Transcript VLSI and Embedded Systems Conference, 5-9 Jan 2014 - EE-IITB

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. Pandey
IIT Bombay
07/Jan/2014
<[email protected]> 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 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
<[email protected]> http://www.ee.iitb.ac.in/~pcpandey
Outline
1. Introduction
2. Design Approach
3. Hardware & Software
4. Test & Results
P. C. Pandey, EE Dept., IIT Bombay
5. Summary
Reference
Hitendra Sahu: “Sensing of impedance cardiogram using
synchronous demodulation”, M. Tech. dissertation, Biomedical
Engineering, Indian Institute of Technology Bombay, June 2013.
3/30
Noninvasive Monitoring of Bioimpedance
o Low-cost diagnostic tool
o Monitoring device
Some Applications
P. C. Pandey, EE Dept., IIT Bombay
o Impedance cardiography: sensing of variation in thoracic
impedance to estimate cardiac output & some other
hemodynamic parameters
o Pneumography: sensing of respiratory parameters
o Plethysmography: sensing of peripheral blood circulation
o Glottography: sensing movement of vocal chords during
speech production
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Instrumentation for Bioimpedance Sensing
o Passing an alternating current of high frequency and low
amplitude through a pair of appropriately placed pair of
electrodes
P. C. Pandey, EE Dept., IIT Bombay
o Amplifier to sense the resulting amplitude modulated voltage
across the same or another pair of appropriately placed
electrodes
o Demodulator to detect the impedance signal
o Signal processing for obtaining the desired parameters
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Example:
Impedance
Cardiograph
P. C. Pandey, EE Dept., IIT Bombay
ICG blocks
• AC excitation current
• Voltage sensing amp.
• Demodulator
• Baseline correction
• ECG extractor
Impedance detector
Differentiator
dZ/dt
Zo(t)
ECG extraction
circuit
ECG
Current source
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.
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Objective
To develop a body-worn bioimpedance sensing device for
• Monitoring the clinically important physiological parameters
during critical care (multi-channel signal acquistion &
processing)
P. C. Pandey, EE Dept., IIT Bombay
• 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)
7/30
Design Approach
P. C. Pandey, EE Dept., IIT Bombay
• 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-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
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Impedance converter AD5933
P. C. Pandey, EE Dept., IIT Bombay
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%.
• 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
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P. C. Pandey, EE Dept., IIT Bombay
Functional block diagram of AD5933
10/30
P. C. Pandey, EE Dept., IIT Bombay
Design using the impedance converter chip
with on-chip sinusoidal source & DFT for synchronous digital
demodulation
11/30
P. C. Pandey, EE Dept., IIT Bombay
Impedance converter circuit
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P. C. Pandey, EE Dept., IIT Bombay
• 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.
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P. C. Pandey, EE Dept., IIT Bombay
V-to-I converter
• Voltage generated from
AD5933 is converted to a
current
• Operational transconductance amplifier
(OTA) based V – I
converter used
• “TI OPA861” from used as
Vin
OTA which offers :
• gm is 90 mA/V at a
quiescent current of 4.7
mA
• +5 V single supply
VSS
operation
• Bandwidth 80 MHz
𝑽
• 𝒊𝒙 = 𝒊𝒏 𝟏
𝑹𝑬 +
OPA 861
VDD
RB
180
3
1
RIQ
180
Vo
7
-V +V C 8
RC
ix 180
B
I
4
E 2
RE
180
VSS
𝒈𝒎
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V – I converter with balanced current outputs
• Complimentary
balanced current
output generated by
using two OTA chips
P. C. Pandey, EE Dept., IIT Bombay
•Quiescent current set
byR1 and R3 connected
at base terminals
• Output current can is
controlled by
changing R2 or
quiescent current
• 𝑰𝒐𝒖𝒕 = 𝑽𝒊𝒏 −𝑽𝒓𝒆𝒇
𝑹𝟐 + 𝟐 𝒈𝒎
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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 E2
• High pass filter
cut-off : 16 kHz
P. C. Pandey, EE Dept., IIT Bombay
E1
R14 10k
R15
A5V 5k
C16 0.1µ
A1V6
R12
R10 1M
8
R8 10k C14 100p 100k 2 7
– 1 AGND
U6
+
6
5
3
R9 10k C15 100p
R13 4
R11 1M
100k
A1V6
A1V6
A1V6
V_IN1_BL
AGND
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Baseline correction
• Subtracting a sinusoidal
reference voltage from the
sensed voltage
P. C. Pandey, EE Dept., IIT Bombay
• 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 is controlled
digitally via SPI interface
𝒗𝟎 = 𝑨𝒔 𝑽𝒔 + 𝑽𝒃𝒄 (𝑨𝒃𝒄 + 𝜶 𝑨𝒄 )
where
𝑨𝒔 = − 𝑹𝟑𝟓 𝑹𝟑𝟒 , 𝑨𝒃𝒄 =
− 𝑹𝟑𝟓 𝑹𝟑𝟑 , 𝑨𝒄 = 𝟏 + 𝑹𝟑𝟓 (𝑹𝟑𝟑 ||𝑹𝟑𝟒 )
𝒗𝟎 = −𝑽𝒃𝒄 𝑲(𝒁 − 𝟐(𝟐𝜶 − 𝟏)/𝑲)
where
K= 𝜷𝑮/(𝑹𝑬 + 𝟐 𝒈𝒎 ),β, and α are
potentiometer ratios from U3 and
U7 respectively
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Demodulation
• Samples the sensed signal at 1.04 MHz.
