Why is the Analog Measurement Front-End of a Spirometer Susceptible to Digital Circuit Interference?

The high-sensitivity flow and pressure sensors in a spirometer output weak analog signals, requiring its analog front-end amplifier to have extremely high gain. High-frequency noise generated by internal digital circuits and switching-mode power supplies can couple into the analog signal path through common ground impedance or spatial radiation coupling. This interference directly superimposes onto the physiological signals, causing signal baseline drift or introducing high-frequency glitches. Consequently, the subsequent ADC sampling values may exhibit jumps, ultimately affecting the calculation accuracy of key parameters such as vital capacity and forced expiratory volume.