Bio-Impedance Circuit Design for Body Worn

Bio-Impedance Circuit Design for
Body Worn Systems
Jose Carlos Conchell
System Applications Engineer
Healthcare Technology
Analog Devices, S.L.
Campus UPV, Edificio 8F, 3rd floor
Camino de Vera s/n 46022 Valencia Spain
ADI speed dial: 6175-9504
Tel: +34 963329504
Fax: +34 963389761
E-mail: [email protected]
Website: www.analog.com/medical
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
TABLE OF CONTENTS
Revision History .............................................................................................................................................................2
1.
INTRODUCTION ................................................................................................................................................3
2.
CHALLENGES .....................................................................................................................................................3
3.
2.1.
Electrode Half-Cell Potential ....................................................................................................................3
2.2.
Electrode Polarization ..............................................................................................................................4
2.3.
Electrode-Skin Impedance ........................................................................................................................4
2.4.
IEC6060-1 .................................................................................................................................................5
SOLUTION .........................................................................................................................................................5
3.1.
Circuit Design ...........................................................................................................................................6
3.2.
Mathematical Study.................................................................................................................................8
3.3.
Design Limitations....................................................................................................................................9
4.
VALIDATION ......................................................................................................................................................9
5.
CONCLUSIONS .................................................................................................................................................10
Revision History
Date
Revision
Author
8/7/2014
Rev. V0.2
Jose Carlos Conchell
3/23/2015
Rev. V0.3
Jose Carlos Conchell
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
1. INTRODUCTION
The proliferation of body worn vital sign monitoring (VSM) sensing devices is transforming the health
care industry. This emerging applications allow us to monitor our vital signs and other key aspects
anytime, anywhere. The most relevant information about some of these key parameters can be obtained
by body-impedance measurements.
Body worn monitoring devices used in these systems have specific requirements such as minimal BOM
(Bill of Material) and low power to work on battery operated systems. The measurement of bioimpedance adds challenges to these type of systems, mostly related to the use of dry electrodes and
safety requirements.
This document describes challenges and solutions in the design of a low power body worn bioimpedance measurement system.
2. CHALLENGES
2.1. Electrode Half-Cell Potential
An electrode is an electrical transducer used to make contact between an electronic circuit and a
nonmetallic object such as the human body. The interaction between an electrode in contact with the
human skin produces a potential difference known as the half-cell potential and its value is varies
depending on the electrode material. Table 1 shows the values of this voltage for the most common
materials1.
The disadvantage of the half-cell potential is it does not provide information about the skin or body
under measurement and it reduces the dynamic range of the ADC.
Metal and Reaction
Al→Al3++3eNi→Ni2++2eAg+Cl-→AgCl+eAg→Ag++eAu→Au++e-
Half-cell Potential (V)
-1.706
-0.230
+0.223
+0.799
+1.680
Table 1Half-cell Potentials
1
Newman, M. R. “Biopotential Electrodes.”
The Biomedical Engineering Handbook: Second Edition.
Ed. Joseph D. Bronzino.
Boca Raton: CRC Press LLC, 2000
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
2.2. Electrode Polarization
When no DC electric current flows between an electrode and the body, the DC potential observed is
only the half-cell potential. If, however, there is a DC current, the half-cell potential will be altered, this
effect is commonly referred to as over-voltage. This over voltage generates the effect known as
polarization and can result in diminished electrode performance, especially under conditions of motion,
because it impedes the current flowing through the electrode into the body. Thus, for most biomedical
measurements, non-polarizable electrodes (wet electrodes) are preferred to those that are polarizable
(dry electrodes). However, most of the portable and consumer applications must use dry electrodes due
to its reusability and low cost.
2.3. Electrode-Skin Impedance
The electrode equivalent circuit is shown in Figure 1. Rd and Cd represent the impedance associated
with the electrode-skin interface and polarization at this interface. Rs is the series resistance associated
with the type of electrode materials. Ehc is the half-cell potential.
Figure 1 Equivalent circuit model for bio-potential electrode.
The electrode-skin impedance 2 is dominated by the series combination of Rs and Rd at low
frequencies. But, this impedance decreases at higher frequencies due to the capacitor’s effect. Table 2
shows the typical values of Rd and Cd for some typical materials and its magnitude impedance at 1 kHz.
The electrode-skin impedance is an important issue when designing the analog front end due to the
high impedances involved.
Material
Wet Ag/AgCl
Metal Plate
Thin Film
MEMS
2
Rd
350kΩ
1.3MΩ
550MΩ
650kΩ
Cd
25nF
12nF
220pF
Negligible
|Rd//Cd| @ 1kHz
6kΩ
13kΩ
724kΩ
650kΩ
Yu Mike Chi, Tzyy-Ping Jung, Gert Cauwenberghs, Dry-Contact and Noncontact Biopotential Electrodes:
Methodological Review. INSPEC Accession Number: 11674853
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
Table 2Electrode-Skin typical impedance
2.4. IEC6060-1
The IEC 60601 is a series of technical standards for the patient/wearer safety and effectiveness of
medical electrical equipment, published by the International Electrotechnical Commission3. This standard
specifies the limits of patient leakage currents and patient auxiliary currents under normal conditions and
single fault conditions4. These current limits are important parameters in the circuit design.
The maximum DC current allowed to be sourced in the body in normal conditions has to be less than
or equal to 10uA and the maximum DC current under single fault condition in the worst scenario is 50uA.
The maximum AC current allowed to be sourced in the body in normal conditions depends on the
frequency. If the excitation frequency is less than or equal to 1 kHz, the maximum allowed current is
10uARMS. If the excitation frequency (FE) is greater than 1 kHz, the maximum current is defined by the
Equation 1.
 =

