ISSCC 2010 / SESSION 27 / DIRECTIONS IN HEALTH,...
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490 • 2010 IEEE International Solid-State Circuits Conference ISSCC 2010 / SESSION 27 / DIRECTIONS IN HEALTH, ENERGY & RF / 27.3 27.3 A 3.9mW 25-Electrode Reconfigured Thoracic Impedance/ECG SoC with Body-Channel Transponder Long Yan, Joonsung Bae, Seulki Lee, Binhee Kim, Taehwan Roh, Kiseok Song, Hoi-Jun Yoo KAIST, Daejeon, Korea Recently, wearable heart monitoring systems have been developed for cardio- vascular-related disease [1] with wearable body sensor network (WBSN) [2-3]. The WBSN introduced in [3] monitored ECG at maximum 48 points, and trans- ferred data using arrayed inductive link for cm-range wireless inter-connectivity. However, most of the previous attempts were limited to sense only ECG signals at limited points [2] on the body with limited network coverage [3]. Thoracic impedance variance (TIV) from the change of aortic blood volume and velocity at each cardiac cycle provides important hemodynamic information (stroke vol- ume, cardiac output). Combined with ECG signals from more than 6 points, it enables the early detection of abnormal symptoms of pandemic diseases like hypertension and heart failure so that the patients can take prophylactic meas- ures [6]. In spite of its importance, the TIV detection was not realized in WBSN due to its requirement of high impedance (<0.2Ω) detection sensitivity which needs to detect AM signal with modulation depth as low as less than 3%. A pure single tone sinusoidal current signal at 1kHz-100kHz [6] is required to realize such a high sensitivity, and only a bulky implementation was reported so far [7]. In this paper, we report a 3.9mW low power SoC with body-channel-transceiver (BCT), which can detect TIV (0.1Ω) and ECG (up to 8 points) concurrently. The chip is integrated on a 4-layer fabric circuit board with thin flexible battery as a poultice-like plaster. In addition, it can reconfigure the 25-electrode array and optimize them in-situ to automatically consider the user dependency of the TIV/ECG signals. The recorded data is transmitted at 1Mbps through body-chan- nel-communication (BCC) [8] with duty cycle modification to extend battery life time and enlarge the network coverage. Figure 27.3.1 shows the proposed wearable TIV/ECG monitoring system. The 15cm x 15cm 4-layer patch fabricated by P-FCB [3] consists of Layer-1, 25-elec- trode array for reconfigurable TIV/ECG sensing, Layer-2, fabric inductor (2.2μH, Q=9.6) for system start-up, Layer-3, thin flexible battery (1.5V, 30mAh), and Layer-4, fabric circuit board on which the SoC is directly wire bonded. Layer-1 has 1) 16 voltage sensing electrodes (each 1.8cm 2 ) divided into 4 sections, 2) 2 anodes and 2 cathodes electrodes (each 3.2cm 2 ) for current injection and 3) 4 ground electrodes (each 1.8cm 2 ) and 1 reference electrode (1.8cm 2 ). A user puts the patch on the chest to monitor the TIV/ECG signals, and he/she can start and stop the system by inductively coupled power switch with ID verification func- tion. BCC is used to upload recorded data to a central base station when on chip storage is full, and download system command when new configuration is required. Figure 27.3.2 shows the overall block diagram of the proposed SoC architecture. It consists of 1) a system start-up module for battery turn on and initial BCC fre- quency allocation, 2) reconfigurable 4 x 4-electrode sensor front ends (RE-FE) for voltage sensing and digitization, 3) a differential sinusoidal current generator (DSCG) for TIV measurement, 4) a digital module containing FSM controller with special purpose registers (SPR), a 20kB data storage, a FIR filter, an 8:1 com- pression block [4], and a packet encoder/decoder, and 5) a duty-cycled BCT for