Auto-Zeroed Sensor Sampling for Time-Varying Parasitic Capacitance
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Solution Overview
Problem
Electronic circuits implementing sensors face performance degradation due to time-varying parasitic capacitances, such as those caused by water droplets on capacitive pressure sensors, which current technologies struggle to mitigate effectively.
Innovation Solution
A sensor system comprising a sense capacitor with variable sense capacitance and a readout integrated circuit using a MASH modulator and auto-zeroing technique to generate ADC output signals, which includes a switched capacitor circuit and sigma-delta ADC to manage and reduce the impact of time-varying parasitic capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional sensor circuits are used, then device simplicity is maintained, but measurement precision deteriorates due to time-varying parasitic capacitances
Solution Approach 1:
The sensor system is divided into distinct functional modules: a capacitive sensor element, a switched-capacitor circuit with separate sampling and integration phases, and a MASH modulator. This segmentation allows each module to handle specific tasks, with the switched-capacitor circuit isolating the sensor from parasitic capacitances during critical measurement phases, thereby improving measurement precision without overwhelming complexity
Solution Approach 2:
A switched-capacitor circuit is introduced as an intermediary between the capacitive sensor and the readout circuitry. This intermediary uses switching mechanisms to connect and disconnect parasitic capacitances at different phases, preventing them from affecting the measurement during critical integration periods while maintaining circuit functionality
2Measurement precision
If sampling frequency is increased to mitigate parasitic effects, then measurement precision improves, but energy consumption increases
Solution Approach 1:
The circuit employs periodic sampling and integration phases controlled by clock signals. During the sampling phase, the sensor is connected to capture the signal; during the integration phase, the signal is integrated while parasitic capacitances are disconnected. This periodic action allows high effective sampling rates for precision while keeping the circuit in low-power states between phases, balancing measurement accuracy with energy consumption
3Reliability
If water droplets are present on the sensor, then parasitic capacitance increases, but the system should maintain measurement reliability
Solution Approach 1:
The circuit performs preliminary actions by establishing a baseline measurement during an initial phase before the actual measurement is taken. This preliminary sampling allows the system to characterize the parasitic capacitance environment (including any water droplet effects) and compensate for it during subsequent measurements, maintaining reliability even when parasitic capacitances are elevated
Solution Approach 2:
The MASH modulator implements feedback mechanisms that continuously monitor the sensor output and adjust the measurement process accordingly. The feedback loop can detect shifts caused by parasitic capacitances and compensate through digital signal processing, ensuring measurement reliability persists even when environmental factors like water droplets increase parasitic effects
Data Source
AI summary
Methods and devices to mitigate time varying impairments in sensors are described. The application of such methods and devices to pressure sensors facing time varying parasitic capacitances due to water droplets is detailed. Benefits of auto-zeroing technique as adopted in disclosed devices is also described.