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AD8233ACBZ-R7 数据表(PDF) 27 Page - Analog Devices |
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AD8233ACBZ-R7 数据表(HTML) 27 Page - Analog Devices |
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27 / 30 page  AD8233 Data Sheet Rev. 0 | Page 26 of 29 APPLICATION CIRCUITS HEART RATE MEASUREMENT (HRM) NEXT TO THE HEART For wearable exercise devices, the AD8233 is typically placed in a pod near the heart. The two sense electrodes are placed under the pectoral muscles; no driven electrode is used. Because the distance from the heart to the AD8233 is small, the heart signal is strong and there is less muscle artifact interference. In this wearable device configuration, space is at a premium. By using as few external components as possible, the circuit in Figure 70 is optimized for size. +VS +VS +IN –IN HPDRIVE +VS HPSENSE IAOUT REFIN GND FR AC/DC RLD SDN SDN LOD RLD RLDFB OUT OPAMP+ OPAMP– REFOUT SW AD8233 180kΩ 180kΩ 10MΩ 10MΩ 0.1µF 10MΩ 1nF 10MΩ ELECTRODE INTERFACE 0.1µF 0.22µF TO DIGITAL INTERFACE SIGNAL OUTPUT 10MΩ Figure 70. Circuit for HRM Next to the Heart A shorter distance from the AD8233 to the heart makes this application less vulnerable to common-mode interference. However, because RLD (Pin D5) is not used to drive an electrode, it can be used to improve the common-mode rejection by maintaining the midscale voltage through the 10 MΩ bias resistors. Alternatively, tie RLD SDN low to save power, and tie the bias resistors to REFOUT. A single-pole, high-pass filter is set at 7 Hz, and there is no low-pass filter. No gain is used on the output op amp, thereby reducing the number of resistors for a total system gain of 100. (see Figure 71). 70 0 0.1 10k FREQUENCY (Hz) 10 20 30 40 50 60 1 10 100 1k Figure 71. Frequency Response for HRM Next to the Heart Circuit The input terminals in this configuration use two 180 kΩ resistors to protect the user from fault conditions. Two 10 MΩ resistors provide input bias. Use higher values for electrodes with high output impedance, such as cloth electrodes. The schematic also shows two 10 MΩ resistors to set the midscale reference voltage. If there is already a reference voltage available, it can be driven into the REFIN input to eliminate these two 10 MΩ resistors. EXERCISE APPLICATION—HEART RATE MEASURED AT THE HANDS In this application, the heart rate signal is measured at the hands with stainless steel electrodes. The arm and upper body movement of the user create large motion artifacts, and the long lead length makes the system susceptible to common-mode interference. A very narrow band-pass characteristic is required to separate the heart signal from the interferers. RL RA LA +VS +VS +IN –IN HPDRIVE +VS HPSENSE IAOUT REFIN GND FR AC/DC RLD SDN LOD RLD RLDFB OUT OPAMP+ OPAMP– REFOUT SW AD8233 22nF 1MΩ 1MΩ 100kΩ 3.3nF 100kΩ 180kΩ 180kΩ 1MΩ 10MΩ 10MΩ 0.1µF 10MΩ 1nF 10MΩ 10MΩ 0.1µF 360kΩ 0.22µF TO DIGITAL INTERFACE SIGNAL OUTPUT 0.22µF SDN +VS Figure 72. Circuit for HRM at Hands The circuit in Figure 72 uses a two-pole, high-pass filter set at 7 Hz. A two-pole, low-pass filter at 24 Hz follows the high-pass filters to eliminate any other artifacts and line noise. 70 0 0.1 1k FREQUENCY (Hz) 10 20 30 40 50 60 1 10 100 Figure 73. Frequency Response for HRM Circuit Taken at the Hands |
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