前言
光体积变化描记图法
（Photoplethysmography，简称PPG）
当一定波长的光束照射到指端皮肤表面，每次心跳时，血管的收缩和扩张都会影响光的透射或是光的反射。当光线透过皮肤组织然后再反射到光敏传感器时，光照会有一定的衰减。像肌肉、骨骼、静脉和其他连接组织对光的吸收是基本不变的 (前提是测量部位没有大幅度的运动)，但是动脉会不同，由于动脉里有血液的脉动，那么对光的吸收自然也会有所变化。当我们把光转换成电信号时，正是由于动脉对光的吸收有变化而其他组织对光的吸收基本不变，得到的信号就可以分为直流DC信号和交流AC信号。提取其中的AC信号，就能反应出血液流动的特点。
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一、产品描述
1.产品特性
EM7028 is a low-power I2C interface module includes Heart Rate Sensor(HRS) and internal LED Current Drivers with 2 Green LED. It can work in both continuous mode and pulse mode. It is designed for applications of Heart Beat Rate detection esp. such as smart watch etc. Heart Rate Sensor in continuous mode is designed to monitor heart rate by optical detection. A 16-bit ADC detects the result constantly with adjustable gain and resolution. Heart Rate Sensor in pulse mode is designed to monitor heart rate by optical detection in low-power. It also acts as a proximity sensor with rejection of ambient light noise. In order to removing component variations and eliminate offset caused by IR reflection, 2 registers were specially defined for compensation and high dynamic range of detection. HRS in continuous mode stores ADC output result into HRS1 registers, while HRS in pulse mode stores ADC output result into HRS2 registers. All pixels are controlled by analog-MUX in different modes.
2.技术规格
二、应用领域
示例：
• 工业控制领域：工业、控制设备美。
• 智能交通设备：收费站、停车场、车内语音提示；
• 广告宣传行业：广告语播报；
三、应用电路图
示例：
