December 2014: Wearable Electronics

December 2014: Wearable Electronics
Wearable Electronic Devices Monitor
Vital Signs, Activity Level, and More
Health Monitoring Is Going Wearable
By Jan-Hein Broeders
When I was a little boy, my mom always made sure that I had
enough change to make a phone call in case of emergency.
Twenty years later, mobile phones allowed us to make calls at
any time and place. After another 20 years of innovation, the
phone is no longer the key feature of our smart devices, which
can take beautiful pictures, stream audio and video, provide
access to a wide variety of services—and are now becoming
our personal trainers. The devices, loaded with sensors or
connected to bodily worn sensors, monitor day-to-day activity
and personal health. An increasing awareness of our health has
fueled interest in measuring vital parameters—such as heart rate,
temperature, oxygen saturation, blood pressure, activity level,
and calories burned—and following their daily trends.
Now, a universal sensor front end with multiple sensors
can monitor these parameters. The biggest challenges are
to minimize size and maximize battery lifetime. This article
discusses solutions for the rapidly growing market for wearable electronics.
The Most Important Vital Sign
Without a heartbeat we would be in serious trouble, so pulse
or heart rate is by far the most important parameter to be monitored. In addition to the number of beats per minute, we want
to check the behavior of the heart as a function of activity. The
rhythm is also important, as rapidly changing heart rates are a
sign of cardiac disease.
Monitoring heart rate and heart activity is classically done
by measuring biopotential with an electrocardiogram (ECG).
Electrodes connected to the body measure signals caused by
electric activity in the cardiac tissue. This principle is used in
professional diagnostic systems, where up to 10 electrodes
HIGH PASS
can be connected to the chest and limbs. ECGs provide
detailed information regarding the various components
(P-, QRS-, and T-wave) of one heartbeat.
Single-lead ECGs are more common in the sports world, with
a two-electrode chest strap measuring heart activity. ECG
waveforms can be detected, but most systems just measure
heart rate. These straps are uncomfortable, so the sports and
wellness industry is looking for alternatives, such as integrating
the electrodes into a sport shirt. The AD8232 single-lead heart
rate monitor front end, shown in Figure 1, was developed for
this kind of low-power wearable application. It includes an
instrumentation amplifier with a gain of 100 V/V and a highpass filter to block the offset voltage generated by the half-cell
potential of the electrodes on the skin. An output buffer and
low-pass filter reject the high-frequency component generated
by muscle activity (EMG signals). This low-power front end,
which draws 170 μA, can be used with the ADuCM350 16-bit
meter-on-a-chip to perform high-performance, single-lead ECG
measurements.
New Method for Measuring Heart Rate
A new trend for measuring heart rate is the photoplethysmogram (PPG), an optical technique that retrieves cardiac
information without measuring the biopotential. PPG has
mainly been used to measure blood oxygen saturation (SpO2),
but can provide cardiac information without a biopotential
measurement. With PPG technology, heart-rate monitors can
be integrated in wearable devices such as wristwatches or
bracelets. This is not possible with biopotential systems due
to the tiny signal levels.
IA
OUT
REF
IN
HP SENSE
HP
DRIVE
+VS
GND
+VS
REF
BUFFER
FAST
RESTORE
SWITCH
10 𝛀
SENSE
ELECTRODES
+IN
10 𝛀
FR
REF
DIGITAL
INPUTS
HP
DRIVE HP
SENSE
AC/DC
IN AMP
G = +100
CM
–IN
SDN
AD8232
RLD FB
LO+
REF
OPTIONAL
DRIVEN LEAD
ELECTRODE
DIGITAL
OUTPUTS
RIGHT
LEG DRIVER
LO–
OP AMP
RLD
REF
FAST
RESTORE
SWITCH
SWITCH
OP
AMP+
REF
REF
OUT
OP
AMP–
OUT
IA
OUT
HIGH PASS
2-POLE LOW PASS AND GAIN
Figure 1. AD8232 single-lead ECG front end.
Analog Dialogue 48-12, December 2014
analog.com/analogdialogue
1
AVDD
DVDD
ADPD142RG/ADPD142RI
AGND
ANALOG BLOCK
VREF
1𝛍F
PHOTODIODE
TIME SLOT A
DATA
TIA
AMBIENT
LIGHT
REJECTION
AFE
GAIN
AFE CONFIGURATION,
TIME SLOT A
VBIAS
VLED
LED2
LED1
14-BIT
ADC
AFE: SIGNAL CONDITIONING
SDA
TIME SLOT B
DATA
SCL
INT
DGND
AFE CONFIGURATION,
TIME SLOT B
DIGITAL
INTERFACE
AND
CONTROL
LED1/NC
LED2/NC
LED1 DRIVER
LED1 LEVEL AND TIMING CONTROL
LED2 DRIVER
LED2 LEVEL AND TIMING CONTROL
LGND
Figure 2. ADPD142 optical module.
