A High Sensitivity Bioimpedance Detector
B. B. Patil, P. C. Pandey, V. K. Pandey, and S. M. M. Naidu
Indian Institute of Technology Bombay, India
<[email protected]>
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%.
1. Introduction
Sensing of the variation in the bioimpedance across a
Impedance
Differentibody segment can be used for noninvasive monitoring
Detector
ator
of the changes in the fluid volume or the underlying
physiological events. The bioimpedance is generally
measured by passing an alternating current through a
Z(t) dZ/dt
Current
pair of surface electrodes. The resulting amplitude
source
ECG
modulated voltage, sensed using the same or another
pair of electrodes, is demodulated to get the impedance
Signal
signal. In order to avoid any physiological effects, the
Acquisition
ECG
measurement is carried out using a low-level current (<5
& Processing
Extraction
mA) in the frequency range of 20 kHz to 1 MHz [1] [3]. Higher the frequency used; easier it is to filter out
Fig. 1 Block diagram of instrumentation for impedance
the carrier ripple from the demodulated output. The
cardiograph
sensed impedance consists of basal impedance, a timevarying component related to the physiological
phenomenon of interest, and various physiological and
non-physiological artifacts.
Impedance cardiography [2] - [6] is a noninvasive method for estimating the stroke volume and some other
cardiovascular indices by sensing the variation in the thoracic impedance during the cardiac cycle. An instrument for
impedance cardiography generally consists of current injecting electrodes, voltage sensing electrodes, a current source,
an impedance detector, and an ECG extraction module, as shown in Fig. 1. A voltage controlled current source can be
used for injecting the low amplitude (< 5 mA) current in the thorax. The impedance detector circuit includes a difference
amplifier and a demodulator. The time varying component of interest in the impedance is generally less than 2 % of the
basal impedance, and therefore the impedance detector should have high sensitivity, external noise suppression, and
carrier ripple rejection. A peak detector circuit has low carrier ripple but it is prone to noise. Precision rectifier based
circuits [5] [7] give better noise rejection but at the cost of higher carrier ripple. Synchronous detection can be employed
for improving the noise rejection but it does not reduce the carrier ripple. Ripple in the output can be reduced by using a
lowpass filter, but it introduces phase distortion. For increasing the sensitivity of the detector for input voltage with very
low modulation index, Fourcin [8] introduced a slicing amplifier which amplifies the signal above a settable reference.
But amplitude demodulation using this circuit increases the carrier ripple in the demodulated output. A sample-and-hold
circuit for sampling the output of the slicing amplifier at the peaks of the sinusoidal excitation can be used for realizing a
detector with high sensitivity and very low ripple. Use of synchronous sampling also reduces external noise. Summation
of the signals obtained by sampling the positive and negative peaks can be used for further reduction of the external
noise.
Here we present a bioimpedance detector circuit developed as part of an instrument for impedance cardiography. It
uses a slicing amplifier for increasing the detection sensitivity for amplitude modulated signals with low modulation
index, and synchronous sample-and-hold for suppression of external noise. Further the carrier ripple is rejected without
lowpass filtering. The current level of excitation and the reference voltage for slicing amplifier can be digitally set for
selecting the sensitivity for sensing the impedance variation. The frequency for measurement can also be digitally
controlled. The circuit provides the variation in the bioimpedance as an analog signal which can be acquired along with
other biosignals by a multichannel signal acquisition system for further processing.
2. Bioimpedance Detector Circuit
A block diagram of the instrumentation for impedance cardiography is shown in Fig. 1. The high frequency current is
injected through the outer pair of electrodes and the resulting amplitude modulated voltage is sensed across the inner pair
of electrodes. The impedance detector consists of a difference amplifier and demodulator. The demodulation is carried
out by two channels of slicing amplifier with synchronous sample-and-hold, one channel for the positive peaks and the
other for the negative peaks. The outputs of the two channels are added together for giving the demodulated output.
