Fingerprint Sensing Circuit With Stored Offset Cancellation
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Solution Overview
Problem
Existing fingerprint recognition systems using capacitive sensing face challenges in completely canceling common-mode signals due to mismatches between compensation and input capacitors, leading to inefficiencies in analog-to-digital conversion and degraded signal processing.
Innovation Solution
A signal processing circuit with a swapping switch configuration and storage capacitors to store and cancel offset information, ensuring complete removal of common-mode signals from the sensing signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If compensation capacitors are used to remove the common-mode part, then most of the unwanted CM part can be removed, but due to process variations, mismatch between compensation capacitors and input capacitors causes the CM part variation to not be entirely canceled
Solution Approach 1:
The patent applies preliminary action by performing offset calibration before normal signal processing. During calibration, the input capacitors are connected to a reference voltage to generate offset information, which is stored in storage capacitors. This preliminary offset storage enables complete cancellation of common-mode signals during subsequent signal processing, resolving the incomplete cancellation issue caused by capacitor mismatch.
Solution Approach 2:
The patent introduces storage capacitors as intermediaries to store offset information generated during calibration. These storage capacitors act as mediators between the input capacitors and the signal processing path, allowing the offset information to be preserved and applied for complete common-mode rejection without being affected by real-time signal variations.
2Measurement precision
If switch configuration is changed to swap input capacitors, then offset information can be generated and stored, but additional switching operations and calibration steps are required
Solution Approach 1:
The patent implements periodic action by performing offset calibration in periodic intervals rather than continuously. The switching operations to swap input capacitor connections are executed only during calibration phases, while normal signal processing uses the stored offset information. This periodic calibration approach reduces the frequency of complex switching operations while maintaining accurate offset cancellation.
Data Source
AI summary
A signal processing circuit for processing a sensing signal from a sensing circuit includes a plurality of input capacitors, an amplifier, an input switch group, a plurality of storage capacitors and a plurality of storage control switches. The plurality of input capacitors are configured to receive the sensing signal from one of a differential input nodes of the signal processing circuit and couple the sensing signal to a plurality of floating nodes. The amplifier, coupled to the plurality of floating nodes, is configured to amplify the sensing signal coupled from the plurality of floating nodes. The input switch group is coupled between the plurality of floating nodes and the plurality of input capacitors. The plurality of storage control switches, coupled between the plurality of floating nodes and the plurality of storage capacitors, are configured to couple offset information of the plurality of input capacitors to the plurality of storage capacitors.