1 Terms and Definitions
| AFE | 模拟前端 |
| 心电图 | 心电图 |
| 人力资源 | Heart rate |
| 集成电路 | 集成电路 |
| OpAmp | Operational amplifier |
| S&H | Sample and hold |
| TIA | 跨阻放大器 |
2 References
For related documents and software, please visit:
//www.xmece.com/亚博电竞菠菜products/greenpak/analog-greenpaks
Download our freeGreenPAK Designer software [1] to open the .gp files [2] and view the proposed circuit design. Use the GreenPAK development tools [3] to freeze the design into your own customized IC in a matter of minutes. Dialog Semiconductor provides a complete library of application notes [4] featuring design examples as well as explanations of features and blocks within the Dialog IC.
- GreenPAK Designer Software, Software Download and User Guide, Dialog Semiconductor
- AN-CM-307心率监视器模拟前端.gp, GreenPAK Design File, Dialog Semiconductor
- GreenPAK Development Tools, GreenPAK Development Tools Webpage, Dialog Semiconductor
- GreenPAK Application Notes, GreenPAK Application Notes Webpage, Dialog Semiconductor
- SLG47004, Datasheet, Dialog Semiconductor
Author: Vladyslav Kozlov
3 Introduction
心率是其中一个重要生命体征humans. The monitoring and tracking of heart rate (HR) is a relatively simple task for modern electronics. That's why modern wearable devices like smartwatches and activity trackers widely utilize a HR monitoring function. In addition, such devices can detect possible diseases like tachycardia (fast HR), bradycardia (slow HR), arrhythmia (often changed HR), and others.
Also, HR monitors are widely used in sports. With the help of an HR measurement, athletes can define appropriate load and rest intervals.
4心率监测技术
有几种不同的方法可以监测心率。所有这些技术都使用不同类型的信号。
4.1 Phonocardiography
这项技术利用心脏瓣膜打开和关闭时产生的声音信号。通常，信号在DSP的帮助下进行滤波和处理。经过适当的过滤，不仅可以检测到心率，还可以检测到心脏的异常情况。
4.2 Pressure Sensing Technique
This technique utilizes a signal from a piezo sensor attached to one of the defined places on the human body, for example, on the wrist. Contraction and relaxation of the heart cause a corresponding change of pressure in the blood vessels.
4.3心电图
这项技术是基于对心脏产生的电场的测量。必须在人体的特定部位放置两个或两个以上的电极。心电图的图形表示由六个峰和谷组成，称为P、Q、R、S、T和U。心率的测量方法是在规定的时间段（例如15、30或60秒）内平均两个R峰值（图1）之间的RR间隔。
4.4 Photoplethysmography
This technique is based on the measurement of the volume variations of light reflected from the skin. When light emitted from a LED passes through a blood vessel, the received light has pulsations that correspond to heartbeat. Typically, a green or infrared LED is used as a light source because of the characteristic of bloods' absorption to light with specific wavelengths. This technique is the most common way of heart rate monitoring in wearables.
The most appropriate wavelength of green light for photoplethysmography is in the range from 540 nm to 570 nm. But as a rule, the LEDs with 530 nm are used, due to the high value of luminous intensity.
5 Proposed Design of Analog Front End
The proposed analog front-end circuit is shown in Figure 2.
The circuit consists of the following parts: LED driver, transimpedance amplifier (TIA), sample and hold circuit, high pass filter, offset correction circuit, biased non-inverting amplifier, analog comparator with digital filter, and analog power supply filter. The structure of the proposed design is shown in Figure 3.
Figure 3: Analog Front End Structure
5.1.1 LED Driver
The LED driver periodically turns on/off two green LEDs. The period is 11.7 ms (85.3 Hz). Turn on time duration is 100 us. So, the duty cycle is 0.1/11.7 = 0.85 %. The operating current through each LED is 12.5 mA, giving 25 mA through one pin of the SLG47004. The average current through the pin is 213 μA.
5.1.2 Transimpedance Amplifier
In this example the TIA operates in photovoltaic mode to convert small photodiode current to voltage. The photodiode part number is BPW34. It's possible to use other LEDs or photodiode, but in that case gain resistor R5 must be changed to provide an appropriate voltage level at the output.
可选的电容器C2增加了TIA的稳定性。建议在TIA阶段出现稳定性问题时放置C2。C2的值可使用以下公式计算：
whereCD，C级CM，C级Diff- are diode capacitance, common-mode operational amplifier capacitance, and differential mode operational amplifier capacitance;
GBWP—运算放大器的增益带宽积；
Rf- is the feedback resistor.
According to the datasheet, amplifiers of the SLG47004 haveCCM= 7 pf andCDiff= 8 pF. Capacitance of photodiodeCD= 40 pF. GBWP = 512 kHz (setting) andRf= 30 kΩ. SoCf= 23.9 pF or 22 pF (nearest standard value).
5.1.3 Sample and Hold Circuit
The S&H circuit consists of an analog switch, capacitor, and buffer. Sampling begins after 30 us pause when LEDs are turned on, see Figure 15. During the pause, the output of the OpAmp is stabilized. When the analog switch is opened (sampling phase), the sampling capacitor C3 is charged to the voltage level at OpAmp0 output. The duration of the sampling time is 70 us.
During the hold phase the analog switch is closed. The sampling capacitor is disconnected from OpAmp0.
The buffer (ACMP buffer) eliminates the effect of the high pass filter on the sampling capacitor.