The slicing amplifier is realized using the voltage clamp amplifier IC AD8037 with internal high speed analog
switches, as shown in Fig. 2. A dc reference voltage VX is obtained using digital potentiometer IC1. The lower clamp
level VL of the voltage clamp amplifier IC3 is set to
R7
(1)
VY 
VX
R7  R6
For VI  VY , the internal switching in the IC connects the input at terminal 3 as the non-inverting input, while for
VI  VY the input at terminal 5 is connected as the non-inverting input. Thus the output is given as
 R  R
VO1  VI 1  3   3 VX , for VI  VY
 R4  R4
 R  R
VY 1  3   3 VX , for VI  VY
 R4  R4
With R7 = R3 and R6 = R4, Eq. (2) can be written as
 R 
VO1  VI  VY  1  3  , for VI  VY
 R4 
(2)
for VI  VY
0,
(3)
VIN
S/H
VDD
VCC+
C1 0.1µ
8 5
VSS 4
1 GND VCC A
CS
3
W6
SDI
2
SCKL
B
IC1
7
MCP4150
R3
4.7k
VI
VCC+
2
7
R4
22
C3
0.1µ
–
6
IC2
3
+ C2 VX
4
0.1µ
VCC-
VCC+
C5 C6
VY
VCC+
C6
8
5
0.1µ 10µ
8 6
– VH 7
0.1µ
VO1 –
IC3
6
VL
3
IC4
+ 4
4
1
+
5
C7
R5
C4 C8
R6
3
22 100 VY
0.1µ 10µ
7
0.1µ
VCCR7
VCC4.7k
2
VO1
VO2
VO2
S/H
Time
Fig. 2 Demodulator using slicing amplifier and sample-and-hold circuit (IC1: MCP4150, IC2: LF356, and IC3: AD8037,
IC4: HA5351) and the output of the slicing amplifier.
Inverting
Amp.
Slicing
Amp.
V2
S/H Ckt
V6 +
V4
Ref.
Control
CH2
+
VO
VSH2
VR
Atten.
Vref
V3
E1
Diff. Amp.
V1
Slicing
Amp.
S/H Ckt
V5
E2
CH1
VΦ
Current source
Monostable
I1
V-to-I
Conv.
Program.
Source
Atten.
I2
Monostable
VS
Amp.
Control
VSH1
Φ Freq.
Control
Fig. 3 Bioimpedance detector with two channel slicing amplifier and sample-and-hold circuit
Thus the circuit works as a slicing amplifier with
a reference of VY and the gain of 1  R3 / R4 . The
figure also shows the waveforms at the input and the
output of the slicing amplifier. With the resistance
values as given in the figure, the circuit has a voltage
gain of around 210 and it gives satisfactory operation
up to 500 kHz. The output of the slicing amplifier is
sampled near the peak of the excitation current, using
the sample-and-hold IC4 (HA5351). The width of the
S/H pulse should be very narrow to minimize the
ripple and the hold edge should be closely aligned to
the peak of the slicing amplifier output.
The amplitude demodulation of the sensed voltage
is carried out by two channels of slicing amplifier with
synchronous sample-and-hold as shown in Fig. 3. In
the first channel, the positive peaks of the output of the
difference amplifier above the set reference are
amplified. In the second channel, the input signal is
inverted and thus the output corresponds to the
negative peaks. The sample-and-hold circuit in each
channel samples the output of the slicing amplifier at
the instant of the corresponding peak, giving a ripplefree output. The outputs of the two channels are added
together for suppression of noise and low frequency
drift in the input.