In optical systems, light is transmitted through the surface of
the skin. Light absorbed by the red blood cells is measured
with a photosensor. As the heart beats, the changing blood
volume scatters the amount of light received. When measured
on a finger or earlobe, where considerable arterial blood is
available, a red or infrared light source provides the best accuracy. Arteries are rarely located on top of the wrist, however,
so with wrist worn devices, pulsatile components must be
detected from veins and capillaries just under the surface of
the skin, making green light better.
The ADPD142 optical module, shown in Figure 2, features a
complete photometric front end, with integrated photosensor,
current sources, and LEDs. Designed for reflective measurement, it can be used to implement a PPG measurement. All
components are mounted in a small module.
Challenges with Optical VSM
The main challenges for measuring PPG on a wrist-worn
device are due to ambient light and motion-generated artifacts. The Sun, which generates dc errors, is relatively easy
to cancel out, but light from fluorescent and energy-saving
lamps carry frequency components that cause ac errors. The
analog front end uses two structures to reject interferers from
dc to 100 kHz. After the analog signal conditioning, a 14-bit,
successive-approximation analog-to-digital converter (ADC)
digitizes the signal, which is transmitted via an I2C interface
to a microcontroller for final post processing.
A synchronized transmit path is integrated in parallel with
the optical receiver. Its independent current sources can drive
two separate LEDs with current levels programmable up to
250 mA. The LED currents are pulsed, with pulse lengths in
the microsecond range, so the average power dissipation is
kept low to maximize battery lifetime.
The LED driving circuit is dynamic and configurable on-thefly, making it independent of environmental conditions such
as ambient light, the tint of the wearer’s skin and hair, or
sweat between the sensor and the skin that would otherwise
2
decrease sensitivity. The excitation LEDs can be configured
easily to build an autoadaptive system. All timing and
synchronization is handled by the analog front end, so no
overhead is required from the system processor.
Two versions of the ADPD142 are available: the ADPD142RG
integrates red and green LEDs to support optical heart-rate
monitoring; and the ADPD142RI integrates red and infrared
LEDs for oxygen saturation (SpO2) measurement.
Influence of Motion
Motion also disturbs optical systems. When optical heart-rate
monitors are used for sleep studies, this may not be an issue,
but sport watches and bracelets worn during exercise have
a hard time cancelling out motion artifacts. Motion between
the optical sensors (LED and photodetector) and the skin will
decrease the sensitivity of the optical signal. In addition, the
frequency components of the motion might be seen as a heartrate measurement, so the motion must be measured and
compensated. The tighter the device is attached to the body,
the lower the impact, but it is nearly impossible to cancel this
out mechanically.
Various methods are used to measure motion. One is optical,
using multiple LED wavelengths. The common signals indicate motion, while the differential signals detect the heart
rate. It’s better to use a real motion sensor, however. Not only
will this allow accurate measurement of motion applied to the
wearable device, but it can also be used for additional features,
such as tracking activity, counting steps, or starting an application when a certain g-force is detected.
The ADXL362 micropower, 3-axis MEMS (microelectromechanical system) accelerometer is ideal for sensing motion
in battery-operated wearable applications. Its 12-bit ADC
converts acceleration into a digital signal with 1-mg resolution.
The power consumption dynamically scales with the sampling
rate, and is only 1.8 μA with a 100-Hz output data rate and
3.0 μA at 400 Hz. These higher data rates are useful for user
interface, such as tap/double-tap detection.
Analog Dialogue 48-12, December 2014
For starting an application when motion is detected, highspeed sampling is not needed, so that the data rate can be
reduced to 6 Hz, resulting in an average power consumption
of 300 nA. This makes this sensor attractive for low-power
applications and implantable devices, in which batteries
cannot be replaced easily. The ADXL362 is available in a
3.0-mm × 3.25-mm package. Figure 3 shows the supply
current vs. output data rate for several supply voltages.
generator powers analog sensors with ac or dc signals. The
high-performance receive signal chain conditions sensor
signals and converts them to digital with a true 16-bit,
160-kSPS ADC, which features ±1-LSB max INL and DNL,
and no missing codes. It can be used with any type of input
signal, including voltage, current, potentiostat, photocurrent,
and complex impedance.
