Put aside the stylish shell A smart watch is essentially a product that relies on various sensors Monitor sleep, monitor heart rate, exercise reminder, GPS positioning... These new applications opened by sensors bring the watch into a new era Which sensors and related modules are used in smart watches? Take stock today How does the vital sign sensor realize blood pressure monitoring? Take the integrated vital signs sensor reference design AS7024 as an example for a brief description. The AS7024 HRM operation is based on photoplethysmography (PPG). The pulse rate is measured by the sampling light adjusted by the blood vessel. When the blood pulse passes through, the blood vessel will expand and contract. Using proprietary software developed in cooperation with partners, it can analyze and synchronize heart rate measurements (HRM) and electrocardiogram (ECG) measurements to calculate blood pressure. I have to admit that the addition of the AS7024 wearable device helps users achieve accurate health tracking, which is a big improvement. The AS7024 has a small appearance, which is especially suitable for fitness wristbands, smart watches, sports watches and smart patches with limited circuit board space. At the same time, it also opened the door to end markets such as wider-scale vital signs monitoring and automotive or personal health monitoring. The AS7024 HRM operation is based on photoplethysmography (PPG). The pulse rate is measured by sampling light adjusted by blood vessels. When blood pulses pass, the blood vessels will expand and contract. This technology is proven. The electrocardiogram is a standard method of measuring the electrical pulses generated by the sinoatrial node of the heart. The AS7024 reference design uses proprietary software developed in cooperation with partners, which can analyze and synchronize HRM and ECG measurements to calculate blood pressure. According to the IEEE standard for wearable cuffless devices, AS7024 has been verified by clinical trials conducted by the Medical University of Graz, Austria, and can provide accurate blood pressure measurement. The results generated by the reference design were also compared with reference measurements made with surgical-grade cuff blood pressure measuring instruments. In more than 1,000 measurements taken by more than 100 18-65-year-old subjects, it was verified that the AS7024 reference design can be accurate to systolic blood pressure <± 7.5 mmHg and diastolic blood pressure <± 5.5 mmHg. Ambient light sensors are currently widely used in many LCD display applications, from consumer electronics to automotive applications. By automatically adjusting the brightness of the display, they can help save device battery power. In addition, these sensors work well under various light sources such as natural daylight, fluorescent and incandescent lamps. Recently, many such products have been released, and they all have a common feature, that is, these newly developed ambient light sensors can match the requirements of the human eye, which is very important for alleviating eye fatigue. The ambient light sensor can perceive the surrounding light conditions and inform the processing chip to automatically adjust the brightness of the display backlight to reduce the power consumption of the product. For example, in mobile applications such as mobile phones, notebooks, and tablets, the display consumes up to 30% of the total battery power, and the use of an ambient light sensor can maximize the working time of the battery. On the other hand, the ambient light sensor helps the display provide a soft picture. When the ambient brightness is high, the LCD display using the ambient light sensor will automatically adjust to high brightness. When the external environment is dark, the display will be adjusted to low brightness. . The realization of an ambient light sensing system requires three major parts: a light sensor that monitors the ambient light intensity, a data processing device (usually a microcontroller), and an actuator that controls the input current of the backlight. Figure 1 is an exemplary block diagram of a system implementing backlight control. In this set of combinations, the light sensor is a key component, because it needs to provide ambient light intensity information to the other modules of the system. The light sensor must have a signal converter (such as a photodiode or CdS photoresistor), a signal amplification and/or adjustment device, and an analog/digital converter (ADC) that converts the light signal into an electrical signal. Figure 1. System block diagram for implementing backlight control Figure 2 shows a discrete photodiode circuit. It can be seen from the figure that the circuit requires one or more operational amplifiers: one for current-to-voltage conversion, and possibly a stage of amplification to provide additional gain. It also includes some branch circuits for power supply to ensure a highly reliable signal chain. In applications where space is extremely valuable, the excessive number of required components may cause space constraints. Figure 2. Discrete design of photodiode circuit There is a more subtle issue here. Specifically, ideally, it should be ensured that the measurement of ambient light simulates the response mechanism of the human eye to light. This is usually based on the visual brightness curve provided by CIE (Figure 3). However, photodiodes are rarely able to fully simulate this response mechanism because they usually have high infrared (IR) sensitivity. In light conditions with high IR intensity (such as incandescent lamps or sunlight), this infrared sensitivity can cause incorrect judgments of light intensity. Figure 3. CIE curve and a typical photodiode One of the solutions to the above-mentioned problem is to use two photodiodes: one uses a component sensitive to visible and infrared light, and the other uses a component sensitive only to infrared light. Finally, the response value of the former is subtracted from the response value of the latter to minimize the infrared interference and obtain an accurate visible light response. The display screen of the smart watch can automatically adjust the brightness of the screen according to different indoor and outdoor light intensity. The biggest contributor is the ambient light sensor. Ams' ambient light sensor TSL2584TSV, 1.145mm x 1.66mm x 0.32mm, this product represents the best level of global ambient light sensor that year. In the display management application, the ambient light sensor is used to automatically control the brightness of the background light, which can extend the battery life while ensuring the best user experience. TSL2584TSV has two major highlights. One is the use of advanced TSV packaging technology, which greatly improves the reliability of the device. It has a strong light response, even if it is behind the dark glass, it can accurately measure the light intensity. The second is to use advanced wafer process technology and precisely installed interference filters to help achieve the excellent performance of the ambient light sensor. The most important thing for battery charging is these three steps: The first step: to judge the voltage "3V, pre-charge first, 0.05C current; Step 2: Determine 3V "voltage "4.2V, constant current charging 0.2C~1C current; The third step: Determine the voltage>4.2V, charge with constant voltage, the voltage is 4.20V, and the current will decrease with the increase of the voltage until it is fully charged. At the beginning of charging, the voltage of the battery to be charged should be checked first. If the voltage is lower than 3V, pre-charge should be carried out first. The charging current is 1/10 of the set current, generally around 0.05C. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is: constant current charging with the set current, when the battery voltage rises to 4.20V, change to constant voltage charging, and keep the charging voltage at 4.20V. At this time, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, charging ends. Generally, the charging current of lithium batteries is set between 0.2C and 1C. The larger the current, the faster the charging and the greater the battery heat. Moreover, charging with an excessive current will result in insufficient capacity because the electrochemical reaction inside the battery takes time. Just like pouring beer, pouring too fast will produce foam, but dissatisfaction. Term explanation: The charging and discharging current is generally referred to by C, which is the value corresponding to the battery capacity. The battery capacity is generally expressed in Ah and mAh. For example, the battery capacity of M8 is 1200mAh, and the corresponding C is 1200mA. 0.2C is equal to 240mA. The following is a typical charging curve diagram of a lithium battery: Only a few points need to be paid attention to when discharging lithium batteries: 1. The discharge current should not be too large. Excessive current will cause heat inside the battery, which may cause permanent damage; 2. Never over discharge! Lithium batteries are most afraid of overdischarge. Once the discharge voltage is lower than 2.7V, the battery may be scrapped. The following is a typical discharge curve of a general lithium battery: It can be seen from the typical discharge curve that the larger the battery discharge current, the smaller the discharge capacity and the faster the voltage drop. Therefore, under normal circumstances, after the battery is under heavy load, the voltage will rise when the load is reduced, which is the so-called "recycle" phenomenon. Take a look at the graph. This discharge curve stopped for a while during the discharge process, and "recycled" appeared. The power output of AS3701 includes two 200mA low dropout linear regulators (LDO), a 500mA step-down DC-DC converter, and two 40mA (maximum) programmable current sinks. The switching frequency of the synchronous buck converter is up to 4M Hz, so only a small inductor and a 10μF output capacitor are required. The integrated lithium battery charger of the micro PMIC can operate in trickle, constant current, and constant voltage modes, and provide a maximum charging current of 500mA. The power path management function enables the device to immediately turn on the device when it is plugged into the charger when the battery is exhausted, and optimizes the power distribution of the battery charging circuit and the system power supply circuit. Many applications can configure the AS3701 through the I2C interface of the main processor or microcontroller. The device also includes a multi-function I/O interface that can be used to achieve power on/off, PWM input and output, and interrupt and control tasks in standby mode. The designer can flexibly realize the power-on sequence required by different processors or different applications through OTP (One Time Programmable Memory). This means that OEM manufacturers can use the same power solution in a variety of terminal product designs, effectively simplifying the design, development and integration process. The device also integrates battery temperature monitoring, power-on reset, and over-current protection functions to ensure operational reliability and safety. AS3701 incorporates many years of experience in designing and manufacturing small high-efficiency power supply circuits by ams. It is the most advanced miniaturized power supply circuit today. It is very suitable for wearable devices with compact space and can effectively extend the battery life of the device. For a smart watch, the importance of long battery life is self-evident. The integrated lithium battery charger of the integrated power management unit AS3701 can operate in trickle, constant current, and constant voltage modes, and provides a maximum charging current of 500mA. The power path management function enables the device to immediately turn on the device when it is plugged into the charger when the battery is exhausted, and optimizes the power distribution of the battery charging circuit and the system power supply circuit. This product can be called a fighter in the long battery life power management module, and has been popular in the wearable world since its release. The AS3921 near field communication analog front end with enhanced NFC technology uses active load modulation: it generates an RFID card response synchronized with the magnetic field of the reader. This makes the coupling factor of card-reader communication an order of magnitude smaller than the passive load modulation (PLM) used by contactless cards. Compared with the use of traditional NFC, this solution can increase the available workplace of the NFC reader by up to 900%. This greatly improves the reliability and perceived speed of NFC transactions. Whether it is a more reliable connection between payment terminals, subway ticket gates or other devices that include NFC card readers, AS3921 can be performed handily. Wearable devices that can detect temperature and humidity are still very cool. ENS210 can provide Kelvin digital temperature output with accuracy up to ±0.2°C in the range of 0°C to 70°C. It can also measure relative humidity and output it in digital form with an accuracy of up to ±3.5%. The product has been calibrated when it is delivered to the customer, so the sensor does not need to be adjusted on the production line. The product provides digital output through an I2C interface, without the application processor or microcontroller of the host device for signal processing. ENS210 consumes only 40nA in standby mode and 7.1μA in active measurement mode (sampling at 1Hz), so it only consumes very little energy from batteries in handheld and portable products. The product can work under a wide input voltage range of 1.71-3.6V, which means that a system equipped with a 1.8V/3.3V dual-voltage power supply does not need to add an additional level-adjusting power supply at the load point. When used in wearable devices, the relative humidity and ambient temperature readings of ENS210 help improve the accuracy and reliability of body movement measurements that are highly dependent on environmental conditions. It can also provide sophisticated automated operation or performance enhancement functions for temperature regulators and interconnected household appliances such as air conditioning and purification systems, refrigerators, dryers, microwaves, kitchen air extractors, and weather monitoring devices. The relative humidity and temperature sensor ENS210 can provide a Kelvin digital temperature output with an accuracy of up to ±0.2°C in the range of 0°C to 70°C. It can also measure relative humidity and output it in digital form with an accuracy of up to ±3.5%.
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