ChrisBox: An Open-Source 4-Channel Portable Charge Amplifier for Research and Education

Piezoelectric sensors convert electric charge in response to mechanical loading and are widely studied for their high sensitivity. In particular, ferroelectret sensors provide the flexibility and material system design that enable biocompatibility. Insole-based gait and pressure sensing remain active research topics, yet most published electronics are limited to conceptual diagrams without practical strategies for noise mitigation or robustness against electromagnetic interference (EMI). Therefore, this work presents a compact, four-channel portable measurement system specifically designed for charge-based force sensing with piezoelectric sensors, called ChrisBox. A comprehensive shielding strategy, including both the circuit board and the sensor connection, via cost-effective USB Type-C cables is implemented. Performance was validated by comparing the output of the developed electronics with a calibrated electrometer under defined mechanical loads applied to a shielded ferroelectret sensor. The system measured the charge with a relative deviation of 8.2%, partly attributed to variability in the verification setup, with a high linearity of R2 > 0.99. In controlled EMI testing within a transverse electromagnetic (TEM) cell, noise levels remained below 0.1 pC RMS, enabling a signal-to-noise ratio (SNR) of 53 dB for a measured charge of 43 pC RMS during human pulse measurement with a ferroelectret. The results confirm the system’s suitability for portable, high-resolution ferroelectret sensing in EMI-prone environments and provide a foundation for further application-specific development.