Ratiometric Capacitance-to-Code Converter Immune to Noise
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitance sensing systems face inaccuracies due to noise and fluctuations in drive voltages, current source outputs, and switching frequencies, leading to poor positioning and touch detection in user interface devices, especially in low power and high-sensitivity applications.
Innovation Solution
A ratiometric mutual-capacitance-to-code converter is implemented, combining charge-transfer and differential sigma-delta converters, which is insensitive to variations in clock frequency, IDAC current, supply voltage, and reference voltage, and has high immunity to external noise, generating an output bitstream proportional to mutual capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance measurement circuits are used, then the device can detect touch events, but the measurement accuracy is degraded by noise and fluctuations in drive voltages, current sources, and switching frequencies
Solution Approach 1:
The patent implements a ratiometric measurement approach that changes the measurement parameters to be ratio-based rather than absolute. The converter measures the ratio of two capacitances (sensor capacitance to reference capacitance), making the measurement immune to common-mode variations in drive voltages, current sources, and switching frequencies. This parameter transformation eliminates the harmful effects of these fluctuations on measurement accuracy.
Solution Approach 2:
The patent introduces a reference capacitor as an intermediary element in the measurement circuit. This reference capacitor serves as a mediator that experiences the same noise and voltage fluctuations as the sensor capacitor, allowing the ratiometric converter to cancel out these common-mode disturbances. The reference capacitor acts as a proxy that enables the system to measure only the differential signal while rejecting common interference.
2Reliability
If higher sensitivity is achieved for touch detection, then touch events can be detected more accurately, but the system becomes more susceptible to noise and environmental interference
Solution Approach 1:
The patent transforms the measurement parameter from absolute capacitance value to a ratiometric value (sensor capacitance divided by reference capacitance). This parameter change enables high sensitivity touch detection while simultaneously providing immunity to external noise, as the ratio measurement cancels out common-mode interference that would otherwise affect sensitive measurements.
Solution Approach 2:
The patent implements a feedback mechanism where the reference capacitor continuously tracks the common-mode variations in the measurement environment. This feedback allows the system to dynamically compensate for noise and voltage fluctuations, maintaining high touch detection sensitivity while rejecting external interference through the ratiometric conversion process.
3Measurement precision
If a ratiometric mutual-capacitance-to-code converter is implemented to achieve immunity to voltage variations, then measurement accuracy improves, but the device complexity increases
Solution Approach 1:
The patent combines charge-transfer conversion and differential sigma-delta conversion into a single integrated ratiometric mutual-capacitance-to-code converter. This merging of conversion techniques achieves voltage variation immunity and high measurement precision while reducing the overall device complexity compared to implementing separate conversion stages. The unified converter structure eliminates the need for multiple independent circuits.
Solution Approach 2:
The patent designs a universal converter circuit that performs multiple functions: it acts as both a charge-transfer converter and a differential sigma-delta converter simultaneously, while also providing ratiometric measurement capability. This multi-functional design achieves high measurement precision with voltage immunity without requiring separate dedicated circuits for each function, thereby managing device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides accurate and reliable capacitance measurements, independent of noise and voltage variations, enhancing the sensitivity and accuracy of touch detection in user interface devices while reducing power consumption.
Implementation Method 1
Capacitance sensing systems are increasingly used for implementing these and other types of user interface devices, and function by sensing electrical signals generated on electrodes that reflect changes in capacitance
Data Source
AI summary
An embodiment of a capacitance sensing circuit includes a set of bridge switches coupled with a reference cell and a sensor cell. The set of bridge switches is configured to, over a first phase, increase a voltage difference between a first modulation capacitor and a second modulation capacitor, and over a second phase, decrease the voltage difference at a rate corresponding to a difference between a capacitance of the sensor cell and a capacitance of the reference cell. The capacitance sensing circuit also includes a comparator configured to generate an output based on comparing a first voltage of the first modulation capacitor with a second voltage of the second modulation capacitor, and initiate a transition between the first phase and the second phase in response to the comparing.