Abstract
This project implements four-wire bioimpedance analysis (BIA) using an STM32F411CEU6 microcontroller and an AD5941 analog front end. By routing excitation through CE0 and switching sense electrodes via AIN1/AIN2/AIN3, we achieve measurements with <1% error across frequencies from 100 Hz to 50 kHz.
Motivation & Overview
BIA is widely used for body composition and tissue characterization. Four-wire measurement eliminates lead resistance errors inherent in two-wire methods, improving accuracy. We designed a compact wearable PCB (40 mm×40 mm) to switch between electrode pairs and capture impedance magnitude and phase in real time.
Hardware Components
- AD5941: Onboard DFT, programmable switch-matrix, calibration features, wide frequency range.
- AP7215: 3.3 V regulator with 55 μA quiescent current (vs. 5 mA for AMS1117).
- STM32F411CEU6: Chosen for available dev hardware and proven peripheral support.
- Solder Bridges: LED disable and BOOT0 selection for power savings and programming flexibility.
- Edge Connector: SWD programming interface.
- Battery Slot: Fits 100 mAh Li-ion for up to ~20 h operation.
- Wearable PCB: 40 × 40 mm form factor with 1 mm square pads for electrode connectors.
Circuit Design
Schematic
Board Layout
Battery Life Estimation
Using Battery Life = Battery Capacity (mAh) / Current Consumption (mA):
- Active (40 mA): 100 mAh / 40 mA ≈ 2.5 h
- Mixed (avg 22.5 mA): 100 mAh / 22.5 mA ≈ 4.4 h
- Idle (5 mA): 100 mAh / 5 mA = 20 h
Switching Technique
The AD5941 SWCON register configures excitation (CE0) and sense pairs (AIN1, AIN2, AIN3):
| Measurement Pair | Excitation (CE0) | V+ | V− | Return |
|---|---|---|---|---|
| Pair 1 | CE0 | AIN1 | AIN2 | AIN3 |
| Pair 2 | CE0 | AIN1 | AIN3 | AIN2 |
| Pair 3 | CE0 | AIN2 | AIN3 | AIN1 |
| Calibration | CE0 | RCAL+ | RCAL− | AIN3 |
Test Methodology
- Verify SPI comms: read register 0x0404 ⇒ expect 0x5502.
- Switch to calibration mode (RCAL = 1 kΩ), measure at 1 kHz, compute gain/phase corrections.
- Test known resistors (500 Ω, 1 kΩ, 10 kΩ) across 100 Hz–50 kHz.
- Compare measured vs. expected magnitude & phase.
Validation & Development
Validation on Human Body: Electrodes placed on forearm and calf yielded tissue impedance profiles matching literature values, confirming safe (<10 μA) and accurate performance across all measured frequencies.
Design & Development Process: A custom AD5941 driver was written in C after vendor libraries proved unreliable. An SWD edge connector was added in the second PCB revision for easier programming. All design choices—component selection, layout, and guarding—adhere to IEC 60601 medical safety standards.