· 10
1000 
Equation 1 Max AC current if FE>1 kHz.
3. SOLUTION
The impedance measurement always requires a voltage/current source and a current/voltage meter,
thus DACs and ADC are common blocks in these systems. Precision voltage reference and voltage/current
control loops are also essential blocks. A microcontroller is typically required to process data and obtain
the real and imaginary part of the impedance values. Additionally, body worn devices are typically
supplied by a unipolar battery. Finally, the BOM is critical, thus the integration of as many components as
possible in a single package is very beneficial.
The ADuCM350 represents a powerful solution in this application since it is an ultralow power,
integrated mixed-signal metering solution that includes a Cortex™-M3 processor and a hardware
accelerator which can perform a single frequency discrete Fourier transform (DFT).
3
http://en.wikipedia.org/wiki/IEC_60601
Single fault condition: Condition in which a single means for protection against hazards is defective or a single
external abnormal hazardous condition is present
4
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
To meet the IEC-60601 standards, the ADuCM350 is used with an external instrumentation amplifier
(AD8226), to complete high precision absolute measurements using a 4-wire measurement technique.
The circuit in Figure 2 solves the polarization effect because there is not DC current flowing between
the electrode and the user, the capacitors CSIO1 and CISO2 block it. The signal which is propagated into the
body is an AC signal generated by the ADuCM350.
The capacitors CISO3 and CISO4 are DC blocking capacitors which solve the problem with the half-cell
potential by blocking this DC level from the ADC, thus maintaining maximum dynamic range at all times.
CISO1, CISO2, CISO3 and CISO4 also ensure the DC current in normal mode and in first case of failure and AC
current in first case of failure are zero because the user is isolated. Finally, the resistor RLIMIT is designed
to guarantee the AC current in normal operation mode is below the limit.
RACCESS symbolizes the skin-electrode contact.
Figure 2 4-Wire isolated measurement circuit using ADuCM350 and AD8226.
3.1. Circuit Design
Equation 2, Equation 3 and Equation 4 describe the DAC’s current (IDAC), the unknown bodyimpedance’s current (IAB) and the TIA’s current (ITIA) relationship in the circuit shown in Figure 2.
The ADuCM350 measures the ITIA current and the output voltage of the AD8226 to calculate the
unknown body-impedance.
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
We want to maintain the largest value as possible for  to maximize the SNR measurement from
the ADuCM350 TIA. This requires that IINAMP- must be as small as possible according to Equation 3. A good
rule of thumb is to make IINAMP- ten times smaller than ITIA. To do this 4 +
least, ten times greater than 2 + 
1
2
is typically equal to 4 + 
1
4
1
4
+ 2 must be, at
+  . This rule can be simplified because 2 + 
1
2
, thus the final equation determines that RCM2 must be, at least, ten
times greater than RTIA.
 =  + +
Equation 2 DAC's current
 =  − −
Equation 3 Measured current by ADuCM350 TIA.
 > 10 · − → 2 > 10 · 
Equation 4 RCM2 must be, at least, 10 times greater than RTIA.
The instrumentation amplifier must measure the voltage in extremes of the unknown impedance, so,
ideally, the voltage in A should be equal to VINAMP+ and the voltage in B should be equal to VINAMP-.
Equation 4 describes VINAMP+, if RCM1 is 10 (or more) times greater than3 + 
1
3 
, the voltage
in A is practically equal to VINAMP+.
+ =
1
1
· ;  1 > 10 · (3 +
) => + = 
1
3 
3 +
+ 1
3 
Equation 5 RCM1 must be, at least, 10 times greater than RACCES3+1/CISO3.
− =
2
1
· ;  2 > 10 · (4 +
) => − = 
1
4 
4 +
+ 2
4 
Equation 6 RCM2 must be, at least, greater than RACCESS4+1/CISO4.
Hence, RCM1 and RCM2 must be as high as possible to ensure most of the current flows through the
unknown impedance and the TIA. Besides, the voltage measured by the instrumentation amplifier is
practically the voltage in extremes of the unknown impedance. The recommended value is 10MΩ
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
3.2. Mathematical Study
The circuit shown in Figure 2 is analyzed from the mathematical point of view in order to understand
its design. The explained assumptions in the previous section are applied in this analysis.
The measured current by the ADuCM350’s TIA (considering IINAMP+ and IINAMP- equal to zero) is:
 = −

