The Challenge
The research team was running clinical trials involving ambulatory ECG and EMG monitoring. Existing wired systems were cumbersome for patients and restricted natural movement, compromising data quality. They needed a device that could wirelessly transmit eight simultaneous biopotential channels to a nearby laptop for real-time analysis and recording.
The constraints were strict: the device had to run on standard AA batteries for at least 8 hours, maintain a signal bandwidth of DC–500Hz with a noise floor below 1µVrms, and fit within a small enclosure patients could wear on a belt clip.
The Process
The design centered on two key ICs: the Texas Instruments ADS1298 (8-channel, 24-bit biopotential AFE) and the Nordic Semiconductor nRF5340 dual-core BLE SoC.
- Analog Front End: The ADS1298 provides eight fully differential input channels with programmable gain (1–12×), integrated right-leg drive, and a 24-bit sigma-delta ADC per channel. Input protection networks with TVS diodes guard against electrostatic discharge from patient leads.
- Digital Core: The nRF5340's network core runs the BLE 5 stack while the application core handles SPI communication with the ADS1298 at 2kHz sample rate per channel. Custom BLE GATT service streams 8 × 16-bit samples per connection event.
- Power: Two AA batteries supply 3V, boosted to 5V for the ADS1298 analog supply via TPS61023, with a separate ultra-low-noise LDO (TPS7A4700) generating the 3.0V analog reference rail to minimize power supply noise coupling into the analog front end.
- PCB Layout: The board was split into analog and digital ground planes joined at a single star point, with the ADS1298 and its surrounding passives isolated from all switching circuitry.
Firmware was developed with Zephyr RTOS, using DMA-driven SPI to ensure deterministic 2kHz sample acquisition with minimal CPU overhead, leaving headroom for BLE stack operations.
The Result
The device achieved all design targets on first prototype. Measured input-referred noise was 0.72µVrms — well below the 1µV specification. BLE connection was stable at 10 meters through typical building walls, and battery life measured 11 hours at full 8-channel, 2kHz operation.
The research team used the device in a 40-patient clinical study, successfully collecting clean biopotential data in fully ambulatory conditions. The wireless design eliminated motion artifacts caused by cable tension — a major improvement over their previous wired system.
- 8 channels simultaneously at 2kHz per channel
- Input-referred noise: 0.72µVrms (DC–500Hz)
- BLE range: 10m+ through walls, 2Mbps PHY
- Battery life: 11 hours on 2× AA
- Enclosure: 65 × 45 × 18mm