Fingerprint Image Sensor Readout for Source Follower Linearity
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Solution Overview
Problem
Nonlinearity in the image sensing pixels of CMOS image sensors, particularly due to the body effect of the source follower, leads to suboptimal fingerprint image quality.
Innovation Solution
The image sensing device incorporates a transistor and a switch to compensate for nonlinearity and voltage drops in the source follower and sensing line switch, ensuring a linear relationship between the sensing voltage and the output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a source follower is used in the image sensing pixel to convert photo diode voltage to sensing voltage, then the sensing function is achieved, but nonlinearity is generated due to the body effect
Solution Approach 1:
An additional transistor is introduced as an intermediary element between the source follower and the readout circuit. This transistor acts as a mediator to compensate for the nonlinearity generated by the source follower's body effect, thereby improving the linearity of the sensing voltage without altering the core sensing mechanism
Solution Approach 2:
The invention changes the electrical parameters (voltage and current) by introducing a compensating transistor that adjusts the sensing voltage levels. This parameter adjustment compensates for the nonlinearity introduced by the source follower, ensuring more accurate fingerprint image capture
2Productivity
If switches are used in the sensing line to transmit sensing voltages at different times, then the scan timing function is achieved, but voltage drops occur in the sensing line
Solution Approach 1:
The additional transistor provides a feedback mechanism that compensates for voltage drops in the sensing line. By monitoring and adjusting the sensing voltage levels, the system maintains accurate voltage transmission despite the presence of switches in the sensing line, thus preserving measurement precision while enabling fast scanning
3Productivity
If multiple sensing lines are used to read pixel arrays, then the reading capacity is increased, but nonlinearity and voltage drops accumulate across multiple lines
Solution Approach 1:
The solution segments the compensation function by assigning individual compensating transistors to different sensing lines or pixel groups. This segmentation allows each line to be independently compensated for nonlinearity and voltage drops, maintaining signal quality across multiple parallel reading channels and thereby supporting high reading capacity without signal degradation
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 compensation method results in a linear output voltage that improves the quality of fingerprint images by correcting nonlinearity issues, enhancing the accuracy of fingerprint identification.
Implementation Method 1
The image sensing pixel may sense light and convert the light intensity into an electronic signal
Data Source
AI summary
An image sensing device for sensing an image sensing pixel array through a plurality of sensing lines is provided. The image sensing device includes a first switch, a second switch, a transistor, and a current source. A first terminal of the first switch is coupled to a first sensing line among the sensing lines. A first terminal of the second switch is coupled to a second sensing line among the sensing lines. A first terminal of the transistor is coupled to a second terminal of the first switch and a second terminal of the second switch. The current source is coupled to a second terminal of the transistor.