For obtaining the sampling pulses for the two
channels, a sinusoidal source with two outputs with a
Vs
V3
V4
VSH1
VSH2
VΦ
Time
Fig. 4 Waveforms in the bioimpedance
detector circuit of Fig. 3.
programmable phase shift is used [9]. It is realized using two direct digital synthesizer chips (AD 9834). The sinusoidal
output of the first synthesizer is used as the input to the voltage-to-current converter for current excitation. The amplitude
level is controlled by a digital attenuator. The square wave output of the second synthesizer is used for obtaining the
sampling pulses for the two channels, by using monostables triggered at the positive and the negative edges. The phase
shift between the outputs of the two synthesizers is set for aligning the holding edge of the sampling pulses to the peaks
of the sensed voltage. The two synthesizers use the same input clock and after loading frequency and phase commands,
are started at the same instants. The relationship between the various waveforms is shown in Fig. 4. A microcontroller is
used to control the two digital synthesizers for setting the phase shift between the outputs and the desired frequency. It
also controls the digital attenuator to set the excitation current level as well as the digital attenuator to set the appropriate
reference voltage for the slicing amplifiers. Thus the circuit provides digital control of frequency and current level of
excitation, and the demodulation parameters.
R1
VDD
C11 0.1µ
VCC+
9
8 5
VSS 4
GND VCC A
E 1
CS
C
3
A 2
–
10
W 6
SDI
R24
33K
SCKL
B
7
VCCVDD
C12 0.1µ
Ex
8 5
VSS 4
D 1 GND VCC A
CS
W 6
C 3
SDI
2
VDD
C10
SCKL
IC3
MCP4150
5
–
+
7 VSS
VSS
8.2k
31
9
RST EA
40
P2.7
VCC
P2.6
IC1
P2.5
AT89S52
P2.4
1
P3.6
P3.4
2
P3.7
P3.5
1
P1.0
2
P1.1
3
P1.2
4
P1.3
R
P1.4 5
10k
P0.1 38
20 GND
P0.0 39
VCC 2
VEE 3
VSS
1
7 Ex2
IC4B
VSS
LCD1 VSS
1
A
14 VCC
0.1µ
B
7
IC4C
R21
33K
6
R20
15K
R23
15K
EN
13
P3.3
C2
VSS
B
2
VDD
10µ
C14
28
DB7 14
27
DB6 13
26
DB5 12
11
25
DB4
6
14
E
4
15
RS
5 VDD
R/W
8
+
Rpot
5K
IC2
MCP4150
A
Vss
Ex1
C1
XTL1
18
Sync. Test
Outputs
A
B Digi. Pot.
C Control
D signals
E
R19
4.7 k
VDD
R22
4.7 k
SW2
XTL2
SW1
19
C3
33p VSS VSS
12MHz
33p
IC8
CD4066
0.1µ VSS
Microcontroller
VDD
R4
2.2K
R10
3.3K
VDD
R2
100
C13 0.1µ
8
VSS 4
GND VCC
1
B
CS
C 3 SDI
R3 A 2
SCKL
100
7
IC5
MCP4150
R14
33
5
R15
150
R6
220
I1
R7
220
E1
1
C9
S1
IC6
7805 3
In
1µ
9V
Out
Com
2
C4
C5
0.1µ
0.1µ
VSS
A
VCC+
W 6
B
JP5
R13
3.3K
JP6
R8
220
R9
220
E2
I2
C16
0.1µ
R5
2.2K
Simulator
R16
2.2K
IC7
1 7805 3
In Out
R11
4.7M
Com
2
R12
Fig.5 Thorax simulator for testing of the bioimpedance detector
4.7M
–
4
IC4
3
C6
0.1µ
Ep
2
1
+
11
IC4A
C7
LM324
0.1µ
Power supply
AGnd
C8
0.1µ
VCC-
3. Test Results
Synch
A bioimpedance simulator [10], [11] can be used for
(a)
measuring various performance parameters of an
Vo
impedance detector: the range of basal resistance,
sensitivity, and frequency response. For this purpose,
Synch
we have developed a thorax simulator which
(b)
provides a basal resistance (settable : 20   200 )
with a periodic resistance variation (settable: 0.1 
Vo
1.2 %) [9] [11]. As shown in Fig.5, terminals I1 and
I2 are the current injection terminals and the voltage
drop is sensed across the terminals E1 and E2. The
Synch
resistance variation is realized using the digital (c)
potentiometer IC5 (MCP4150-502E/P). The base
resistance values can be selected through jumpers by
Vo
connecting R14 and R15. Use of a microcontroller
and a digital potentiometer permits the selection of
Synch
the frequency (1  250 Hz) and waveshape (square, (d)
sinusoidal) of the variation in the resistance. The
Vo
resistance is varied in accordance with the selected
waveshape by providing the appropriate sequence of
digital control words to the digital potentiometer.
