Datasheet AD8232 (Analog Devices) - 24

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AD8232 Data Sheet APPLICATION CIRCUITS HEART RATE MEASUREMENT NEXT TO THE HEART
The input terminals in this configuration use two 180 kΩ For wearable exercise devices, the AD8232 is typically placed in resistors, to protect the user from fault conditions. Two 10 MΩ a pod near the heart. The two sense electrodes are placed under- resistors provide input bias. Use higher values for electrodes neath the pectoral muscles; no driven electrode is used. Because with high output impedance, such as cloth electrodes. the distance from the heart to the AD8232 is small, the heart The schematic also shows two 10 MΩ resistors to set the signal is strong and there is less muscle artifact interference. midscale reference voltage. If there is already a reference voltage In this configuration, space is at a premium. By using as few available, it can be driven into the REFIN input to eliminate external components as possible, the circuit in Figure 62 is these two 10 MΩ resistors. optimized for size.
EXERCISE APPLICATION: HEART RATE MEASURED AT THE HANDS 0.22µF
In this application, the heart rate signal is measured at the
ELECTRODE
hands with stainless steel electrodes. The user’s arm and upper
HPDRIVE HPSENSE INTERFACE 180kΩ 10MΩ
body movement create large motion artifacts and the long lead
+IN IAOUT +VS 10MΩ
length makes the system susceptible to common-mode inter-
180kΩ –IN REFIN
ference. A very narrow band-pass characteristic is required to
10MΩ 0.1µF 10MΩ RLDFB +VS 10MΩ
separate the heart signal from the interferers.
1nF 0.1µF RLD GND AD8232 +VS 0.22µF SW FR OPAMP+ AC/DC +V 10MΩ S HPDRIVE HPSENSE REFOUT SDN 10MΩ 180kΩ 10MΩ LA +IN IAOUT +VS OPAMP– LO+ TO DIGITAL 180kΩ INTERFACE RA –IN REFIN OUT LO– 10MΩ 0.1µF 0.22µF
1
RLDFB +VS 10MΩ
16
1nF 0.1µF 360kΩ
66-
SIGNAL RL OUTPUT RLD GND
108
AD8232
Figure 62. Circuit for Heart Rate Measurement Next to Heart
SW FR +VS 1MΩ 1MΩ
A shorter distance from the AD8232 to the heart makes this
OPAMP+ AC/DC 100kΩ 22nF
application less vulnerable to common-mode interference.
REFOUT SDN 100kΩ
However, since RLD is not used to drive an electrode, it can be
OPAMP– LO+ TO DIGITAL 3.3nF INTERFACE
used to improve the common-mode rejection by maintaining
1MΩ OUT LO–
the midscale voltage through the 10 MΩ bias resistors. -262
SIGNAL OUTPUT
A single-pole high-pass filter is set at 7 Hz, and there is no low- 0866 1 Figure 64. Circuit for Heart Rate Measurement at Hands pass filter. No gain is used on the output op amp thereby reducing the number of resistors for a total system gain of 100. The circuit in Figure 64 uses a two-pole high-pass filter set at
70
7 Hz. A two-pole low-pass filter at 24 Hz follows the high-pass filters to eliminate any other artifacts and line noise.
60 70 50 60 B) (d E 40 50 UD IT B) N d 30 ( G E 40 A M UD 20 IT N 30 G A M 10 20 0 10 0.1 1 10 100 1k 10k
057 66-
FREQUENCY (Hz)
108
0
Figure 63. Frequency Response for HRM Next to Heart Circuit
0.1 1 10 100 1k
059 66-
FREQUENCY (Hz)
108 Figure 65. Frequency Response for HRM Circuit Taken at the Hands Rev. C | Page 24 of 28 Document Outline Features Applications Functional Block Diagram General Description Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Instrumentation Amplifier Performance Curves Operational Amplifier Performance Curves Right Leg Drive (RLD) Amplifier Performance Curves Reference Buffer Performance Curves System Performance Curves Theory of Operation Architecture Overview Instrumentation Amplifier Operational Amplifier Right Leg Drive Amplifier Reference Buffer Fast Restore Circuit Leads Off Detection DC Leads Off Detection AC Leads Off Detection Standby Operation Input Protection Radio Frequency Interference (RFI) Power Supply Regulation and Bypassing Input Referred Offsets Layout Recommendations Applications Information Eliminating Electrode Offsets High-Pass Filtering Two-Pole High-Pass Filter Additional High-Pass Filtering Options Low-Pass Filtering and Gain Driving Analog-to-Digital Converters Driven Electrode Application Circuits Heart Rate Measurement Next to the Heart Exercise Application: Heart Rate Measured at the Hands Cardiac Monitor Configuration Portable Cardiac Monitor with Elimination of Motion Artifacts Packaging and Ordering Information Outline Dimensions Ordering Guide