The AFE can be operated in standalone mode without involvement of the Cortex-M3 processor. A programmable sequencer
controls the measurement engine, with results stored into
memory via DMA. Before starting a measurement, a calibration routine can be performed to correct offset and drift
errors in the transmit-and-receive signal chains. For complex
impedance measurements such as blood glucose, body mass
index (BMI), or tissue discrimination applications, a built-in
DSP accelerator provides a 2048-point, single-frequency discrete Fourier transform (DFT) without involvement of the M3
processor. These high-performance AFE features make the
ADuCM350 unique vs. other integrated solutions.
CURRENT CONSUMPTION (𝛍A)
6
VS = 1.6
VS = 2.0
VS = 2.5
VS = 3.0
VS = 3.5
5
4
3
2
1
0
0
100
200
300
OUTPUT DATA RATE (Hz)
The Cortex processor supports various communication ports
including I2S, USB, MIPI, and an LCD display driver (static). In
addition, it includes flash memory, SRAM, and EEPROM, and
supports five different power modes to maximize battery life.
400
Figure 3. ADXL362 supply current as function of the output
data rate.
Designed for use with ultralow-power sensors, the ADuCM350
is limited to low-speed devices. Applications that require
more processing power could use M3 cores that operate up
to 80 MHz or Cortex-M4 processor cores.
Connecting the Sensors in the System
The heart of the system that connects all these sensors, runs
the required software, and stores, displays, or transmits
the results is the ADuCM350 mixed-signal meter-on-a-chip,
which integrates a high-performance analog front end (AFE)
with a 16-MHz ARM® Cortex®-M3 processor core, as shown in
Figure 4. The flexibility of the AFE and rich feature set of the
microprocessor make this chip ideal for portable and wearable applications. The configurable AFE allows it to be used
with nearly any sensor, and its programmable waveform
PLL
What About Power?
Power is always a critical factor in portable and wearable
devices. The devices described in this article are designed
for high performance, small size, and low power, but integrating everything, including the battery, in a small package
is still a challenge. Despite new battery technologies that
bring more capacity per mm3, the battery is still large compared to the electronics.
SW/JTAG
1 × 256kB
1 × 128kB
LF XTAL
HF XTAL
ARM
CORTEX-M3
FLASH
HF OSC
LF OSC
NVIC
16kB
EEPROM
TRACE
DMA
AFE
• 16-BIT PRECISION ADC
• PRECISION REFERENCE
• SWITCH MATRIX
• 12-BIT DAC
• IN-AMP CONTROL LOOP
• TIA
SIGNAL
GENERATION
AFE
CONTROLLER
DFT
RECEIVE
FILTERS
USB PHY
AMBA
BUS
MATRIX
SRAM0
(16kB)
SRAM1
(16kB)
POR
USB
PSM
PDI
LP LDO
CapTouch
HP LDO
SPIH
UART
SPI0
SPI1
I2C
AHB-APB
BRIDGE
ABP-0
I2S
LCD
TMR0
GPIO
CRC
TMR1
PMU
BEEP
RTC
MISC
ABP-1
TMR2
WDT
Figure 4. Cortex-M3 with integrated AFE.
Analog Dialogue 48-12, December 2014
3
BACK_UP_M1
BACK_UP
ADP5090
SYS
RSYS
BACK_UP_M2
CSYS
SYS SWITCH
PHOTOVOLTAIC
CELL
SW
–
SDB
–
CIN
VIN
BAT
+
BAT SWITCH
BSTO
HS
+
BAT
COLD-START
CHARGE PUMP
PG
REF
SYS
MPPT
MPPT
CONTROLLER
TERM_REF
EN_BST
CBP
BOOST
CONTROLLER
SETSD
SDB
BATTERY
BOOST
CONTROL
PGOOD
MINOP
SETPG
SYS
DIS_SW
CLK
PG
TERM_REF
TRM
BIAS REFERENCE
AND OSCILLATOR
VREF
TERM
CONTROL
TERM
2R
R
PGND
BAT
AGND
Figure 5. ADP5090 energy harvester.
Energy harvesting can reduce battery size and extend battery
life. Various technologies are used to harvest energy, including
thermoelectric, piezoelectric, electromagnetic, and photovoltaic, with light and heat being the most appropriate for
wearable devices. The sensors usually don’t provide a lot of
output power, so every joule generated should be caught and
used. The ADP5090 ultralow-power boost regulator, shown in
Figure 5, bridges the gap between harvester and battery. This
efficient switch-mode power supply boosts input voltages
from as low as 100 mV up to 3 V. During a cold start, with
the battery completely discharged, a 380-mV minimum input
voltage is required, but in normal operation, where either the
battery is not completely drawn down or some energy is left
in the super capacitor, any input signal down to 100 mV can
be converted to a higher potential and stored for later use.
