Capacitive Sensor Read-Out Circuit With Offset Noise Cancellation
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Solution Overview
Problem
Capacitive sensors face challenges in generating accurate signals due to noise introduced when a terminal is grounded, affecting the measurement of capacitance changes.
Innovation Solution
A read-out circuit design incorporating an operational amplifier, feedback capacitor, sensor charging/discharging circuit, and switching circuits to manage the charging and discharging of sensor capacitors, along with an offset removal circuit to cancel noise effects, allowing for precise capacitance measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a terminal of the capacitor is grounded in the capacitive sensor, then noise may be introduced through the terminal, making it difficult to generate an accurate signal
Solution Approach 1:
The patent extracts the harmful ground connection from the capacitive sensor circuit by using a differential amplifier configuration that references both capacitor terminals to virtual ground potentials without physically grounding either terminal. This removes the noise introduction path while maintaining circuit functionality.
Solution Approach 2:
The patent introduces virtual ground nodes created by the differential amplifier as intermediaries between the capacitor terminals and the reference potential. These virtual grounds provide stable reference points without creating direct physical ground connections that would introduce noise.
2Measurement precision
If a switching circuit is used to charge/discharge the sensor capacitor, then capacitance measurement can be performed, but switching noise may affect measurement precision
Solution Approach 1:
The patent employs periodic charging and discharging cycles of the sensor capacitor through the switching circuit, synchronized with the differential amplifier's measurement phases. This periodic action allows for correlated double sampling that eliminates switching noise from the measurement.
Solution Approach 2:
The differential amplifier provides feedback through its operational amplification and virtual ground creation, allowing the circuit to compensate for switching noise by continuously adjusting the virtual ground potentials to maintain balance during switching transitions.
3Measurement precision
If an offset removal circuit is added to cancel noise effects, then measurement accuracy improves, but device complexity increases
Solution Approach 1:
The differential amplifier serves multiple functions simultaneously: it amplifies the differential signal from the capacitor, creates virtual ground references, and performs offset removal through its balanced configuration. This multi-functionality achieves noise cancellation without requiring separate dedicated offset removal circuits.
Solution Approach 2:
The patent merges the signal amplification function and the offset removal function into a single differential amplifier stage. The amplifier's inherent differential configuration naturally rejects common-mode offsets and noise, combining multiple benefits in one circuit element rather than requiring separate stages.
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 cancels noise from power supply and ground voltage sources, enabling accurate measurement of capacitance changes and reducing the need for additional correction operations.
Implementation Method 1
an operational amplifier configured to receive input voltage via a positive input terminal
Implementation Method 2
a feedback capacitor connected between an output terminal of the operational amplifier and a negative input terminal of the operational amplifier
Implementation Method 3
a sensor charging/discharging circuit configured to charge or to discharge a sensor capacitor included in a sensor during a first time
Implementation Method 4
a switching circuit configured to connect the sensor capacitor and the operational amplifier during a second time after the sensor capacitor is charged or discharged
Implementation Method 5
an offset removal circuit including an offset capacitor connected between a first node and a second node and configured to charge or discharge the offset capacitor during the first time
Data Source
AI summary
A read-out circuit includes an operational amplifier configured to receive input voltage via a positive input terminal; a feedback capacitor connected between an output terminal of the operational amplifier and a negative input terminal of the operational amplifier; a sensor charging/discharging circuit configured to charge or to discharge a sensor capacitor included in a sensor during a first time; and a switching circuit configured to connect the sensor capacitor and the operational amplifier during a second time after the sensor capacitor is charged or discharged.