Half-Bridge Fingerprint Sensing Circuit Noise Rejection
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Solution Overview
Problem
Existing fingerprint sensing technologies face challenges in accurately measuring differential capacitance while rejecting common mode noise, especially when a conductive object, such as a finger, is placed on the sensing surface, due to variations in manufacturing tolerances and non-repeatability of capacitance changes.
Innovation Solution
A differential capacitance measurement circuit using a half-bridge configuration with buried capacitors driven by complementary signals and a listener electrode for noise rejection, coupled with a differential amplifier to provide a digital representation of capacitance, ensuring balanced capacitance measurements despite the presence of a conductive object.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conductive object is placed on the sensing surface for fingerprint detection, then the capacitance measurement is affected by common mode noise, but the measurement accuracy deteriorates due to variations in manufacturing tolerances and non-repeatability of capacitance changes
Solution Approach 1:
The sensing surface is segmented into multiple independent sensing elements arranged in a matrix of row electrodes and column electrodes. Each intersection forms an independent mutual capacitor that can be measured separately, allowing the system to process capacitance changes locally rather than as a whole, which improves measurement precision while maintaining reliability
Solution Approach 2:
Buried electrodes are introduced as intermediary elements to provide a stable reference capacitance. These buried capacitors serve as a mediator between the variable sensing capacitance and the measurement circuit, providing a consistent baseline that compensates for manufacturing tolerances and environmental variations, thereby improving measurement precision
2Object-affected harmful factors
If traditional capacitance measurement circuits are used, then the circuit complexity is low, but the common mode noise rejection capability deteriorates
Solution Approach 1:
The measurement circuit uses differential voltage measurements instead of single-ended measurements. By measuring the voltage difference between two points rather than absolute voltage, the circuit inherently rejects common mode noise while maintaining relatively simple circuit implementation, thus improving noise rejection without significantly increasing complexity
Solution Approach 2:
The buried electrodes are configured to maintain equipotential conditions that provide a stable reference level. By ensuring that the buried capacitors maintain consistent potential relationships, the circuit achieves better common mode noise rejection while keeping the overall circuit design relatively simple through symmetric configuration
3Manufacturing precision
If manufacturing tolerances are relaxed to reduce production cost, then the ease of manufacture improves, but the capacitance balance between sensing elements deteriorates
Solution Approach 1:
The measurement system performs self-calibration using the buried electrodes as reference. The system automatically measures and compensates for capacitance variations in each sensing element relative to the stable buried reference, eliminating the need for high-precision manual matching during manufacturing. This self-service approach maintains high capacitance balance precision while significantly improving ease of manufacture
Solution Approach 2:
The system implements feedback compensation where the measured capacitance values are compared against reference values from buried electrodes, and adjustment signals are applied to compensate for manufacturing variations. This feedback mechanism ensures high capacitance balance precision is achieved automatically during operation, relaxing manufacturing precision requirements while maintaining overall system performance
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 solution effectively rejects common mode noise and maintains accurate capacitance measurement balance, even with variations in capacitance due to the presence of a conductive object, enhancing the reliability of fingerprint detection.
Implementation Method 1
A first mutual capacitor is formed between a row electrode and a column electrode of an array and a second mutual capacitor that is buried, or not alterable by a user
Implementation Method 2
The shared node between the first and second mutual capacitances may be coupled to a differential amplifier at a first input
Implementation Method 3
In one embodiment, the listener electrodes may be configured to provide enable mode noise rejection with the differential input stage of the differential amplifier
Implementation Method 4
The first mutual capacitor may be driven with a first signal and the second mutual capacitor may be driven with a signal that is complementary to the first signal
Data Source
AI summary
Fingerprint detection circuits with common mode noise rejection are described. The Fingerprint detection circuit includes a half-bridge circuit coupled to a receive (RX) electrode of an array of fingerprint detection electrodes and to a buried capacitance that is unalterable by the presence of a conductive object on the array. The fingerprint detection circuit may also include a listener electrode configured to enable common mode noise rejection through a differential input stage of a low noise amplifier (LNA).