Fingerprint Sensor Readout Circuit With Drive Signal Nulling
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Solution Overview
Problem
Fingerprint sensors face challenges in capturing images from fingers farther away due to weakened spatial field strength variations, leading to difficulties in measurement resolution and noise interference, especially when high voltages are required, causing discomfort or damage.
Innovation Solution
A finger biometric sensing device with drive signal nulling circuitry and error compensation circuitry, including digital-to-analog converters and gain stages, to reduce the large drive signal component and compensate for errors, enhancing measurement sensitivity and reducing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the voltage on the finger is increased to compensate for weakened spatial field strength variations at greater distances, then the fingerprint pattern strength increases, but persons with sensitive fingers may feel a tingling sensation
Solution Approach 1:
The patent introduces a driven ground electrode as an intermediary element between the finger and the sensor array. This electrode creates an intermediate reference potential that allows the sensor to operate at higher voltages without directly exposing the finger to high voltage stress, thereby maintaining measurement precision while reducing the harmful tingling sensation.
Solution Approach 2:
The patent changes the voltage distribution parameters by implementing a driven ground system where the ground reference is actively controlled rather than fixed. This allows dynamic adjustment of voltage parameters to optimize the balance between signal strength and user comfort, enabling higher effective voltages without proportional increases in perceived discomfort.
2Measurement precision
If the voltage on the sensor array is increased to compensate for weakened spatial field strength variations at greater distances, then the fingerprint pattern strength increases, but the sensor readout electronics may saturate, generate noise, or be damaged
Solution Approach 1:
The patent segments the voltage handling functions by separating the high-voltage drive signal generation from the low-voltage signal readout. The sensor array elements can be driven at higher voltages to maintain field strength, while the readout electronics operate at safe low voltages, thus preserving measurement precision without compromising electronics reliability.
Solution Approach 2:
The patent introduces intermediate signal conditioning stages between the sensor array and the readout electronics. These intermediary circuits buffer and condition the signals, allowing the sensor array to operate at higher voltages for improved measurement precision while protecting the sensitive readout electronics from voltage-induced saturation, noise, or damage.
3Ease of operation
If the finger is placed farther away from the sensor array, then user comfort is improved, but the spatial field strength variations become weaker making measurement difficult
Solution Approach 1:
The patent implements a driven ground system that dynamically adjusts the reference potential based on the finger-sensor distance. This dynamic adaptation allows the system to maintain optimal measurement precision even when the finger is placed at comfortable distances, effectively decoupling user comfort from measurement quality.
Solution Approach 2:
The patent changes the operational parameters by actively controlling the ground reference voltage rather than keeping it fixed. This parameter change enables the system to compensate for distance-induced signal weakening, allowing users to place their fingers at more comfortable distances while maintaining measurement precision through real-time parameter adjustment.
4Reliability
If small spatial voltage differences are measured on top of a large common mode voltage, then finger drive can be maintained close to ground, but measurement of the small spatial voltage differences becomes difficult
Solution Approach 1:
The patent employs feedback mechanisms through the driven ground electrode that continuously monitors and adjusts the common mode voltage level. This feedback allows the system to maintain stable finger drive voltages close to ground while simultaneously compensating for the large common mode voltage, thereby preserving both reliability and measurement precision of small spatial voltage differences.
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
The solution effectively compensates for noise and increases measurement sensitivity, allowing for accurate fingerprint capture without discomfort or damage to the sensor electronics, even at greater distances and with varying finger contact.
Implementation Method 1
These systems measure the fingerprint pattern by establishing an electric field between the finger and the sensor array, and measuring the spatial fluctuations in field strength at the sensor array caused by the shape of the fingerprint ridge and valley pattern.
Implementation Method 2
The drive signal nulling circuitry may include a first digital-to-analog converter (DAC) capable of generating an inverted scaled replica of the drive signal for the at least one gain stage.
Implementation Method 3
At least one gain stage may be coupled to the array of finger biometric sensing pixel electrodes.
Data Source
AI summary
A finger biometric sensing device may include drive circuitry for generating a drive signal and an array of finger biometric sensing pixel electrodes cooperating with the drive circuitry and generating a detected signal based upon placement of a finger adjacent the array. The detected signal may include a drive signal component and a sense signal component superimposed thereon. A gain stage may be coupled to the array and drive signal nulling circuitry may be coupled to the gain stage for reducing the drive signal component from the detected signal. The drive signal nulling circuitry may include a first digital-to-analog converter (DAC) generating an inverted scaled replica of the drive signal for the gain stage. Error compensation circuitry includes a memory storing error compensation data and a second DAC coupled in series with the first DAC compensating an error in the inverted scaled replica based upon the error compensation data.