Bioimpedance Circuit Design
Challenges for Body-Worn Systems
By José Carlos Conchell
Introduction
Wearable devices for vital-sign monitoring (VSM) are
transforming the healthcare industry, allowing us to monitor our vital signs and activity anytime, anywhere. The
most relevant information about some of these key parameters can be obtained by measuring body impedance.
To be effective, wearable devices must be small, low cost,
and low power. In addition, measuring bioimpedance
entails challenges related to the use of dry electrodes and
safety requirements. This article provides some solutions
to these issues.
4
Housed in a tiny 3-mm × 3-mm package, the chip is programmable for use with various harvester sensors. It draws 250-nA
maximum quiescent current and works with almost any battery technology from Li-Ion to thin-film batteries and super
capacitors. Integrated protection circuits ensure safe operation.
Conclusion
This article describes some low-power products for wearable
and personal health applications, but this fast growing market
is changing rapidly. ADI technology can convert challenging
problems into complete products and turnkey solutions.
Watch for more to come.
References
www.analog.com/healthcare
Electrode Half-Cell Potential
The electrode is an electrical transducer that makes contact between an electronic circuit and a nonmetallic object
such as human skin. This interaction produces a voltage, known as the half-cell potential, which reduces the
dynamic range of the ADC. The half-cell potential varies
with the electrode material, as shown in Table 1.
Table 1. Half-Cell Potentials for Common Materials
Metal and Reaction
Al→Al +3e
3+
Ni→Ni +2e
2+
Half-Cell Potential (V)
–1.706
–
–0.230
–
Ag+Cl →AgCl+e
+0.223
–
Ag→Ag +e
+0.799
Au→Au +e
+1.680
+
–
+
+
–
Analog Dialogue 48-12, December 2014
Electrode Polarization
IEC 60601
When no current flows through the electrode, the halfcell potential is observed. The measured voltage increases
when dc current flows. This overvoltage impedes current
flow, polarizing the electrode, and diminishing its performance, especially under conditions of motion. For most
biomedical measurements, nonpolarizable (wet) electrodes are preferred over polarizable (dry) electrodes,
but portable and consumer devices typically use dry
electrodes due to their low cost and reusability.
IEC 60601 is a series of technical standards for the safety
and effectiveness of medical electrical equipment published by the International Electrotechnical Commission.
It specifies 10 µA maximum dc-leakage current through
the body under normal conditions and 50 µA maximum
under worst-case, single-fault conditions. The maximum
ac-leakage current depends on the excitation frequency.
If the frequency (fE) is less than or equal to 1 kHz, the maximum allowed current is 10 µA rms. If the frequency is
greater than 1 kHz, the maximum allowed current is
fE × 10 μA rms. These patient current limits are
1000 Hz
important circuit design parameters.
Electrode-Skin Impedance
Figure 1 shows an equivalent circuit of the electrode. 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, and Ehc is the half-cell potential.
Circuit Design Solution
The impedance measurement requires a voltage/current
source and a current/voltage meter, so DACs and ADCs
are commonly used. A precision voltage reference and
voltage/current control loops are essential, and a microcontroller is typically required to process data and obtain the
real and imaginary parts of the impedance. Additionally,
wearable devices are typically powered by a unipolar battery.
Finally, integration of as many components as possible
in a single package is very beneficial. The ADuCM350
ultralow-power, integrated, mixed-signal meter-on-a-chip
includes a Cortex-M3 processor and a hardware accelerator that can perform a single-frequency discrete Fourier
transform (DFT), making it a powerful solution for wearable devices.
Rd
Rs
Cd
Figure 1. Equivalent circuit model for biopotential electrode.
The electrode-skin impedance is important when designing
the analog front end due to the high impedances involved.
Dominated by the series combination of Rs and Rd at low
frequencies, the impedance decreases to Rd at high frequencies due to the capacitor’s effect. Table 2 shows typical
values for Rd, Cd, and the impedance at 1 kHz.
Table 2. Typical Electrode-Skin Impedance
Material
Rd
Cd
|Rd//Cd| @ 1 kHz
Wet Ag/AgCl 350 kΩ
25 nF
6 kΩ
Metal Plate
12 nF
13 kΩ
1.3 MΩ
Thin Film
550 MΩ
220 pF
724 kΩ
MEMS
650 kΩ
Negligible
650 kΩ
To meet IEC 60601 standards, the ADuCM350 is used with
the AD8226 instrumentation amplifier to make high-precision measurements using a 4-wire technique, as shown in
Figure 2. Capacitors CSIO1 and CISO2 block dc current flow
between the electrode and the user, eliminating the polarization effect. An ac signal generated by the ADuCM350 is
propagated into the body.
