Capacitive Sensor Amplifier Feedback for Parasitic Capacitance
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Solution Overview
Problem
Conventional capacitive sensor biasing schemes are sensitive to parasitic capacitances, leading to reduced sensitivity and increased noise due to charge redistribution and parasitic capacitance effects, which necessitate higher amplification factors.
Innovation Solution
A capacitive sensor amplifier circuit with a feedback capacitor connected across the amplifier, effectively counteracting static and parasitic capacitances, thereby increasing sensitivity and reducing the need to minimize parasitic capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional biasing scheme with a bias resistor is used, then the capacitive sensor can be biased to produce an output voltage signal, but the sensitivity is reduced due to parasitic capacitances at the output and amplifier input
Solution Approach 1:
A feedback capacitor is introduced as an intermediary element between the amplifier output and input. This feedback capacitor mediates the interaction between the amplifier and the capacitive sensor, effectively counteracting the harmful parasitic capacitances and restoring sensitivity without requiring changes to the sensor or amplifier themselves.
Solution Approach 2:
The patent implements a feedback mechanism by connecting a capacitor from the amplifier output back to the input. This feedback path allows the amplifier to compensate for the voltage division effect caused by parasitic capacitances, thereby maintaining high sensitivity despite the presence of unwanted capacitance at the output and input terminals.
2Measurement precision
If the amplification factor is increased to compensate for reduced sensitivity, then the output signal magnitude is improved, but noise is also increased
Solution Approach 1:
The feedback capacitor converts the harmful effect of parasitic capacitances into a beneficial effect. By carefully selecting the feedback capacitor value, the circuit transforms the voltage division caused by parasitic capacitance into a signal boosting mechanism, improving output magnitude without the need for high amplification that would simultaneously amplify noise.
3Reliability
If the bias resistor value is increased to reduce current leakage, then charge is better maintained on the capacitor, but the circuit becomes more sensitive to parasitic capacitances
Solution Approach 1:
The feedback capacitor serves as an intermediary that decouples the relationship between the high-value bias resistor and the parasitic capacitances. This allows the bias resistor to maintain its high value for good charge retention while the feedback capacitor compensates for the sensitivity to parasitic capacitances, resolving the contradiction between reliability and sensitivity to harmful factors.
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 enhances sensitivity and robustness to interference by boosting the output signal magnitude, making it less susceptible to noise and interference sources.
Implementation Method 1
A feedback capacitor is connected across the amplifier, using positive feedback, i.e. one plate of the feedback capacitor is connected to the non-inverting input 6 of the amplifier 4 and the other plate is connected to the non-inverting output 7 of the amplifier 4. By use of positive feedback a part of the output voltage signal is fed back to the capacitive sensor 1.
Implementation Method 2
In a general aspect therefore, the feedback capacitor is connected between an input and an output of the amplifier where the input and output are of the same sign. Other known arrangements in which a capacitor is provided in a feedback loop across an operational amplifier for amplifying signals from a capacitive sensor are known
Data Source
AI summary
A capacitive sensor amplifier circuit comprising: a capacitive sensor; a bias voltage supply connected across the capacitive sensor via a bias resistor; an operational amplifier having an input connected to the capacitive sensor; and a feedback capacitor connected between the input and an output of the amplifier, the input and output being of the same sign.