Capacitive Sensor Ring Oscillator Circuit for Precision Measurement
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Solution Overview
Problem
Existing pixellated capacitive sensing systems face challenges in achieving accurate capacitance measurements with simple circuitry, as they often require conversions that introduce noise and distortion, limiting their sensitivity and precision.
Innovation Solution
The use of an array of ring oscillator sensor circuits, where the capacitance to be measured is part of the frequency-determining components, allowing for direct measurement of time and thus accurate capacitance determination through changes in ring oscillator frequency, with a measuring circuit that includes a row selector, counters, and a reference oscillator to derive capacitance from frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If AC voltage drive with measurement of AC current through capacitor is used, then the measurement can be implemented with simple circuitry, but the measurement accuracy and sensitivity are limited due to noise and distortion from conversion processes
Solution Approach 1:
The patent replaces traditional voltage/current-based capacitive measurement circuits with a ring oscillator-based time measurement system. The capacitance to be measured is converted into a frequency-determining element of the ring oscillator, allowing direct time measurement through frequency counting. This substitution of measurement domain (from electrical voltage/current to time/frequency) eliminates the need for complex analog-to-digital conversions and reduces noise and distortion, thereby improving measurement accuracy while maintaining circuit simplicity
Solution Approach 2:
The patent changes the measurement parameter from voltage or current to frequency/time. By making the capacitance under measurement part of the frequency-determining components of the ring oscillator, the system measures time directly through frequency counting. This parameter transformation allows accurate capacitance determination without complex conversion circuits, resolving the contradiction between measurement precision and device complexity
2Productivity
If ring oscillator is used instead of LC oscillator, then many sensor circuits can be integrated in compact form, but the oscillator becomes noisier and more power hungry
Solution Approach 1:
The patent uses multiple identical ring oscillator circuits in an array configuration, where each oscillator is a copy of the same compact design. This allows mass integration of sensor circuits (hundreds or thousands of pixels) on a single chip. While individual ring oscillators are noisier than LC oscillators, the standardized copying enables high integration density that would be impossible with larger LC oscillators, and the noise is managed through the differential measurement approach and frequency counting methodology
Data Source
AI summary
The invention provides a capacitive sensor circuit in which a capacitance to be sensed is selectively coupled into a ring oscillator circuit. The ring oscillator frequency is measured with the capacitance coupled and not coupled, and a capacitance is derived from the change in ring oscillator frequency.