CN-0308: Powering an ECG Front End in Battery Powered Patient Monitoring

CN-0308: Powering an ECG Front End in Battery Powered Patient Monitoring
Circuit Note
CN-0308
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0308.
ADAS1000
Electrocardiogram (ECG) Front End with
Respiration and Pace Detection
ADP151
Ultralow Noise, 200 mA CMOS Linear
Regulator
Powering an ECG Front End in Battery Powered Patient Monitoring Applications
EVALUATION AND DESIGN SUPPORT
CIRCUIT DESCRIPTION
Circuit Evaluation Board
Evaluation board (EVAL-ADAS1000SDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The ADAS1000 five-electrode ECG analog front end (AFE)
addresses the challenges of next generation, low power, low
noise, high performance, tethered, and portable ECG systems.
CIRCUIT FUNCTION AND BENEFITS
This circuit is a highly integrated electrocardiogram (ECG) front
end for use in battery powered patient monitoring applications.
Figure 1 shows a top level diagram of the physical connections
in a typical 5-lead (four limb and one precordial chest lead) ECG
measurement system, including features such as respiration and
pace detection. The configuration is typical for portable telemetry
ECG measurements or a minimum lead set from a line powered
bedside instrument.
An ECG signal has a small amplitude of typically around 1 mV
when measured on the surface of the skin. Important information
about the health or other characteristics of the patient is stored
within that small signal; therefore, requiring measurement
sensitivity in the µV level. At the system level, various medical
standards call for a maximum of 30 µV p-p noise; however,
designers typically target less than this. As a result, when designing
a solution suitable for system level requirements, all noise sources
must be considered.
The noise performance of the ADAS1000 is specified across
various different operating conditions. The power supply must
be designed to ensure that it does not degrade the overall
performance. Selection of the ADP151 linear regulator was based
on its ultralow noise performance (9 µV rms typical, 10 Hz to
100 kHz), coupled with the power supply rejection of the
ADAS1000, ensures that the noise of the ADP151 does not
degrade the overall noise performance.
The ADAS1000 is a highly integrated chip consisting of five
electrode inputs and a dedicated right leg drive (RLD) output
reference electrode and is designed for both monitor and
diagnostic quality ECG measurements.
In addition to supporting the essential elements of monitoring
ECG signals, the ADAS1000 is equipped with functionality such
as respiration measurement (thoracic impedance measurement),
pace artifact detection, lead/electrode connection status, and
internal calibration features.
A single ADAS1000 supports five electrode inputs, easily enabling
a traditional 6-lead ECG measurement. Paralleling a second
ADAS1000 slave device allows scaling of the system to a true
12-lead measurement (made up of nine electrodes and one RLD),
while adding multiple more slave devices (three and above)
scales the system to a 15-lead measurement and beyond.
Respiration
The ADAS1000 has an integrated digital-to-analog converter
(DAC) for respiration drive at a programmable frequency of
46 kHz to 64 kHz, and an analog-to-digital converter (ADC)
that simplifies this difficult measurement. The measurement is
demodulated and converted to magnitude and phase from
which respiration can be determined, given the specific cable
parameters. The circuit has a resolution of 200 mΩ using the
internal capacitor, and higher resolutions (<200 mΩ) using an
external capacitor. The circuit has a flexible switching scheme
allowing measurement on one of three leads (I, II, or III).
