Capacitive Image Sensor Symmetrical Readout Circuit Noise Cancellation

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Solution Overview

Problem

Capacitive image sensing devices are affected by system voltage noise, leading to incorrect sensing signals due to the amplification of common-mode disturbances in readout circuits.

Innovation Solution

A capacitive image sensing device with a symmetrical readout circuit structure using two charge amplifiers and a differential amplifier, where the second charge amplifier provides a reference voltage signal to offset common-mode noise, effectively eliminating interference from power noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a general readout circuit is used to convert charge to sensing signals, then the circuit structure is simple, but the sensing signal is affected by system voltage noise resulting in incorrect sensing

Engineering Contradiction:

Improvesensing signal accuracyVSAvoidreadout circuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The readout circuit is segmented into two symmetrical charge amplifier paths: one for sensing signals and one for reference voltage signals. This segmentation allows independent processing of sensing and reference paths, enabling noise cancellation while maintaining circuit modularity and manageable complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent converts the harmful system voltage noise into a beneficial reference signal. By introducing a reference charge amplifier that experiences the same noise and passing its output through a subtractor, the noise is subtracted from the sensing signal, transforming the harmful noise into a useful reference for cancellation.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Object-affected harmful factors

If a symmetrical readout circuit with two charge amplifiers is used, then power noise interference is eliminated, but the device complexity increases

Engineering Contradiction:

Improvepower noise interferenceVSAvoidreadout circuit structure

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

While the overall circuit structure is symmetrical, the patent introduces asymmetry in the signal processing path by applying the reference signal through a subtractor to cancel noise. This controlled asymmetry in the feedback and subtraction paths enables noise elimination while maintaining the symmetrical charge amplifier configuration.

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The patent introduces a subtractor as an intermediary component between the sensing charge amplifier and the output. This subtractor mediates the noise cancellation process by subtracting the reference voltage signal (containing noise) from the sensing signal, thereby eliminating power noise interference.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Reliability

If reference sensing electrodes are disconnected from charge amplifiers during heavy pressing, then noise from heavy pressing is reduced, but the circuit switching complexity increases

Engineering Contradiction:

Improvesensing accuracy under heavy pressingVSAvoidcircuit switching control

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent implements dynamic switching control where reference sensing electrodes are selectively connected or disconnected based on pressing conditions. The switching circuit responds to detected heavy pressing by dynamically reconfiguring the circuit topology, connecting reference electrodes in parallel to provide a stable reference voltage that is insensitive to pressing variations.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent changes the electrical connection parameters of reference sensing electrodes based on pressing conditions. During normal operation, reference electrodes are connected individually to charge amplifiers. During heavy pressing, the switching circuit changes the parameter by connecting reference electrodes in parallel, thereby altering the equivalent capacitance and providing noise-resistant reference signals.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS11015914B2Capacitive image sensing device and capacitive image sensing method

Publication Date: 2021.05.25 NOVATEK MICROELECTRONICS CORP

AI summary

A capacitive image sensing device and a capacitive image sensing method are provided. The capacitive image sensing device includes a sensor array, a first charge amplifier, a second charge amplifier, a differential amplifier and a first switching circuit. The sensor array includes a plurality of sensing electrodes and a first reference sensing electrode. An input terminal of the first charge amplifier is coupled to one of the sensing electrodes. A first input terminal of the differential amplifier is selectively coupled to an output terminal of the first charge amplifier. A second input terminal of the differential amplifier is coupled to an output terminal of the second charge amplifier. The first switching circuit is configured to selectively electrically connect and disconnect the first reference sensing electrode and the input terminal of the second charge amplifier.