Capacitive Sensor Electrode Parasitic Capacitance Mitigation
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Solution Overview
Problem
Common mode coupling and parasitic capacitances in fingerprint sensors using capacitive sensing interfere with measuring smaller capacitance values between sensor electrodes and a finger, making it difficult to accurately detect ridges and valleys.
Innovation Solution
A processing system that applies a charging voltage to a first sensor electrode and a neighboring sensor electrode, then drives the first sensor electrode to a reference voltage during a read stage, mitigating the effects of parasitic capacitance by ensuring no voltage difference between the electrodes, allowing for accurate measurement of charge stored on the first sensor electrode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sensor electrode is used to measure capacitance changes, then fingerprint detection is enabled, but common mode coupling from neighboring electrodes interferes with measurement accuracy
Solution Approach 1:
The patent applies equipotentiality by driving neighboring sensor electrodes to the same potential (reference voltage or charging voltage) during the read stage. This eliminates voltage differences between neighboring electrodes, thereby eliminating common mode coupling effects. Specifically, the controller circuit drives a first neighboring sensor electrode and a second neighboring sensor electrode to the same potential, which cancels out the capacitive coupling interference from these neighboring electrodes.
2Measurement precision
If sensor electrodes are used to detect finger capacitance, then biometric sensing is enabled, but parasitic capacitance from output lines overwhelms the small finger capacitance signal
Solution Approach 1:
The patent extracts and separately measures the parasitic capacitance component by driving the sensor electrode to the reference voltage and measuring the charge stored on the electrode. This extracted measurement represents the parasitic capacitance from the output line and other fixed capacitances. The system then subtracts this extracted parasitic capacitance value from the total capacitance measurement to obtain the finger capacitance signal, effectively removing the harmful parasitic effect.
Solution Approach 2:
The patent uses feedback by measuring the charge stored on the sensor electrode during the read stage and using this measurement to calculate and subtract the parasitic capacitance component. The controller circuit uses the measured charge information to compensate for parasitic effects, creating a feedback loop that continuously corrects for parasitic capacitance interference.
3Measurement precision
If the sensor electrode is driven to measure charge, then capacitance measurement is achieved, but voltage differences with neighboring electrodes create measurement errors
Solution Approach 1:
The patent maintains equipotential conditions by driving both the sensor electrode and neighboring electrodes to the same potential (either charging voltage or reference voltage) during different stages. This eliminates voltage differences that would cause capacitive coupling errors, ensuring accurate charge measurement without requiring complex compensation circuits.
Solution Approach 2:
The patent uses periodic action by alternating between a charge stage and a read stage. During the charge stage, electrodes are driven to charging voltage to charge the sensor electrode. During the read stage, electrodes are driven to reference voltage to measure the stored charge. This periodic switching enables separation of the charging function from the measurement function, simplifying the control approach.
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 improves the accuracy of capacitive sensing by isolating the capacitance between the sensor electrode and the finger from parasitic capacitance, enabling better detection of finger features like ridges and valleys.
Implementation Method 1
measure a first charge stored on the first sensor electrode resulting from applying the charging voltage
Data Source
AI summary
Embodiments herein describe an input device that includes a rectangular array of sensor electrodes connected to sensor modules that measure capacitive sensing signals corresponding to the electrodes. During a charge stage, the input device applies a charging voltage to neighboring sensor electrodes in the array. The input device then drives the neighboring sensor electrodes to a reference voltage and measures the amount of charge accumulated on at least one of the sensor electrodes. Because of the parasitic capacitance between the neighboring electrodes, driving these electrodes (even the ones not being measured) to the same charging and reference voltages reduces the effect of the parasitic capacitance on the capacitive sensing measurement. Thus, during the read stage, the measured charge is affected primarily by the capacitance between the sensor electrodes and an input object (e.g., a finger).