external communication. The system starts with 8b ID check which is ASK modulated at 13.56MHz through fabric inductor coupling [3]. A CMOS rectifier generates power-on-reset (POR) trigger signal to demodulate incoming ID packet, and an ID checker (8b- XOR) authenticates the ID with on-chip preprogrammed 8bit code for the flexi- ble battery to turn on the SoC. The start-up module downloads the BCC frequen- cy allocation packet at 0.8kb/s through the same link. ECG is measured (Mode 0) through 8 electrodes (impedance less than 120kΩ in 0.4Hz-1.1kHz) in section 0 and 2 in time multiplexing while TIV is monitored (Mode 1) through 8 elec- trodes (impedance of 150Ω at 90kHz) in section 1 and section 3. In order to find the optimum sensing points automatically, it scans 8 different electrodes in 2 sections for 8s using the same injection electrodes. Then the current injection electrodes are changed from section 0-2 (Anode0-Cathode0) to section 1-3 (Anode1-Cathode1) with mode swapping between Mode 0 and Mode 1 as shown in Fig. 27.3.3. The BCT is activated every 4s to listen from base station for 0.1s, and it updates channel assessment with a new system command from the base station. When the 20kB storage is full, the stored data is transmitted through BCT to the base station, and then system resumes the sensing operation. To eliminate the mismatch between 2 pairs of current injection electrodes, a tetra-polar electrode configuration [6] is adopted for TIV measurement. 2 fully differential amplifiers and RC bridge are used to generate a 90kHz (determined by RC time constant) differential sinusoidal voltage as shown in Fig. 27.3.4. The 2 nd harmonic is reduced by the differential signaling, and variable voltage swing (V sw ) is achieved by adjusting the gate voltage of M 1,2 which is controlled by 2b DAC. The V sw is converted to differential current (V sw /R I ) in the range of 100- 350μA p-p to accommodate the differences of the individual users. The measured spectrum of DSCG shows that the 2 nd harmonic is suppressed down to -58dBc, and it equals to 0.81% THD at 250uA p-p . The reconfigurable electrode instrumentation amplifier (REIA) of Fig. 27.3.5, shared by ECG and TIV, is proposed to reconfigure electrodes as shown in Fig. 27.3.3. The 4 negative inputs of REIA share 1 reference electrode for Mode=0 while it connects to 4 distinct electrodes for Mode=1. The current balanced IA [5] keeps V ds of M 3,4 constant to obtain 96dB CMRR. To amplify both ECG and TIV signal with the same circuit, a dual band operation is realized by adjusting HPF and controlling the load capacitor at R 2 in Fig. 27.3.5. 28.5dB gain in 0.4Hz- 1.1kHz and 27.8dB gain in 20kHz-280kHz are realized for ECG and TIV, respec- tively. The 90kHz modulated TIV is additionally amplified by 20dB before down conversion by recovered in-phase carriers. As shown in Fig. 27.3.6, a 0.1Ω of TIV is measured at sensitivity of 3.17V/Ω. The FSK modulated BCT of Fig. 27.3.2 operates in 20MHz-40MHz with 8b capac- itor bank LC digitally controlled oscillator (LC-DCO). Its initial communication frequency is allocated at system start-up. The BCT wakes up within 1μs, and it is 10 3 faster than [8]. On the receiver side, an extra 20dB gain stage is cascad- ed to LNA to enhance noise figure by 3dB for robust operation in the unstable receiver FSK spectrum. The BCT shows -75dBm sensitivity with BER 10 -8 at 1Mbps which is 10dB enhancement compared to [8] while consuming only 3.2mW. The 5mm x 5mm chip is fabricated by 0.18μm 1P6M CMOS process. Figure 27.3.7 shows the chip micrograph and its performance summary. It dissipates only 3.9mW peak power when BCT operates at 1Mbps with sensitivity of -75dBm, and consumes 2.4mW static power when it operates in TIV detection mode with sensitivity of 3.17V/Ω. The