Rev. 0
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CN-0308
Circuit Note
ADP151
VIN = 3.7V MINIMUM
1
VIN
2
GND
3
EN
VOUT = 3.3V*
VOUT
10µF
5
0.1µF
10µF
BATTERY
NC
4
ADP151
VOUT = 3.3V
1
VIN
VOUT
10µF
PATIENT
PROTECTION
5
0.1µF
ON
2
GND
3
EN
NC
10µF
4
OFF
RZ
CZ
2.2nF 100kΩ
RLD GAIN
SETTING
0.1µF
10µF
REFIN
RFB
4MΩ
RIN
40kΩ
RLD_OUT
REFOUT CAL_DAC_IO RLD_SJ
CM_IN
DRIVEN
LEAD
AMP
VREF
CALIBRATION
DAC
CM_OUT/WCT
SHIELD
DRIVE
AMP
VCM_REF
(1.3V)
ECG1_LA
SIX ELECTRODES
(FIVE ECG + ONE RLD)
LEAD I
RL
10kΩ
10kΩ
COMMONMODE AMP
ADCVDD, DVDD
1.8V
REGULATORS
ADCVDD
DVDD
VCM
AC
LEAD-OFF
DETECTION
VREF
PACE
DETECTION
DC LEADOFF/MUXES
ECG PATH
AMP
ADC
CS
V1
LEAD II
IOVDD
LA
LEAD III
ESIS FILTER
RA
AVDD
ADAS1000
RESPIRATION
DAC
AC
LEAD-OFF
DAC
SHIELD
ECG2_LL
AMP
SCLK
ADC
SDI
ECG3_RA
LL
AMP
FILTERS,
CONTROL,
AND
INTERFACE
LOGIC
ADC
ADC
AMP
ADC
DRDY
PD
RESET
SYNC_GANG
ECG4_V1
AMP
SDO
ECG5_V2
GPIO0/MCS
GPIO1/MSCLK
GPIO2/MSDO
GPIO3
EXT_RESP_LA
EXT_RESP_LL
AMP
EXT_RESP_RA
ADC
MUX
RESPDAC_LA
RESPIRATION PATH
RESPDAC_LL
CLOCK GEN/OSC/
EXTERNAL CLK
SOURCE
CLK_IO
RESPDAC_RA
REFGND
AGND
DGND
XTAL1
XTAL2
8.192MHz
NOTES
1. IN THIS CIRCUIT, THE AGND, DGND AND REFGND ARE ALL TIED INTO ONE GROUND PLANE. THE DIGITAL GROUND
AND DIGITAL CIRCUITRY CAN BE SPLIT INTO A SEPERATE GROUND, WITH CONNECTION MADE TO AGND AT THE ADAS1000 DEVICE.
THE CONFIGURATION SHOWN HERE USES RA, LA, LL, V1 AND RLD, LEAVING 1 SPARE ADC PATH THAT CAN BE USED FOR OTHER
MEASUREMENTS. ALTERNATIVELY, THIS CAN BE GANGED WITH A SLAVE ADAS1000 DEVICE TO ACHIEVE A NINE ECG + ONE RLD OR 12-LEAD MEASUREMENT.
11108-001
*IOVDD CAN BE OPERATED FROM 1.65V TO 3.6V. IN THIS CIRCUIT, 3.3V IS USED FOR INTERFACING PURPOSES.
Figure 1. Simplified Block Diagram Showing the ADAS1000 as Used in a Typical 4-Electrode + RLD or 5-Lead Configuration (All Connections and Decoupling Not Shown)
Rev. 0 | Page 2 of 6
Circuit Note
CN-0308
Pace Detection Algorithm
The pace detection algorithm runs three instances of a digital
algorithm on three of four possible leads (I, II, III, or aVF). It
runs on the high frequency ECG data in parallel with the internal
decimation and filtering. It has been designed to detect and
measure pacing artifacts of widths ranging from 100 µs to 2 ms
and amplitudes of 400 µV to 1000 mV. The ADAS1000 returns a
flag that indicates pace was detected on one or more of the leads, as
well as the measured height and width of the detected signal.
When users wish to run their own digital pace algorithm, the
ADAS1000 has a high speed pace interface providing the ECG data
at a fast data rate (128 kHz), with the filtered and decimated ECG
data remaining on the standard interface.
Low Power
The ADAS1000 is designed for low power and requires only
21 mW to operate five ECG electrode measurements. To further
minimize overall power dissipation in applications such as batteryoperated Holter and telemetry, any unused channels or features
can be easily disabled to further reduce power to as low as 11 mW
for one ECG lead.
