14)

Session: B-2 Embedded Platform, Venue: VMCC-21, Session Time: 4:30 pm to 6:30 pm

P. C. Pandey: An embedded system design for a synchronous demodulation based noninvasive bioimpedance sensor (invited talk)

Abstract: A long-duration 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 for sensing peripheral blood circulation, glottography (for sensing movement of vocal chords), etc. These

instruments pass an alternating current of high frequency and low amplitude through a pair of appropriately placed pair of electrodes,

an amplifier to sense the resulting amplitude modulated voltage across the same or another pair of appropriately placed electrodes, a

demodulator to detect the impedance signal, and signal processing for obtaining the desired parameters. An embedded system design

approach is used to develop a body-worn device to be used for monitoring the clinically important physiological 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. It senses the basal value and time-varying

component of the impedance waveform, with settable excitation frequency and with very low noise and demodulation relatedcomponent of the impedance waveform, with settable excitation frequency and with very low noise and demodulation related

distortions. A microcontroller and an impedance converter chip are used for stable sinusoidal source with programmable frequency

control and a digital synchronous demodulation. 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 have been 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.

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

Outline

1. Introduction

2. Design Approach

3. Hardware & Software

4. Test & Results

5. Summary

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5. Summary

Reference

Hitendra Sahu: “Sensing of impedance cardiogram using

synchronous demodulation”, M. Tech. dissertation, Biomedical

Engineering, Indian Institute of Technology Bombay, June 2013.

Noninvasive Monitoring of Bioimpedance

oLow-cost diagnostic tool

oMonitoring device

Some Applications

o Impedance cardiography: sensing of variation in thoracic

impedance to estimate cardiac output & some other

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impedance to estimate cardiac output & some other

hemodynamic parameters

oPneumography: sensing of respiratory parameters

oPlethysmography: sensing of peripheral blood circulation

oGlottography: sensing movement of vocal chords during

speech production

Instrumentation for Bioimpedance Sensing

o Passing an alternating current of high frequency and low

amplitude through a pair of appropriately placed pair of

electrodes

o Amplifier to sense the resulting amplitude modulated voltage

across the same or another pair of appropriately placed

electrodes

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electrodes

o Demodulator to detect the impedance signal

o Signal processing for obtaining the desired parameters

ICG blocks

• AC excitation current

• Voltage sensing amp.

• Demodulator

• Baseline correction

Example:

Impedance

Cardiograph

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• Baseline correction

• ECG extractor

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.

Objective

To develop a body-worn bioimpedance sensing device for

• Monitoring the clinically important physiological parameters

during critical care (multi-channel signal acquistion &

processing)

• Ambulatory recording for early diagnosis of cardiovascular

disorders and for post-operative care (recording in the presence

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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)

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”

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• Impedance converter chip “Analog Devices AD5933”

• V-to-I convertor and amplitude control

• Voltage sensing amplifier and baseline correction

• PC-based GUI with isolated serial communication for setting

parameters and data acquisition

Impedance converter AD5933

Features

• Excitation voltage generator & digital synch. demodulator

• Programmable voltage with a settable frequency up to 100 kHz

• Impedance measurement range from 1 kΩ to 10 MΩ

• Internal system clock

• DC rejection, error averaging, phase measurement

• Accuracy: ± 0.5%.

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• 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

Functional block diagram of AD5933P.

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Design using the impedance converter chip

with on-chip sinusoidal source & DFT for synchronous digital

demodulation

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Impedance converter circuitP.

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• Digital pot. AD8400 (U3, U7) used for controlling the amplitudes the

excitation current and baseline correction voltage.

• Total resistance 1 K with 8 bit resolution.

• Wiper position changed via SPI interface.

• Supply range : 2.6 – 5.5 V.

V-to-I converterP.

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V – I converter with balanced current outputsP.

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Voltage sensing amplifier

• Instr. amp. INA155

for amplifying the

sensed voltage

• BW: 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

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cut-off : 16 kHz

Baseline correction

• Subtracting a 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

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• Baseline correction output

tracked by microcontroller using

ADC.

• Potentiometer ratio is controlled

digitally via SPI interface

DemodulationP.

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Microcontroller

• 44-pin

PIC24F64GB004

used

• Supply range : 3.0

– 3.6 V

• 16 MHz clock

• 64 KB program

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• 64 KB program

memory,

8 KB RAM

• Single channel 10

bit ADCs

• UART module

• USB module

• SPI module

• I2C module

Power supply features

• Separate analog & digital supplies of 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.

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• Li-ion charge control IC MCP73833 used for battery charging.

• Total current consumption ~60 mA.

• Low battery indication.

• Provision for powering through USB.

Power supply cktP.

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

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Signal acquisition interface

LabWindows CVI software for signal acquisition using RS232

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Test & Results

• Exc.: 65.5

kHz, 0.9

A) Voltage

sensing

amplifier:

output

linearity

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kHz, 0.9

mA

• Lin.

range: up

to 400 J

B) Interference

Significant only over a b.w. of 3 kHz

C) Automatic Sensitivity AdjustmentVoltage sensing amplifier output vs test resistances

for excitation current of 0.6 − 1.5 mA, set by varying β

1200

1400

1600

1800

2000

Voltage o

utp

ut (m

V)

B=0.08

B=0.04

B=0.12

B=0.16

B=0.20

B=0.24

B=0.28

b=0.32

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0

200

400

600

800

1000

10 20 30 40 50 60 70 80 90 100 110 120

Voltage o

utp

ut (m

V)

Test resistance (Ω)

b=0.32

B=0.36

B=0.40

B=0.44

B=0.48

B=0.51

B=0.55

B=0.60

B=0.63

B=0.67

B=0.70

D) Validation using thoracic impedance simulator

Excitation:

0.6 mA, 65.56 kHz

Simulator settings:

R = 20 Ω, ∆R = 0.8 Ω,

f = 0.1 Hz

Sampling freq.: 10 Hz 0 5 10 15 20 25 30 35 40

20.6

20.7

20.8

20.9

21

21.1

21.2

21.3

21.4

21.5

Impedance (ohm)

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Excitation:

0.6 mA, 65.56 kHz

Simulator settings:

R = 49 Ω, ∆R = 0.5Ω ,

f = 1 Hz

Sampling freq.: 200 Hz

0 5 10 15 20 25 30 35 40

Time (s)Figure 5.3 Impedance Vs Time

Excitation:

0.6 mA, 65.56 kHz

Simulator settings:

R = 30 Ω, ∆R = 0.8 Ω,

f = 0.1 Hz

Sampling freq. : 200 Hz

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Excitation:

0.6 mA, 65.56 kHz

Simulator settings :

R = 19 Ω, ∆R = 0.5 Ω ,

f = 5 Hz

Sampling freq.: 200 Hz

Summary

Developed

A bioimpedance sensor using an impedance converter chip

using digital synchronous demodulation

Further work

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• Median filtering for further carrier ripple rejection without

smearing transitions

• Adaptation for for specific applications

• Integration with the signal processing software

• Field testing

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

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[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.

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