user-friendly reconfigurable TIV/ECG monitoring patch enables wearable hemodynamic monitoring for possible healthcare applications. References: [1] Heart Disease and Stroke Statistics 2008 Update: A Report from the American Heart Association Statistics Committee and Stroke Statistics Subcommittee. [2] A. C-W. Wong, et al., “A 1V, Micropower System-on-Chip for Vital-Sign Monitoring in Wireless Body Sensor Networks,” ISSCC Dig. Tech. Papers, pp. 138-139, Feb., 2008. [3] J. Yoo, et al., “A 5.2mW Self-Configured Wearable Body Sensor Network Controller and a 12μW 54.9% Efficiency Wirelessly Powered Sensor for Continuous Health Monitoring System,” ISSCC Dig. Tech. Papers, pp.290-291, Feb., 2009. [4] H. Kim, et al., “A Low Cost Quadratic Level ECG Compression Algorithm and Its Hardware Optimization for Body Sensor Network System,” Proc. IEEE EMBC, pp. 5490-5493, Aug., 2008. [5] R.Yazicioglu, et al., “A 60μW 60nV√Hz readout Front-End for Portable Biopotential Acquisition Systems,” ISSCC Dig. Tech. Papers, pp. 56-57, Feb., 2006. [6] A. Nowakowski, et al., “Advances in electrical impedance methods in med- ical diagnostics,” Bulletin of the Polish Academy of Sciences, Tech. Papers, Vol.53, No.3, pp.231-243, 2005. [7] M-C. Cho, et al., “A Bio-Impedance Measurement System for Portable Monitoring of Heart Rate and Pulse Wave Velocity Using Small Body Area,” Proc. IEEE ISCAS, pp. 3106-3109, May, 2009. [8] N. Cho, et al, “A 60kb/s-to-10Mb/s 0.37nJ/b Adaptive-Frequency-Hopping Transceiver for Body-Area Network,” ISSCC Dig. Tech. Papers, pp. 132-133, Feb., 2008. 978-1-4244-6034-2/10/$26.00 ©2010 IEEE
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490 • 2010 IEEE International Solid-State Circuits Conference
ISSCC 2010 / SESSION 27 / DIRECTIONS IN HEALTH, ENERGY & RF / 27.3
27.3 A 3.9mW 25-Electrode Reconfigured Thoracic Impedance/ECG SoC with Body-Channel Transponder
Long Yan, Joonsung Bae, Seulki Lee, Binhee Kim, Taehwan Roh, Kiseok Song, Hoi-Jun Yoo
KAIST, Daejeon, Korea
Recently, wearable heart monitoring systems have been developed for cardio-vascular-related disease [1] with wearable body sensor network (WBSN) [2-3].The WBSN introduced in [3] monitored ECG at maximum 48 points, and trans-ferred data using arrayed inductive link for cm-range wireless inter-connectivity.However, most of the previous attempts were limited to sense only ECG signalsat limited points [2] on the body with limited network coverage [3]. Thoracicimpedance variance (TIV) from the change of aortic blood volume and velocityat each cardiac cycle provides important hemodynamic information (stroke vol-ume, cardiac output). Combined with ECG signals from more than 6 points, itenables the early detection of abnormal symptoms of pandemic diseases likehypertension and heart failure so that the patients can take prophylactic meas-ures [6]. In spite of its importance, the TIV detection was not realized in WBSNdue to its requirement of high impedance (<0.2Ω) detection sensitivity whichneeds to detect AM signal with modulation depth as low as less than 3%. A puresingle tone sinusoidal current signal at 1kHz-100kHz [6] is required to realizesuch a high sensitivity, and only a bulky implementation was reported so far [7].In this paper, we report a 3.9mW low power SoC with body-channel-transceiver(BCT), which can detect TIV (0.1Ω) and ECG (up to 8 points) concurrently. Thechip is integrated on a 4-layer fabric circuit board with thin flexible battery as apoultice-like plaster. In addition, it can reconfigure the 25-electrode array andoptimize them in-situ to automatically consider the user dependency of theTIV/ECG signals. The recorded data is transmitted at 1Mbps through body-chan-nel-communication (BCC) [8] with duty cycle modification to extend battery lifetime and enlarge the network coverage.
