Capacitance Sensor Noise Rejection via Differential Sampling
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive touch interfaces face significant noise interference from power supply conduction and external radiation, which affects the accuracy of capacitance sensing, especially in noisy environments.
Innovation Solution
A capacitance sensor circuit with a noise rejection mechanism, utilizing a charger, sampler, and accumulator with current mirrors and a noise cancellation module to differentiateially process sample current signals, effectively canceling low-frequency noise and averaging out high-frequency components, while using a spread spectrum clock to reduce noise at specific frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing is used, then the interface can detect capacitance variations, but noise from power supply conduction and external radiation significantly degrades measurement accuracy
Solution Approach 1:
The patent applies differential sensing that converts the harmful noise effect into a benefit by measuring and subtracting the noise component. The first and second sample current signals are obtained by charging the sensed capacitance to different voltages, and the differential processing converts the common-mode noise into a cancelable component, transforming the noise problem into a solvable mathematical operation.
Solution Approach 2:
The patent changes the voltage parameter applied to the sensed capacitance during different phases. By charging the capacitance to a first voltage during a first phase and to a second voltage during a second phase, the system creates distinguishable signal states that allow noise cancellation through differential processing, improving measurement precision in noisy environments.
2Reliability
If noise cancellation processing is implemented, then low-frequency noise is rejected, but the device complexity increases due to additional circuit components
Solution Approach 1:
The patent segments the sensing process into distinct phases: a first phase for obtaining a first sample current signal by charging to a first voltage, and a second phase for obtaining a second sample current signal by charging to a second voltage. This temporal segmentation allows differential noise cancellation while using a relatively simple circuit structure, balancing reliability improvement with device complexity management.
3Object-affected harmful factors
If spread spectrum clocking is used, then noise at specific frequencies is reduced, but the manufacturing precision requirements increase
Solution Approach 1:
The patent employs periodic charging phases where the sensed capacitance is charged to different voltages in alternating first and second phases. This periodic action creates a structured signal pattern that enables noise cancellation through differential processing. The regular timing and voltage switching provide robust noise rejection that is relatively tolerant to manufacturing variations, reducing the impact of manufacturing precision limitations.
Data Source
AI summary
In a touch interface, a sensor provides an output signal that is a function of a sensed capacitance. The sensor includes a charger for repetitively applying first and second voltages to charge the sensed capacitance to first and second charge values in first and second phases respectively. A sampler includes a first current mirror for providing first and second sample current signals that are a function of the first and second charge values respectively. An accumulator uses an accumulator signal to provide the output signal. The accumulator repetitively uses the first and second sample current signals differentially to modify a charge on an accumulator capacitor and provide the accumulator signal. The accumulator signal is a progressive function of the sensed capacitance but cancels noise in the first and second sample signals at frequencies less than a repetition rate of operation of the accumulator.