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AD8232ACPZ-R7 数据表(PDF) 24 Page - Analog Devices |
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AD8232ACPZ-R7 数据表(HTML) 24 Page - Analog Devices |
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24 / 28 page  AD8232 Data Sheet Rev. A | Page 24 of 28 APPLICATION CIRCUITS HEART RATE MEASUREMENT NEXT TO THE HEART For wearable exercise devices, the AD8232 is typically placed in a pod near the heart. The two sense electrodes are placed under- neath the pectoral muscles; no driven electrode is used. Because the distance from the heart to the AD8232 is small, the heart signal is strong and there is less muscle artifact interference. In this configuration, space is at a premium. By using as few external components as possible, the circuit in Figure 62 is optimized for size. Figure 62. Circuit for Heart Rate Measurement Next to Heart A shorter distance from the AD8232 to the heart makes this application less vulnerable to common-mode interference. However, since RLD 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. 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. Figure 63. Frequency Response for HRM Next to 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 user’s arm and upper body movement create large motion artifacts and the long lead length makes the system susceptible to common-mode inter- ference. A very narrow band-pass characteristic is required to separate the heart signal from the interferers. Figure 64. Circuit for Heart Rate Measurement at Hands The circuit in Figure 64 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. Figure 65. Frequency Response for HRM Circuit Taken at the Hands +VS +VS +IN –IN HPDRIVE +VS HPSENSE IAOUT REFIN GND FR AC/DC SDN LO+ LO– RLD RLDFB OUT OPAMP+ OPAMP– REFOUT SW AD8232 180kΩ 180kΩ 10MΩ 10MΩ 0.1µF 10MΩ 1nF 10MΩ ELECTRODE INTERFACE 0.1µF 0.22µF TO DIGITAL INTERFACE SIGNAL OUTPUT 10MΩ 70 0 0.1 10k FREQUENCY (Hz) 10 20 30 40 50 60 1 10 100 1k RL RA LA +VS +VS +VS +IN –IN HPDRIVE +VS HPSENSE IAOUT REFIN GND FR AC/DC SDN LO+ LO– RLD RLDFB OUT OPAMP+ OPAMP– REFOUT SW AD8232 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 70 0 0.1 1k FREQUENCY (Hz) 10 20 30 40 50 60 1 10 100 |
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