• 1024-point DFT is calculated at the excitation frequency.
𝟏𝟎𝟐𝟑
𝑿 (𝒇) =
𝒙 𝒏
𝒇
−𝒋𝟐𝝅𝒏
𝒇
𝒔
𝒆𝒙𝒑
𝒏=𝟎
• The DFT algorithm returns a real (R) and imaginary (I) data values.
P. C. Pandey, EE Dept., IIT Bombay
• Data values are transferred to microcontroller over an I2C.
• Impedance values send to PC over UART or USB.
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P. C. Pandey, EE Dept., IIT Bombay
Microcontroller
• 44-pin
PIC24F64GB004
used
• Supply range : 3.0
– 3.6 V
• 16 MHz clock
• 64 KB program
memory,
8 KB RAM
• Single channel 10
bit ADCs
• UART module
• USB module
• SPI module
• I2C module
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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.
P. C. Pandey, EE Dept., IIT Bombay
• Li-ion charge control IC MCP73833 used for battery charging.
• Total current consumption ~60 mA.
• Low battery indication.
• Provision for powering through USB.
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P. C. Pandey, EE Dept., IIT Bombay
Power supply ckt
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Assembly
P. C. Pandey, EE Dept., IIT Bombay
Two-layer PCB (102 mm x 64 mm) with SMD components
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Signal acquisition interface
P. C. Pandey, EE Dept., IIT Bombay
LabWindows CVI software for signal acquisition using RS232
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Test & Results
P. C. Pandey, EE Dept., IIT Bombay
A) Voltage
sensing
amplifier:
output
linearity
• Exc.: 65.5
kHz, 0.9
mA
• Lin.
range: up
to 400 Ω
Plot between Voltage sensing amplifier output and test resistances
B) Interference
Significant only over a b.w. of 3 kHz
24/30
C) Automatic Sensitivity Adjustment
Voltage sensing amplifier output vs test resistances
for excitation current of 0.6 − 1.5 mA, set by varying β
2000
B=0.08
1800
B=0.04
B=0.12
1600
B=0.16
P. C. Pandey, EE Dept., IIT Bombay
Voltage output (mV)
1400
B=0.20
B=0.24
1200
B=0.28
b=0.32
1000
B=0.36
B=0.40
800
B=0.44
B=0.48
600
B=0.51
400
B=0.55
B=0.60
200
B=0.63
B=0.67
0
10
20
30
40
50
60
70
Test resistance (Ω)
80
90
100
110
120
B=0.70
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D) Validation using thoracic impedance simulator
21.5
21.4
21.3
Impedance (ohm)
Excitation:
0.6 mA, 65.56 kHz
Simulator settings:
R = 20 Ω, ∆R = 0.8 Ω,
f = 0.1 Hz
Sampling freq.: 10 Hz
21.2
21.1
21
20.9
20.8
20.7
20.6
0
5
10
15
20
25
P. C. Pandey, EE Dept., IIT Bombay
Figure 5.3 Impedance Vs Time
30
35
40
Time (s)
Excitation:
0.6 mA, 65.56 kHz
Simulator settings:
R = 49 Ω, ∆R = 0.5Ω ,
f = 1 Hz
Sampling freq.: 200 Hz
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P. C. Pandey, EE Dept., IIT Bombay
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
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Summary
Developed
A bioimpedance sensor using an impedance converter chip
using digital synchronous demodulation
P. C. Pandey, EE Dept., IIT Bombay
Further work
• Median filtering for further carrier ripple rejection without
smearing transitions
• Adaptation for for specific applications
• Integration with the signal processing software
• Field testing
28/30
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.
P. C. Pandey, EE Dept., IIT Bombay
[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.
29/30
P. C. Pandey, EE Dept., IIT Bombay
30/30