Low Noise
Device noise performance can be optimized using its high
performance mode, where the sample rate of the on-board SAR
ADCs is increased to 2 MSPS, thereby giving a higher signal-tonoise ratio.
Flexible Data Rates
The standard serial interface outputs all the information related to
the ECG, including LEADS OFF status, pace, respiration, and
other auxiliary functions. The large number of 32-bit or 16-bit
data words, collectively known as a packet or frame, is output
on the serial SDO pin of the data bus. Different data frame rates
(2 kHz, 16 kHz, or 128 kHz) are available to ensure ultimate
ease in data capture. The slowest data rate of 2 kHz allows for
more decimation and is the optimum frame data rate for low
noise performance. It is also possible to read data in the skip
mode that reads the packet or frame from the device every
second or third word. The slowest data rate is 500 Hz.
A photograph of the ADAS1000 evaluation board connected to
the SDP board is shown in Figure 2.
The evaluation board has been designed to offer 1-lead to 12-lead
ECG measurement.
Low noise performance is critical for appropriate diagnosis of
different conditions. The ADAS1000 noise performance is required
to support end equipment regulatory standards. The ADAS1000
allows tradeoffs between noise performance, power, and data rate,
making it suitable for a wide variety of products. The ADAS1000
performance also excels in line-powered ECG systems where
power is not a major concern.
Rev. 0 | Page 3 of 6
CN-0308
Circuit Note
POWER
CONNECTOR,
5V (J9)
EXTRA LINKS/CIRCUIT RELATED
TO EXTERNAL RESPIRATION
MEASUREMENTS
OPTIONAL BENCH
SUPPLY FOR ADAS
CHIPS (J6)
OPTIONAL BENCH
SUPPLY (5V)
(J7)
ADAS1000
EVALUATION BOARD
5V WALL
ADAPTOR
EXTERNAL RESPIRATION
CONNECTOR (J8)
ON BOARD
DC-DC
REGULATORS
RESET FOR
ADAS CIRCUIT
ELECTRODE CONNECTOR
DB15 (J1)
SDP BOARD
PATIENT CABLE
ADAS1000 MASTER
ELECTRODES LA, RA, LL, V1, V2, RLD
RLD EXT
COMPONENTS
ADAS1000 SLAVE
ELECTRODES V3,
V4, V5, V6 SPARE
ADAS1000 SPI
INTERFACE (J4)
RESET BUTTON
FOR SDP BOARD
USB
CABLE
TO PC
USB
CONNECTOR
11108-002
SPACE FOR
CABLE/ESIS
MODELLING
Figure 2. ADAS1000 Evaluation Board/SDP Board (EVAL-SDP-CB1Z)
Batteries Used in Portable ECG Applications
Selecting the Right Power Solution
The types of batteries used in portable ECG equipment vary,
and, in some cases, AA or AAA batteries are found, allowing for
easy replacement or recharging.
The ADAS1000 requires a minimum of two power rails, AVDD
and IOVDD. As shown in Table 1, the ADCVDD and DVDD
rails are optional, and these supplies may be derived from the
AVDD or IOVDD rails, respectively, using the on-chip LDOs
within the ADAS1000.
Batteries contribute to the overall weight of the instrument.
Because patient comfort is important, minimizing overall
solution size and weight while maintaining battery life is a
prime concern in portable ECG applications.
Table 1. Power Supplies Needed by the ADAS1000
Newer products are likely to use battery chemistries, such as
lithium ion, and achieve operating time from a few hours to a
number of days, depending on the product.
The battery voltage range depends on the supply range of the
components within the system. An AVDD of 3.3 V is required
for the ADAS1000. Therefore, if the ADP151regulator is used,
the battery must supply at least a 3.7 V minimum, which
requires a 400 mV headroom. The nominal cell voltage of a
lithium ion or lithium polymer battery is 3.7 V; however, the
discharge voltage is about 3.2 V. Therefore, a stack of two is
required to guarantee a 3.7 V minimum for the ADP151.
