Capacitive Touch Measuring Circuit Guard Electrode Isolation
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Solution Overview
Problem
Capacitive touch-sensitive panels are sensitive to parasitic capacitors, which can affect measurement accuracy and introduce unwanted signals from the LCD screen and other sources, making it difficult to accurately detect finger proximity.
Innovation Solution
A measuring circuit that uses a floating reference potential and a guard electrode to isolate the capacitive electrodes from parasitic capacitors, and varies the modulation frequency to smooth the transfer function and reduce interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a charge amplifier with feedback capacitor is used to measure capacitive touch electrodes, then the measurement can be performed in the analog domain with direct voltage variation detection, but the circuit becomes extremely sensitive to parasitic capacitors between electrode input node and ground
Solution Approach 1:
The patent introduces a guard electrode as an intermediary element positioned between the capacitive touch electrode and the charge amplifier input. This guard electrode is driven at the same potential as the virtual ground of the charge amplifier, creating an equipotential region that eliminates voltage differences across parasitic capacitors. By placing this intermediary guard structure, the harmful voltage variations across parasitic capacitances are prevented, thereby eliminating their interfering effect on the measurement while preserving the direct analog detection capability.
2Measurement precision
If all voltages in the measuring circuitry are varied with respect to ground to achieve insensitivity to parasitic capacitors, then measurement precision improves, but the device complexity increases due to floating reference potential and guard electrode requirements
Solution Approach 1:
The patent applies the equipotentiality principle by driving the guard electrode at the same potential as the virtual ground of the charge amplifier. This creates an equipotential region between the guard electrode and the input node, ensuring that no voltage difference exists across parasitic capacitors connected to these nodes. By maintaining equipotential conditions, the circuit achieves insensitivity to parasitic capacitors without requiring complex floating reference potential circuitry, as the guard electrode simply follows the virtual ground potential.
3Measurement precision
If guard electrode is used to isolate capacitive electrode from external ground, then parasitic capacitors are eliminated, but unwanted signals from LCD screen and other sources can still affect measurement
Solution Approach 1:
The patent employs periodic modulation by varying the voltage of the guard electrode in a periodic manner (e.g., alternating between different potential levels). This periodic modulation allows the measurement system to distinguish between signals caused by finger proximity (which modulate at the guard electrode frequency) and unwanted signals from the LCD screen or other sources (which typically occur at different frequencies). By using synchronous detection tuned to the modulation frequency, the system can filter out non-modulated interference while preserving the desired capacitive touch signal.
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 effectively reduces the impact of parasitic capacitors and unwanted signals, improving measurement accuracy and sensitivity to finger proximity while minimizing interference from the LCD screen and other sources.
Implementation Method 1
the wire connecting Cin to the measurement circuitry may be uncoupled from the external ground by using a guard electrode. This guard electrode must then be connected to the internal or floating ground VF or to a node biased at a constant voltage with respect to VF, such that the capacitor between capacitive electrode and guard remain biased at a constant voltage and does not affect the measurement result
Implementation Method 2
it consists into varying the voltage of the capacitive electrode and detecting the corresponding charge variation across Cin. Since the current across the capacitor Cin only flows towards Cfb (the amplifier having high impedance inputs), the charge variation across Cin (and thus the value itself of Cin) may be determined from the voltage variation across feedback capacitor Cfb
Implementation Method 3
involving varying the modulation frequency of a modulating voltage so as to smooth the peaks in the transfer function
Data Source
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Figure 5~6
AI summary
A measuring circuit connectable to a capacitive touch-sensitive panel, the panel including a plurality of sense electrodes and optionally a common guard electrode, adapted to measure variations in the instantaneous electric capacity of the sense electrodes in response to proximity to conductive bodies, wherein the sense electrodes are biased at a fixed voltage relative to the common guard electrode, the measuring circuit comprising: a power management integrated circuit comprising a voltage source generating a modulation voltage that is available at a guard terminal of the power management integrated circuit that is in electric connection with the guard electrode; one or more slave integrated circuits, each connected to a plurality of sense electrodes and comprising a Capacity-to-Digital converter or a plurality of Capacity-to-Digital converters that are operatively arranged for generating digital measure codes representing the instantaneous electric capacity of sense electrodes; a means for varying the frequency of the modulation voltage. There is further provided a corresponding method for measuring the instantaneous electric capacity of the sense electrodes in a touch-sensitive panel.