1
 + 1 + 
1
 +  + 2 + 
· 350
1
2 
Equation 7 Current measurement carried out byADuCM350’s TIA
The voltage in VINAMP+ (considering the voltage in A = VINAMP+) is:
+ =
 + 2 + 
1
 + 1 + 
1
2 
1 
1
+  + 2 + 
· 350
2 
Equation 8 Voltage in the instrumentation amplifier’s positive input
The voltage in VINAMP- (considering the voltage in B = VINAMP-) is:
− =
2 + 
1
 + 1 + 
1
2 
1 
1
+  + 2 + 
· 350
2 
Equation 9 Voltage in the instrumentation amplifier’s negative input
The measured voltage by the instrumentation is:
 =  · (+ − − )
 =  ·
 + 1 + 
1

1
 +  + 2 + 
1
· 350
2 
Equation 10 Voltage measurement carried out by the instrumentation amplifier
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
The unknown impedance is obtained by the division of the Vvoltage and Vcurrent:

 · 
=−


→
 = −
1 
·
·
  
Equation 11 Body Impedance Measurement
| | =
 | |
·
 | |
Equation 12 Body Impedance Magnitude
ℎ( ) = 180 + (ℎ( ) − ℎ( ))
Equation 13 Body Impedance Phase
3.3. Design Limitations
This design presents some limitations when the electrode-skin impedance is close to 10MΩ at the
excitation frequency. If the electrode-skin impedance is not significantly smaller than RCM1 and RCM2
(10MΩ), VINAMP+ cannot be considered equal to A and VINAMP- cannot be considered equal to B (see Equation
5 and Equation 6). Thus, the measurement accuracy is degraded. However, the electro-skin impedance is
typically much smaller than 1MΩ when the excitation frequency is greater than 1 kHz (see Table 2).
4. VALIDATION
To prove the accuracy of this design, the system has been tested with different unknown impedances.
The values of the components in Figure 2 are listed below.
Component
RACCESS1, RACCESS2, RACCESS3 and RACCESS4
CISO1, CISO2, CISO3 and CISO4
RCM1 and RCM2
RG
RLIMIT
RTIA
Table 3 Component values
Value
220Ω
47nF
10MΩ
43kΩ
1kΩ
1.8kΩ
BIO-IMPEDANCE CIRCUIT
DESIGN FOR BODY WORN
SYSTEMS
JOSE CARLOS CONCHELL
August 7, 2014
The unknown impedances have been measured by an Agilent 4294A impedance analyzer. Results are
shown in Table 4. The magnitude error is less than ±1% in all the tests.
Absolute phase error is less than 1 degree at 500 Hz and 5 kHz. The circuit under test presents an
offset of 9 degrees in the phase measurements at 50 kHz which may be corrected by software
Test 1
Theoretical
Freq. (Hz)
value
500
5000
2kΩ
50000
Test 2
Theoretical
Freq. (Hz)
value
500
200Ω
5000
50000
Test 3
Theoretical
Freq. (Hz)
value
500
(2k+22nF)//
5000
1kΩ
50000
Test 4
Theoretical
Freq. (Hz)
value
500
(2k+44nF)//
5000
1kΩ
50000
RESULTS
Agilent 4294A
Body-Imp. Circuit
Mag. (Ω)
Phase (º)
Mag. (Ω)
1995.22
1995.60
1995.13
-0.002
-0.100
-0.400
1989.50
1989.37
1982.81
Agilent 4294A
-0.062
-0.900
-9.187
Body-Imp. Circuit
Mag. (Ω)
Phase (º)
Mag. (Ω)
201.09
201.04
201.10
-0.006
-0.008
-0.020
199.687
199.812
199.250
Agilent 4294A
Phase (º)
Phase (º)
-0.250
-0.875
-9.437
Body-Imp. Circuit
Mag. (Ω)
Phase (º)
Mag. (Ω)
Phase (º)
996.80
862.00
671.70
-01.77
-11.00
-03.20
993.500
861.625
666.937
-01.750
-11.875
-12.300
Agilent 4294A
Body-Imp. Circuit
Mag. (Ω)
Phase (º)
Mag. (Ω)
Phase (º)
991.531
773
668
-03.095
-11.000
-01.975
988.56
773.19
663.43
-03.000
-12.000
-11.000
Errors
Error (%) Abs. Error
Mag
Phase
-0.286
-0.060
-0.311
-0.800
-0.617
-8.787
Errors
Error (%) Abs. Error
Mag
Phase
-0.697
-0.244
-0.610
-0.867
-0.919
-9.417
Errors
Error (%)
Abs Error
Mag
Phase
0.025
-0.331
-0.875
-0.044
-9.100
-0.709
Errors
Error (%)
Abs Error
Mag
Phase
0.095
-0.299
0.024
-1.000
-0.683
-9.025
Table 4 Test results
5. CONCLUSIONS
Designing a battery operated body worn bio-impedance measurement solution must consider low
power, high SNR, electrode polarization and the IEC60601 safety requirements. The solution using the
ADuCM350 and AD8226 described in Figure 2 meets these requirements.