Power Rail
AVDD
IOVDD
ADCVDD
(optional)
DVDD
(optional)
Voltage
Range
3.3 V ± 5%
1.65 V to 3.6 V
1.8 V ± 5%
1.8 V ± 5%
Function
Analog supply rail
Digital interface supply rail
ADC supply rail; can be derived
from AVDD using internal LDO
Digital supply rail; can be derived
from IOVDD using internal LDO
On the evaluation board, AVDD and IOVDD are powered with
3.3 V. The 3.3 V was chosen for the IOVDD rail to maintain
compatibility with the SPORT interface on the EVAL-SDP-CB1Z.
If interfaced to a microcontroller operating at a lower supply
voltage, the IOVDD supply voltage can be as low as 1.65 V.
Rev. 0 | Page 4 of 6
Circuit Note
CN-0308
CIRCUIT EVALUATION AND TEST
Alternatively, where a more power efficient solution is required,
the ADCVDD and DVDD internal rails can be disabled using
the hardware pin (VREG_EN) on the ADAS1000, which allows
the ADCVDD and DVDD rails to be driven by the external
supplies. Because the ADCVDD rail powers the ADCs on the
chip, it must be kept as clean as possible, and it must not be shared
with noisy digital supplies.
Equipment List
The following equipment is needed:
• The EVAL-ADAS1000SDZ kit, which includes the EVALADAS1000SDZ evaluation board, a 5 V wall wart supply,
and a CD with the ADAS1000 evaluation software
• The EVAL-SDP-CB1Z System Demonstration Board
• A PC with a USB port and the ADAS1000 evaluation
software installed
• A patient simulator or function generator can be used for
signal capture
Depending on operational mode, the supply current on the
AVDD rail for a single ADAS1000 is typically between 8 mA
and 15 mA with all five channels enabled; unused channels can
be disabled to lower power.
A dedicated ADP151 is used for both the AVDD and IOVDD
supplies on the evaluation board. Note that each ADP151 is
capable of driving 200 mA and can therefore supply other
components within the system. The input to the ADP151
regulators comes from a 5 V rail generated on the board for
other purposes.
A detailed discussions of the use of the ADAS1000 evaluation
board is available in the ADAS1000SDZ User Guide. Figure 3
shows a typical screen capture using the evaluation board software
with a patient simulator attached to the evaluation board.
A single ADP151 can supply both the AVDD and IOVDD rails
if appropriate filtering is added to ensure the AVDD rail is kept
free of any digital noise on the IOVDD rail.
The EVAL-ADAS1000SDZ evaluation board has been designed
to provide power to the EVAL-SDP-CB1Z board that requires 5 V
at approximately 250 mA. The ADP2503 buck boost dc-to-dc
converter generates the 5 V rail from an input supply to the board
of 4.5 V to 5.5 V.
11108-003
If this hardware is battery powered with the SDP board connected,
the overall power consumption quickly discharges the batteries.
COMMON VARIATIONS
Figure 3. Screen Capture of ADAS1000 with Patient Simulator Connected,
Heart Rate = 70 BPM
Other pin compatible ECG front ends in the ADAS1000 family offer
less functionality. For example, the ADAS1000-4 is a 3-channel
version with pace and respiration, the ADAS1000-3 offers three
ECG channels without pace or respiration. The ADAS1000-2 is
a companion device with five ECG channels that is intended for use
in a gang configuration to support a 12-lead ECG measurement
(nine ECG electrodes and one RLD). Table 2 shows the differences
between the members of the family. The range of products ensures
flexible configurations for expansion from small lead counts all
the way to 15-lead measurements and beyond.
For higher efficiency, a dc-to-dc converter can be used for the
supplies if care is taken with respect to layout and ripple noise.