Figure 27.3.1 shows the proposed wearable TIV/ECG monitoring system. The15cm x 15cm 4-layer patch fabricated by P-FCB [3] consists of Layer-1, 25-elec-trode array for reconfigurable TIV/ECG sensing, Layer-2, fabric inductor (2.2µH,Q=9.6) for system start-up, Layer-3, thin flexible battery (1.5V, 30mAh), andLayer-4, fabric circuit board on which the SoC is directly wire bonded. Layer-1has 1) 16 voltage sensing electrodes (each 1.8cm2) divided into 4 sections, 2) 2anodes and 2 cathodes electrodes (each 3.2cm2) for current injection and 3) 4ground electrodes (each 1.8cm2) and 1 reference electrode (1.8cm2). A user putsthe patch on the chest to monitor the TIV/ECG signals, and he/she can start andstop the system by inductively coupled power switch with ID verification func-tion. BCC is used to upload recorded data to a central base station when on chipstorage is full, and download system command when new configuration isrequired.
Figure 27.3.2 shows the overall block diagram of the proposed SoC architecture.It consists of 1) a system start-up module for battery turn on and initial BCC fre-quency allocation, 2) reconfigurable 4 x 4-electrode sensor front ends (RE-FE)for voltage sensing and digitization, 3) a differential sinusoidal current generator(DSCG) for TIV measurement, 4) a digital module containing FSM controller withspecial purpose registers (SPR), a 20kB data storage, a FIR filter, an 8:1 com-pression block [4], and a packet encoder/decoder, and 5) a duty-cycled BCT forexternal communication.
The system starts with 8b ID check which is ASK modulated at 13.56MHzthrough fabric inductor coupling [3]. A CMOS rectifier generates power-on-reset(POR) trigger signal to demodulate incoming ID packet, and an ID checker (8b-XOR) authenticates the ID with on-chip preprogrammed 8bit code for the flexi-ble battery to turn on the SoC. The start-up module downloads the BCC frequen-cy allocation packet at 0.8kb/s through the same link. ECG is measured (Mode0) through 8 electrodes (impedance less than 120kΩ in 0.4Hz-1.1kHz) in section0 and 2 in time multiplexing while TIV is monitored (Mode 1) through 8 elec-trodes (impedance of 150Ω at 90kHz) in section 1 and section 3. In order to findthe optimum sensing points automatically, it scans 8 different electrodes in 2sections for 8s using the same injection electrodes. Then the current injectionelectrodes are changed from section 0-2 (Anode0-Cathode0) to section 1-3(Anode1-Cathode1) with mode swapping between Mode 0 and Mode 1 as shownin Fig. 27.3.3. The BCT is activated every 4s to listen from base station for 0.1s,
and it updates channel assessment with a new system command from the basestation. When the 20kB storage is full, the stored data is transmitted throughBCT to the base station, and then system resumes the sensing operation.
To eliminate the mismatch between 2 pairs of current injection electrodes, atetra-polar electrode configuration [6] is adopted for TIV measurement. 2 fullydifferential amplifiers and RC bridge are used to generate a 90kHz (determinedby RC time constant) differential sinusoidal voltage as shown in Fig. 27.3.4. The2nd harmonic is reduced by the differential signaling, and variable voltage swing(Vsw) is achieved by adjusting the gate voltage of M1,2 which is controlled by 2bDAC. The Vsw is converted to differential current (Vsw/RI) in the range of 100-350µAp-p to accommodate the differences of the individual users. The measuredspectrum of DSCG shows that the 2nd harmonic is suppressed down to -58dBc,and it equals to 0.81% THD at 250uAp-p.
The reconfigurable electrode instrumentation amplifier (REIA) of Fig. 27.3.5,shared by ECG and TIV, is proposed to reconfigure electrodes as shown in Fig.27.3.3. The 4 negative inputs of REIA share 1 reference electrode for Mode=0while it connects to 4 distinct electrodes for Mode=1. The current balanced IA[5] keeps Vds of M3,4 constant to obtain 96dB CMRR. To amplify both ECG andTIV signal with the same circuit, a dual band operation is realized by adjustingHPF and controlling the load capacitor at R2 in Fig. 27.3.5. 28.5dB gain in 0.4Hz-1.1kHz and 27.8dB gain in 20kHz-280kHz are realized for ECG and TIV, respec-tively. The 90kHz modulated TIV is additionally amplified by 20dB before downconversion by recovered in-phase carriers. As shown in Fig. 27.3.6, a 0.1Ω ofTIV is measured at sensitivity of 3.17V/Ω.
