Capacitive Input Sensing Circuit for High-Sensitivity Low-Power Detection
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Solution Overview
Problem
Existing self-capacitance type sensors face challenges in increasing sensitivity while maintaining low power consumption and preventing output voltage from exceeding the power supply range, due to limitations in amplifying the AC voltage and the influence of parasitic capacitors.
Innovation Solution
The input device incorporates a current output circuit that generates a periodically changing detection voltage, amplifies the drive current, and includes a detection current correction circuit to adjust for parasitic capacitors, allowing for increased sensitivity without exceeding the power supply voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the amplitude of the AC voltage Vs is increased to increase sensitivity, then the detection sensitivity is improved, but the output voltage Vout exceeds the power supply voltage range causing distortion
Solution Approach 1:
The patent introduces a shield electrode as an intermediary element between the detection electrode and the object. This shield electrode is driven at the same potential as the detection electrode, creating an electrostatic shield that reduces parasitic capacitance effects. By placing this intermediary shield, the system can achieve higher detection sensitivity without the output voltage exceeding the power supply range, as the parasitic capacitance that causes voltage distortion is minimized.
Solution Approach 2:
The patent changes the electrical parameters of the system by introducing a shield electrode driven at the same potential as the detection electrode. This parameter change (adding a controlled electrostatic field) modifies the capacitance distribution in the system, reducing the parasitic capacitance between the detection electrode and surrounding conductive structures. This allows the AC voltage amplitude to be increased for higher sensitivity without causing output voltage distortion.
2Measurement precision
If the amplitude of the AC voltage Vs is increased to increase sensitivity, then the detection sensitivity is improved, but the power consumption increases
Solution Approach 1:
The shield electrode acts as an intermediary that reduces parasitic capacitance, allowing the system to achieve high detection sensitivity with lower AC voltage amplitudes. By minimizing the parasitic capacitance through this electrostatic shield, the required drive voltage can be reduced, thereby lowering power consumption while maintaining high detection sensitivity.
3Device complexity
If parasitic capacitors are not corrected, then the circuit is simpler, but the detection accuracy decreases due to parasitic capacitor influence
Solution Approach 1:
The patent applies preliminary action by introducing a shield electrode that is driven at the same potential as the detection electrode before the actual detection process. This preliminary electrostatic shielding configuration proactively reduces the parasitic capacitance effects, preventing detection accuracy degradation without requiring complex post-processing correction circuits. The shield electrode is configured in advance to counteract the parasitic capacitance influence.
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 configuration enables high sensitivity in detecting minute electrostatic capacitance changes while maintaining low power consumption and correcting for parasitic capacitor errors, thereby improving the overall detection capability.
Implementation Method 1
an electrostatic capacitance value of a detection-target capacitor formed between the detection electrode and the object changes according to the proximity of the object
Implementation Method 2
a current output circuit which outputs a drive current to the detection electrode such that a predetermined detection voltage having periodically changing level is generated in the detection electrode
Data Source
Figure 1~2
Figure 3
Figure 4A~4B
AI summary
In a case where the amplitude of a detection voltage Vs is increased, the amplitude of a drive current Is flowing in a detection-target capacitor Cs also increases, and thus, the amplitude of a detection current Izm also increases. In this case, the increase in amplitude of the detection current Izm is not directly restricted by the condition of a power supply voltage range (GND to VDD) that enables a circuit to operate. For this reason, for example, a current conversion ratio "α" of a current output circuit 10, a capacitance value "C" of a capacitor Cf of a current-voltage conversion circuit 20, or the like is set to an appropriate value so as not to cause a voltage Vo to exceed the power supply voltage range, whereby it is possible to prevent the amplitude of the detection current Izm from being restricted by the condition of the power supply voltage range. Therefore, it is possible to increase the amplitude of the detection voltage Vs to a maximum within the power supply voltage range.