Capacitance-to-Digital Converter for Single-Sensor Noise Rejection
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Solution Overview
Problem
Capacitance-sensing technologies for power buttons face challenges in noise immunity and spectral emissions due to the integration of noise during scanning, with existing solutions requiring two sensors for differential operation or being susceptible to supply noise when using a single sensor.
Innovation Solution
The implementation of a pseudo-differential sensing method using a single sensor with auto-zero refresh and spread-spectrum asynchronous clocking, along with a power-cycled shield buffer, to reduce electromagnetic susceptibility and emissions, and a half-wave sense method for improved supply rejection and tunable sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If two sensors are used for differential operation, then noise immunity is improved, but device complexity increases
Solution Approach 1:
The patent segments the sensing operation into two distinct phases: a first phase where the sensor is coupled to a first input of the comparator, and a second phase where the sensor is coupled to a second input of the comparator. This temporal segmentation allows a single sensor to achieve differential-like noise rejection without requiring two simultaneous sensors, thereby reducing device complexity while maintaining noise immunity.
Solution Approach 2:
The patent implements periodic switching between the two sensing phases using non-overlapping clock signals. The sensor alternates between coupling to the first input and second input in a periodic manner, enabling noise rejection through differential operation while using only one physical sensor. This periodic action resolves the contradiction by achieving the benefits of two sensors without the hardware complexity.
2Measurement precision
If scanning is performed to detect capacitance changes, then touch detection capability is improved, but electromagnetic emissions increase
Solution Approach 1:
The patent employs periodic scanning with non-overlapping clock signals that alternate between two phases. This periodic action distributes the electromagnetic emissions over time rather than concentrating them in continuous operation, reducing peak emissions while maintaining touch detection capability. The scanning is performed in discrete intervals rather than continuously.
Solution Approach 2:
The patent extracts and removes the harmful electromagnetic emissions by using asynchronous clocking with non-overlapping phases. The scanning operation is taken out of continuous operation and performed in separated, non-overlapping intervals, which reduces the overall electromagnetic emission profile while preserving the ability to detect capacitance changes for touch events.
3Device complexity
If single sensor operation is used, then device complexity is reduced, but susceptibility to supply noise increases
Solution Approach 1:
The patent segments the single sensor operation into two distinct phases with different coupling configurations. During the first phase, the sensor couples to the first input; during the second phase, it couples to the second input. This segmentation allows noise cancellation through differential comparison even with a single sensor, reducing susceptibility to supply noise without requiring multiple sensors.
Solution Approach 2:
The patent implements a feedback mechanism where the output of the comparator feeds back to control the switching of the sensor between the two inputs. This feedback loop enables the system to maintain balance and reject supply noise by continuously comparing and adjusting the sensor coupling, achieving noise rejection with only one physical sensor.
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
This approach enhances noise rejection and sensitivity while reducing electromagnetic emissions, enabling robust and low-power operation for capacitive power buttons.
Implementation Method 1
Capacitance sensing systems can sense electrical signals generated on electrodes that reflect changes in capacitance. When a conductive object (e.g., a finger, hand, or other objects) comes into contact with or close proximity to a capacitive sense element, the capacitance changes, and the conductive object is detected.
Data Source
AI summary
Apparatuses and methods of capacitance-to-digital code conversion are described. One capacitance-to-digital converter (CDC) includes front-end circuitry, including a comparator. The CDC further includes a first capacitive digital-to-analog converter (CDAC) coupled to a first input of the comparator and, in a first phase, to a sensor cell. The CDC further includes a second CDAC coupled to a second input of the comparator and, in a second phase, to the sensor cell. The front-end circuitry provides a digital output. The digital output is proportional to a sensor capacitance of the sensor cell.