The FSK modulated BCT of Fig. 27.3.2 operates in 20MHz-40MHz with 8b capac-itor bank LC digitally controlled oscillator (LC-DCO). Its initial communicationfrequency is allocated at system start-up. The BCT wakes up within 1µs, and itis 103 faster than [8]. On the receiver side, an extra 20dB gain stage is cascad-ed to LNA to enhance noise figure by 3dB for robust operation in the unstablereceiver FSK spectrum. The BCT shows -75dBm sensitivity with BER 10-8 at1Mbps which is 10dB enhancement compared to [8] while consuming only3.2mW.
The 5mm x 5mm chip is fabricated by 0.18µm 1P6M CMOS process. Figure27.3.7 shows the chip micrograph and its performance summary. It dissipatesonly 3.9mW peak power when BCT operates at 1Mbps with sensitivity of -75dBm, and consumes 2.4mW static power when it operates in TIV detectionmode with sensitivity of 3.17V/Ω. The user-friendly reconfigurable TIV/ECGmonitoring patch enables wearable hemodynamic monitoring for possiblehealthcare applications.
References:[1] Heart Disease and Stroke Statistics 2008 Update: A Report from theAmerican Heart Association Statistics Committee and Stroke StatisticsSubcommittee.[2] A. C-W. Wong, et al., “A 1V, Micropower System-on-Chip for Vital-SignMonitoring in Wireless Body Sensor Networks,” ISSCC Dig. Tech. Papers, pp.138-139, Feb., 2008.[3] J. Yoo, et al., “A 5.2mW Self-Configured Wearable Body Sensor NetworkController and a 12µW 54.9% Efficiency Wirelessly Powered Sensor forContinuous Health Monitoring System,” ISSCC Dig. Tech. Papers, pp.290-291,Feb., 2009.[4] H. Kim, et al., “A Low Cost Quadratic Level ECG Compression Algorithm andIts Hardware Optimization for Body Sensor Network System,” Proc. IEEE EMBC,pp. 5490-5493, Aug., 2008.[5] R.Yazicioglu, et al., “A 60µW 60nV√Hz readout Front-End for PortableBiopotential Acquisition Systems,” ISSCC Dig. Tech. Papers, pp. 56-57, Feb.,2006.[6] A. Nowakowski, et al., “Advances in electrical impedance methods in med-ical diagnostics,” Bulletin of the Polish Academy of Sciences, Tech. Papers,Vol.53, No.3, pp.231-243, 2005.[7] M-C. Cho, et al., “A Bio-Impedance Measurement System for PortableMonitoring of Heart Rate and Pulse Wave Velocity Using Small Body Area,” Proc.IEEE ISCAS, pp. 3106-3109, May, 2009.[8] N. Cho, et al, “A 60kb/s-to-10Mb/s 0.37nJ/b Adaptive-Frequency-HoppingTransceiver for Body-Area Network,” ISSCC Dig. Tech. Papers, pp. 132-133,Feb., 2008.
978-1-4244-6034-2/10/$26.00 ©2010 IEEE
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491DIGEST OF TECHNICAL PAPERS •
ISSCC 2010 / February 10, 2010 / 2:30 PM
Figure 27.3.1: Proposed wearable thoracic impedance variance (TIV)/ECGmonitoring system.
Figure 27.3.2: Overall block diagram of thoracic impedance variance(TIV)/ECG SoC architecture.
Figure 27.3.3: Reconfigurable operation in thoracic impedance variance(TIV)/ECG sensing.
Figure 27.3.5: Reconfigurable electrode instrumentation amplifier (REIA). Figure 27.3.6: Measured thoracic impedance variance (TIV) with ECG.
Figure 27.3.4: Differential sinusoidal current generator (DSCG).
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• 2010 IEEE International Solid-State Circuits Conference 978-1-4244-6034-2/10/$26.00 ©2010 IEEE
ISSCC 2010 PAPER CONTINUATIONS
Figure 27.3.7: Chip micrograph and its performance summary.
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