Table 2. Overview of Features Available from the Different Members of the ADAS1000 Family
Generic
ADAS1000
ADAS1000-1
ADAS1000-2
ADAS1000-3
ADAS1000-4
1
ECG
Five ECG channels
Five ECG channels
Five ECG channels
Three ECG channels
Three ECG channels
Operation
Master/slave
Master/slave
Slave
Master/slave
Master/slave
Right Leg
Drive
Yes
Yes
Yes
Yes
Respiration
Yes
Pace Detection
Yes
Yes
Yes
Shield
Driver
Yes
Yes
Master
Interface 1
Yes
Yes
Yes
Yes
Yes
Yes
Package
Option
LFCSP, LQFP
LFCSP
LFCSP, LQFP
LFCSP, LQFP
LFCSP, LQFP
Master interface is provided for users wishing to use their own digital pace algorithm; see the Secondary Serial Interface section of ADAS1000 data sheet.
Rev. 0 | Page 5 of 6
CN-0308
Circuit Note
Noise Measurements
The evaluation board software was used to capture the peak-topeak noise performance of the ECG lead path using the ADAS1000
evaluation board. The results are shown in Figure 4.
The device was configured using the following conditions:
•
•
•
•
•
in personal injury or death. This board must not be used for
diagnostic purposes and must not be connected to a human being
or animal. It must not be used with a defibrillator or other
equipment that produces high voltages in excess of the supply rails
on the board.
This evaluation board is provided for evaluation and development
purposes only. It is not intended for use or as part of an end
product. Any use of the evaluation board or design in such
applications is at your own risk and you shall fully indemnify
Analog Devices, its subsidiaries, employees, directors, officers,
servants and agents for all liability and expenses arising from
such unauthorized usage. You are solely responsible for compliance
with all legal and regulatory requirements connected to such use.
A gain setting of 1.4
An ADC sample rate 2 MSPS (high performance mode)
A data rate of 2 kHz
In digital lead mode (digitally calculated leads)
ECG channels connected to internal test tone of 1.3 V
LEARN MORE
CN-0308-Design Support Package:
http://www.analog.com/CN0308-DesignSupport
MS-2160 Technical Article, Mitigation Strategies for ECG Design
Challenges, Analog Devices
MS-2125 Technical Article, Common-Mode Rejection: How It
Relates to ECG Subsystems and the Techniques Used to Provide
Superior Performance, Analog Devices
MS-2126 Technical Article, Multipysiological Parameter Patient
Monitoring, Analog Devices
Figure 4. Screen Capture of ADAS1000 Noise Measurements in Lead Mode
Using ADAS1000 Evaluation Board and Evaluation Software
The x-axis is time based, showing a capture over a number of
seconds, and the y-axis scale is in µV, showing noise performance
on the order of ±7 µV for these conditions. This was in line with
expectations of the ADAS1000 performance and aligned with
performance gathered on the same hardware using a low noise
linear bench-top supply. This confirmed that the ADP151 power
supply circuit on the evaluation board was not causing a significant
increase in the overall noise from the ADAS1000.
MS-2066 Technical Article, Low Noise Signal Conditioning for
Sensor-Based Circuits, Analog Devices
Video AFE for Diagnostic-Quality ECG Applications
MT-021 Tutorial, Successive Approximation ADCs, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Data Sheets and Evaluation Boards
Conditions Regarding the Use of This Evaluation Board
and Circuit Note
ADAS1000 Data Sheet
See complete disclaimer in the ADAS1000SDZ User Guide.
ADAS1000-2 Data Sheet
This evaluation board design is being provided “as is” without
any express or implied representations or warranties of any kind
and the use of this board or design shall impose no legal obligation
on Analog Devices, Inc., and its subsidiaries, employees, directors,
officers, servants and agents. In addition, it is understood and
agreed to that the evaluation board or design is not authorized for
use in safety critical healthcare applications (such as life support)
where malfunction or failure of a product can be expected to result
ADAS1000-3 Data Sheet
ADAS1000-1 Data Sheet
ADAS1000-4 Data Sheet
ADP151 Data Sheet
REVISION HISTORY
10/12—Revision 0: Initial Version
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©2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN11108-0-10/12(0)
Rev. 0 | Page 6 of 6
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