5.1.4 High Pass Filter
The first order high pass filter has a cutoff frequency of 2.3 Hz. The DC component of the signal after that filter is VDD/2伏。
5.1.5 Offset Correction Circuit
通过改变分压器的一个分支（R9、R10、R11和数字变阻器RH0）的电阻来实现放大器和缓冲器偏移电压的校正。分压器的配置能够以100 mV的输出电压精度（最坏情况）微调系统。如果不需要高精度，可以简化分频器。
5.1.6偏置非反相放大器
The non-inverting amplifier has a gain of 151 and a bias voltage of VDD/2 (2.5 V for VDD= 5 V).
5.1.7 Analog Comparator with Digital Filter
Digital filtering of the signal is performed by Delay5, configured as rising edge delay.
5.1.8 Analog Power Supply Filter and Grounding Considerations
混合信号电路的一种常见做法是提供模拟电源电压，从原理图数字部分产生的噪声中滤除。在当前的项目中，这是在L-C过滤器的帮助下完成的。
布线接地迹线时必须特别小心。模拟和数字接地应连接在一个点上。从公共接地到电源的迹线电阻应尽可能小。此外，建议使用低输出阻抗的电源。
6 Internal Blocks Configuration
6.1 OpAmp0 and OpAmp1 Configurations
OpAmp配置如图4所示。
Figure 4: Operational Amplifier 0, 1 Settings
6.2 Chopper ACMP Configuration
Chopper ACMP configuration is shown in Figure 5.
6.3 HD Buffer Configuration
HD缓冲器与OpAmp0宏单元共享内部参考电压。注意，在当前项目中，内部Vref与OpAmp0断开。HD缓冲器的通电源是连接矩阵信号。HD缓冲区和OpAmp0 Vref配置如图6所示。
Figure 6: HD Buffer and OpAmp0 Vref Settings
6.4 ACMP Buffer Configuration
To use ACMP buffer as a voltage follower, the ACMP1 and Vref1 macrocells should be configured as shown in Figure 7:
Figure 7: ACMP1L and Vref1 Settings
6.5 Oscillators Configurations
Oscillator1 uses default settings. Oscillator0 configuration is shown in Figure 8.
Figure 8: Oscillator0 Settings
6.6 Delay Macrocells Configurations
Delay configurations are shown in Figure 9.
Figure 9: Delay Macrocells Settings
6.7 P配置
P DLY配置如图10所示。
6.8 LUTs Configurations
LUT配置如图11所示。
6.8 Digital Rheostat 0 Configuration
数字变阻器0配置如图12所示。
Figure 12: Digital Rheostat 0 Settings
6.10模拟开关1配置
模拟开关1的配置如图13所示。
6.11一²C宏单元配置
I²C macrocell uses default configurations.
6.12 GPIOs Configurations
GPIOs configurations are shown in Figure 14.
7 Normal Operation Mode
当上电输入（GPI0）的电压变为高电平或I²C主集合I²C虚拟输入0到1。
每次通电输入（I²C病毒。输入0）变为高。自动微调过程的最大持续时间为512（变阻器代码）/2048（千赫时钟）=250毫秒。
During the calibration procedure the analog switch is turned off and the resistance of the high pass filter divider is being changed until the voltage at the inverting input of Chopper ACMP (Vref=AVDD/2) isn't equal to the voltage at the non-inverting input of Chopper ACMP (OpAmp1 output).
After the Auto-Trim process is done AFE begins to sense heart rate. LEDs are turned on 85 times per second during short intervals (100 us). The sample and hold circuit takes samples when LEDs are turned on after a pause of 30 us, see Figure 15. The sample and hold principle greatly minimizes the average current consumed by the LEDs.
Figure 15: Sample and Hold Phases During Normal Operation Mode
8 Software Simulation and Hardware Prototype Testing
Figure 16 and Figure 17 show the simulation results of the first four seconds of AFE operation. The sensor (photodiode) is modeled as a sine voltage source with 10 mV amplitude. As shown in Figure 17, the desired point of trim (VDD/2） 在自动微调过程开始后达到114 ms。然后，自动微调系统继续工作，并在所需微调点附近切换变阻器，直到释放内部设置信号。
The waveforms of the hardware prototype testing are shown in Figure 18.
Figure 18: Analog (Yellow) and Digital (Pink) Output Signals of AFE Applied to Finger
The frequency of pulses at the digital output (Figure 18) is f输出=1.063赫兹。这个频率对应于（f）的心率输出*60) = 64 beats per minute.
The short voltage spikes on the analog output signal are caused by the LED's turn on/off. These spikes don't affect digital output signal since they are filtered by digital filter inside the SLG47004. If needed, these spikes can be also filtered by 1st order RC low pass filter placed at the OpAmp1 output.
模拟输出信号的饱和区是由HD缓冲器引起的。此缓冲区的当前接收能力有限。但HD缓冲区的这种限制并不影响数字输出信号。
9结论
提出的心率监护仪是基于测量皮肤反射的光。信号处理和LED控制均由单个SLG47004 IC执行。额外的采样保持电路可以大大降低电流消耗。
The performance of the heart rate monitor highly depends on the optical system setup: the direction of light from LEDs, the wavelength of LEDs, the distance and angle between LEDs and photodiode, and others. Also, the measurements are affected by ambient light and small movements of the sensor.
如果心率信号相对较小，例如来自人类手腕的信号，则可能需要额外的软件信号处理。