Overview
Wearable products are changing daily life, and smart bands are among the most common wearable devices. A smart band can record exercise, sleep, and in some cases dietary data, and synchronize this data with mobile phones, tablets, or iPod touch to support health tracking. As portable consumer devices, smart bands receive significant attention for product quality, stability, and electromagnetic compatibility (EMC) performance.
Common EMC Issues
- Bluetooth pairing or data transfer between the band and mobile devices is unstable or fails.
- Internal accelerometer sensors are susceptible to high-frequency electromagnetic interference, causing inaccurate step counting.
- Built-in batteries require charging; exposed charging interfaces are vulnerable to electrostatic discharge and other disturbances that can cause damage.
Hardware designers must address EMC during product development to mitigate these issues.
EMC Filter Design Points
External charging ports require protection and filtering to improve ESD tolerance and to suppress interference that could affect downstream power circuitry. Recommended circuit elements include a TVS diode (BV05C) on the power input and magnetic beads such as BLM0402-600. An example circuit is shown below.
Motion sensing typically uses compact, low-power three-axis accelerometers with high integration, which are sensitive to interference. Filter the analog power supply for the accelerometer using high-frequency magnetic beads and high-frequency capacitors. A PI-type filter topology is preferred. Example magnetic bead parts: FBMA-11-100505-121A10T; FBMA-10-100505-600T.
Smart bands usually use ultra-low-power application processors and a Bluetooth radio for communication. To prevent high-frequency spurious coupling into the Bluetooth radio, apply filtering on the Bluetooth power rail using high-frequency magnetic beads and capacitors. Recommended magnetic bead: FBMA-10-100505-600.
Output stages often require a pi network for impedance matching. Adjust capacitances or inductances at the three positions to tune the impedance toward the center point and achieve maximum power transfer.
PCB Design Considerations for EMC
Given the small size of smart bands and dense PCB layouts, component placement must still satisfy EMC requirements. Keep the power conversion chip, CPU module, Bluetooth module, and sensor circuits separated to reduce mutual coupling, and maintain physical distance where possible between the power conversion module, Bluetooth module, and sensor module.
On the PCB, separate the digital, analog, and RF areas. You may use either a single ground plane or split grounds connected with 0 ohm resistors, but reference each domain to an appropriate ground plane to reduce coupling. When using split ground planes, avoid routing signals across ground splits to prevent large loop areas that are susceptible to external high-frequency interference.
Also consider a ground pour around the board edge and recess critical signals inward to reduce ESD coupling through structural gaps and to lower board sensitivity to external discharges.
Mechanical Design Considerations for EMC
Enclosure design should emphasize sealing. Increase the spacing between the PCB and structural parts around buttons to prevent discharges from directly affecting the board, which improves the board's ESD tolerance.
Conclusion
Addressing EMC through filtering, ESD protection, PCB layout, ground partitioning, and careful routing improves smart band reliability and stability. These measures reduce interference and the likelihood of damage in real-world use.
