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.

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