SWITCH
MATRIX
RCAL 1
IDAC
VBIAS
REF
AD8226
VBIAS
RCM1
IINAMP+
CISO3
RCM2
VBIAS
CISO4
AFE 7
CISO1
AFE 6
IAB
A
UNKNOWN Z
B
RACCESS4 RACCESS2
IINAMP+
AFE 8
RLIMIT
RACCESS1
RACCESS3
RG
RCAL 2
DAC_ATTEN_EN
EXCITATION
AMPLIFIER
DACP
LOOP
DACN 0.6V PGA
GAIN
1/0.025
AFE 4
RCF
50kHz
VREFDAC 1.8V
1.0V
12-BIT
DAC
0.2V
REFNHIZ 0.6V
DACCLK
P
AFE 5
×1.5
N
VBIAS
AFE 3
AFE 2
CISO2
D
CLOSED-LOOP
GAIN = 2
×1.5
T
MUX
Ehc
ADC 16-BITS, DFT
160kSPS
78SPS
AFE 1
ITIA
AN_A
×1
RTIA
Figure 2. Four-wire isolated measurement circuit using ADuCM350 and AD8226.
Analog Dialogue 48-12, December 2014
5
Capacitors CISO3 and CISO4 block the dc level from the ADC,
solving the half-cell potential problem and maintaining
maximum dynamic range at all times. CISO1, CISO2, CISO3, and
CISO4 isolate the user, ensuring zero dc current in normal
mode and in the first case of failure, and zero ac current in
the first case of failure. Finally, resistor RLIMIT is designed to
guarantee that the ac current in normal operation is below
the limit. RACCESS symbolizes the skin-electrode contact.
The ADuCM350 measures the current from the transimpedance amplifier (TIA) and the output voltage of the
AD8226 to calculate the unknown bodyimpedance. RCM1
and RCM2 must be as high as possible to ensure that most
of the current flows through the unknown impedance
and the TIA. The recommended value is 10 MΩ.
Design Limitations
This design presents some limitations when the eletrodeskin impedance is close to 10 MΩ at the excitation frequency. The electrode-skin impedance must be significantly
smaller than RCM1 and RCM2 (10 MΩ), or VINAMP+ will not be
equal to A and VINAMP– will not be equal to B, and the measurement accuracy will be degraded. The electrode-skin
impedance is typically much smaller than 1 MΩ when
the excitation frequency is greater than 1 kHz, as shown
in Table 2.
Validation
To prove the accuracy of this design, the system was
tested with different unknown impedances, with the
results compared to those measured by an Agilent 4294A
impedance analyzer. The magnitude error was less than
±1% in all the tests. The absolute phase error was less
than 1° at 500 Hz and 5 kHz. The 9° phase offset error
at 50 kHz could be corrected in software.
Conclusion
Designs for battery-powered, body-worn devices that
measure bioimpedance must consider low power, high
SNR, electrode polarization, and IEC 60601 safety requirements. A solution using the ADuCM350 and AD8226 was
described here. Additional details, including complete
design equations, can be found at www.analog.com/
library/analogdialogue/archives/48-12/bio_imp.pdf.
References
Neuman, Michael R. “Biopotential Electrodes.”The Biomedical Engineering Handbook, Fourth Edition. CRC Press, 2015
Chi, Mike Yu, Tzyy-Ping Jung, and Gert Cauwenberghs.
“Dry-Contact and Noncontact Biopotential Electrodes: Methodological Review.” IEEE Reviews in Biomedical Engineering,
Volume 3, 2010.
http://en.wikipedia.org/wiki/IEC_60601
Jan-Hein Broeders
Jan-Hein Broeders [[email protected]] is ADI’s healthcare
business development manager in Europe, the Middle East, and Africa.
Working closely with healthcare professionals, he translates their
present and future requirements into solutions. Jan-Hein has more
than 20 years of experience in the semiconductor industry. He joined
ADI as global FAE for Philips in 2005, and has been in his current role
since 2008. He holds a bachelor’s degree in electrical engineering
from the University of s-Hertogenbosch, the Netherlands.
José Carlos Conchell
José Carlos Conchell [ [email protected]] received his degree in
electronic engineering from Universidad de Valencia, Spain, in 2010, then
joined ADI as a system applications engineer in the Healthcare Group.
6
Analog Dialogue 48-12, December 2014
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