Ultrasonic Fingerprint Sensor Common-Mode Cancellation for Foldable Displays

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Solution Overview

Problem

Designing under-display ultrasonic sensor systems, particularly for foldable devices, is challenging due to signal trapping within display layers, leading to a lowered signal-to-noise ratio and ADC saturation, which impairs fingerprint detection accuracy.

Innovation Solution

Incorporating a resistor-inductor-capacitor (RLC) network and a piezoelectric layer with common mode canceling capacitive elements or circuitry to attenuate or cancel the common mode signal before it reaches the pixel array, enhancing the signal-to-noise ratio and preventing ADC saturation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If under-display ultrasonic sensor systems are designed for foldable devices, then device integration and form factor are improved, but signal trapping within display layers causes lowered signal-to-noise ratio and ADC saturation

Engineering Contradiction:

Improvedevice integrationVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Volume of moving objectVSReliability

Solution Approach 1:

A common mode canceling circuit is introduced as an intermediary component between the ultrasonic sensor and the pixel array. This circuit includes a capacitive element connected in parallel with the pixel array and a feedback path that generates a feedback signal to cancel the common mode signal, thereby mediating the harmful signal trapping effect while maintaining the compact device integration

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The common mode canceling circuit employs feedback mechanism where the received acoustic signal is processed to generate a feedback signal that is fed back through the RLC network to counteract the common mode component. This feedback loop continuously adjusts to maintain optimal signal-to-noise ratio while preserving the integrated device structure

Inventive Principle:

Principle #23Feedback

2Reliability

If common mode canceling circuitry is added to attenuate common mode signal, then signal-to-noise ratio is improved, but device complexity increases

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidcircuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The common mode canceling circuit is designed to be self-regulating through its feedback mechanism. The capacitive element automatically adjusts the feedback signal based on the received acoustic signal characteristics, eliminating the need for external control systems. This self-service approach improves signal-to-noise ratio while minimizing the increase in device complexity

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The circuit uses a capacitive element with specific capacitance value (Cpiezo/N) that is predetermined based on the piezoelectric layer properties. By optimizing this parameter, the circuit achieves effective common mode cancellation with minimal complexity. The RLC network parameters are also tuned to match the acoustic signal characteristics, achieving efficient noise reduction without requiring complex control logic

Inventive Principle:

Principle #35Parameter changes

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 significantly improves the fidelity and reliability of ultrasonic fingerprint sensing by mitigating common mode noise, allowing for accurate fingerprint pattern detection.

Implementation Method 1

a piezoelectric layer, wherein each pixel within the pixel array is coupled with at least a portion of the piezoelectric layer

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

a common mode canceling capacitive element electrically coupled with the RLC network in parallel with the pixel array and configured to attenuate at least a portion of the common mode component

Methodology Applied

Scientific EffectCapacitance filtering: Capacitance

Data Source

PatentUS20260073727A1Ultrasonic fingerprint sensor with passive and active common mode cancellation

Publication Date: 2026.03.12 QUALCOMM INC

AI summary

An apparatus for common mode noise cancellation in an ultrasonic fingerprint sensor comprising a resistor-inductor-capacitor (RLC) network, a piezoelectric layer, a pixel array electrically coupled with the RLC network at a first end of the RLC network and either: (A) a common mode canceling capacitive element electrically coupled with the RLC network in parallel with the pixel array and configured to attenuate at least a portion of the common mode component from the acoustic signal before the acoustic signal is received by the pixel array or (B) a common mode canceling circuitry electrically coupled with the RLC network at a second end of the RLC network and configured to introduce a feedback signal to the pixel array through the RLC network, wherein the feedback signal is determined based on the received acoustic signal at each pixel within the pixel array.