Differential Capacitive Sensing Circuit for RF-Resistant Measurement

The patent introduces a transimpedance amplifier as an intermediary device between the sense electrode and the measurement circuit. This amplifier converts the capacitive current from the sense electrode into a voltage signal, effectively isolating the measurement circuit from the parasitic impedances between sense and guard electrodes. The amplifier's high input impedance prevents loading effects while its low output impedance drives the subsequent circuitry, thereby maintaining measurement accuracy despite the presence of guard electrode parasitics.

The patent employs phase modulation of the excitation signal applied to the guard electrode. By modulating the guard electrode signal at a specific frequency and using synchronous detection, the system can distinguish between signals of interest and parasitic coupling effects. This parameter change in the excitation signal allows the measurement circuit to reject parasitic impedances through frequency-selective detection while maintaining the guard electrode's directional masking function.

The patent applies synchronous detection techniques that convert the harmful RF interference into a useful signal processing advantage. By modulating the sensor excitation at a known frequency and using phase-sensitive detection, the system creates a narrow bandwidth detection window that rejects out-of-band RF interference. The harmful RF currents are effectively filtered out by the phase-locked detection process, which only responds to signals at the modulation frequency, thereby converting the RF-hostile environment into an advantage through selective frequency response.

The patent uses periodic modulation of the excitation signal applied to the sense and guard electrodes. This periodic action creates a time-varying measurement signal that can be easily distinguished from random RF interference through synchronous rectification and low-pass filtering. The modulated measurement signal appears at a specific frequency, allowing the system to reject broadband RF noise while maintaining sensitivity to the periodic capacitive coupling signal from target objects.

The patent merges multiple functions into a single integrated transimpedance amplifier circuit. The amplifier simultaneously performs current-to-voltage conversion, provides impedance transformation, enables phase modulation, and implements rejection of parasitic impedances through its differential input configuration. By combining these functions in one circuit block rather than using separate stages, the design maintains relative simplicity while achieving high reliability against RF interference and parasitic effects.

The transimpedance amplifier in the patent serves multiple functions: it converts capacitive current to voltage, provides impedance buffering, enables phase-sensitive detection, and rejects common-mode RF interference. This multi-functional design eliminates the need for additional separate circuits for each function, maintaining design simplicity while achieving robust performance. The single amplifier stage performs what would traditionally require multiple discrete circuit blocks, thereby simplifying the overall design without sacrificing reliability.