Wearable products are changing people's lives. The smart bracelet is the most popular product in wearable devices and has been widely acclaimed by white-collar consumers. Through this bracelet, users can record real-time data such as exercise, sleep, and some diets in daily life, and synchronize these data with mobile phones, tablets, and iPod touches to guide healthy living through data. The bracelets sold in the market are also a wide variety. As a portable product for service life, the quality, stability and electromagnetic compatibility of the bracelet have received great attention.
Smart bracelet products often encounter the following electromagnetic compatibility interference problems during daily use, which plagues people's use of the ring:
1. Communication equipment such as mobile phones cannot be smoothly connected to the wristband via Bluetooth for data transmission and communication;
2. The internal acceleration sensor is subjected to high frequency electromagnetic interference in the high frequency electromagnetic field environment, and the step counting is not accurate;
3, smart bracelet built-in battery, need to charge when the power is not enough, but the charging interface is vulnerable to static electricity and other damage due to external exposure.
Therefore, as a hardware designer of the smart bracelet, electromagnetic compatibility design must be carried out during product development.
Intelligent bracelet electromagnetic compatibility filter design points
The external charging port of the product needs to be designed with protection filtering to enhance the anti-static capability of the interface and suppress the influence of interference on the power supply of the latter stage. The proposed circuit design is shown in the figure below, in which the power port TVS tube is BV05C and the magnetic bead is selected as BLM0402-600;
Since the acquisition of the amount of motion uses a small, thin, low-power three-axis resolution accelerator sensor, the sensor has high integration and is highly susceptible to interference. In the design, the analog power supply needs to be filtered, and the analog power supply needs to adopt a high frequency. Magnetic beads and high frequency capacitor filtering, preferably adopting PI type filtering method;
Magnetic beads = FBMA-11-100505-121A10T; FBMA-10-100505-600T
The bracelet usually uses an ultra-low-power processor chip, and the amount of data is very small, and it communicates with the mobile device through the Bluetooth chip. When designing the Bluetooth chip circuit, it is necessary to prevent other high-frequency stray crosstalk and affect the communication of Bluetooth. It is necessary to filter the design of the Bluetooth power supply, and adopt high-frequency magnetic beads and capacitors for high-frequency interference filtering. The recommended filter design circuit is as follows: Beads = FBMA-10-100505-600
The output circuit needs a π-shaped circuit (acting as impedance matching). It is necessary to add capacitance or inductance to the three positions according to the actual situation, and adjust the impedance to the center point to achieve the maximum power output effect.
Intelligent bracelet electromagnetic compatibility filter PCB design points
The smart bracelet is small in size and the layout on the PCB is relatively compact, but the device layout also needs to meet the electromagnetic compatibility design requirements. Special attention should be paid to the smooth layout. The power conversion chip, CPU module, Bluetooth module, and sensor circuit are laid out separately. They cannot cross each other. At the same time, pay attention to the power conversion module, Bluetooth module and sensor module to keep a certain distance.
On the PCB board, it is necessary to distinguish between the digital ground area, the analog ground area, and the RF ground area. It can be designed without any ground or by ground and connected by a 0 ohm resistor. The corresponding digital, analog, and RF circuits are respectively referenced to the corresponding ground. Plane, reducing mutual coupling interference. When designing the ground, pay special attention to the fact that the signal wiring cannot be divided across the ground. Otherwise, the routing loop will be large and it will be vulnerable to external high frequency interference.
In other aspects, when designing a single-board PCB, pay attention to the network where the board is laid, and the signal is retracted to reduce the interference of static electricity from the structure gap to the board, and improve the sensitivity of the board.
Design points of intelligent bracelet electromagnetic compatibility structure
The structural design pays attention to the airtightness. Pay attention to the key and increase the distance between the single board and the structural part. When the structural part is discharged, it will not act on the single board, which indirectly improves the static electricity discharge capability of the single board.
Through the electromagnetic compatibility design considerations from the aspects of filtering, anti-static, PCB layout, grounding, wiring, etc., the quality and stability of the smart bracelet can be improved! In practical applications, it can be more reliable, undisturbed, and not easily damaged, so that it can truly become a health assistant in our lives!
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