Fig. 6 Waveforms from measurements on the thorax
The microcontroller provides "Sync. Test" square
simulator. Upper trace: synchronization test output of
wave output in synchronism to the impedance
simulator, Lower trace: output of the impedance detector
variation cycle.
(excitation frequency = 100 kHz and current = approx. 1
The bioimpedance detector presented in the
mA) for f = 8 Hz, R = 196 . and. resistance variations of
previous section has been extensively tested using
(a) 1.2 % sinusoidal, (b) 0.6 % sinusoidal, (c) 1.2 %
the thorax simulator, for frequencies of the
square, (d) 0.6 % square. Time scale: 40 ms/div, Ch1
impedance variation ranging from 1 Hz to 250 Hz
scale: 5 V/div, Ch2 scale: 500 mV/div.
and impedance variations well below 2%. Outputs
for the excitation current of approximately 1 mA and
frequency of 100 kHz as captured using a DSO are shown in Fig. 6, for basal resistance of 196  and resistance variation
in the range of 0.1 to 1.2%. The figure shows the detector output along with the "Sync. Test" output of the thorax
simulator. We see that the output of the impedance detector tracks the variation in the resistance and the carrier ripple has
been rejected without lowpass filtering the output.
4. Conclusion
A bioimpedance detector is developed as part of instrumentation for impedance cardiography. It uses slicing amplifier for
detection of very low modulation index and synchronous sampling for ripple-free output. The circuit provides digital
control of frequency and current level of excitation and sensitivity for measurement of the variation in the bioimpedance.
The circuit operation has been verified using a thorax simulator for detecting variations well below 2%.and for the
impedance variation frequencies from 1 Hz to 250 Hz. As the circuit does not require lowpass filtering for rejection of
the carrier ripple, it faithfully detects the variation in the bioimpedance.
References
[1] J. Nyboer, Electrical Impedance Plethysmography. 2nd ed., Springfield, Massachusetts: Charles C. Thomas, 1970.
[2] L. E. Baker, "Principles of impedance technique", IEEE Eng. Med. Biol. Mag., vol. 8, pp. 11 - 15, 1989.
[3] M. Min, T. Parve, A. Ronk, P. Annus, and T. Paavle, "Synchronous sampling and demodulation in an instrument
for multifrequency bioimpedance measurement", IEEE Trans. Inst. Measurements, vol. 56, pp. 1365 - 1372, 2007.
[4] W. G. Kubicek, F. J. Kottke, and M. U. Ramos, "The Minnesota impedance cardiograph – theory and
applications", Biomed. Eng., vol. 9, pp. 410 - 416, 1974.
[5] 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. 1029 - 1036, 1986.
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[7] J. N. Sarvaiya, P. C. Pandey, and V. K. Pandey, “An impedance detector for glottography”, IETE J. Research, vol
55, no. 3, pp 100-105, 2009.
[8] A. J. Fourcin, "Apparatus for speech pattern derivation", U. S. Patent No. 4,139,732, Feb. 13, 1979.
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Institute of Technology Bombay, 2009.
[10] V. K. Pandey, P. C. Pandey, and J. N. Sarvaiya, "Impedance simulator for testing of instruments for bioimpedance
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of impedance cardiographs", in Proc. Int. Symp. Emerging Areas in Biotechnology & Bioengineering (ISEABB),
Mumbai, India, 2009, pp. 122-125.