Capacitive Sensor Circuit With Boxcar Sampling and Feedback
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Solution Overview
Problem
Existing capacitive sensor circuits face challenges in achieving high linearity, stability against vibrations and electromagnetic interference, while maintaining a compact design and low power consumption, particularly in applications requiring differential signal processing.
Innovation Solution
A circuit design that operates in alternating modes, incorporating a GM stage, integrator, and hold circuit, with boxcar sampling and differential architecture, to convert sensor voltage to current, integrate and hold the signal, and provide feedback for noise reduction and differential signal processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a closed-loop architecture with force feedback is used to improve linearity and sensitivity, then measurement precision is improved, but device complexity and power consumption increase
Solution Approach 1:
The circuit operates in discrete time intervals with alternating modes (first mode for resetting/discharge, second mode for measurement/integration), segmenting the continuous operation into manageable phases that reduce complexity while maintaining precision
Solution Approach 2:
The circuit employs periodic boxcar sampling with alternating operational modes, where the GM stage and integrator are activated in specific time intervals to achieve high linearity and sensitivity through periodic measurement cycles rather than continuous operation
2Measurement precision
If boxcar sampling with GM stage and integrator is implemented to reduce noise and improve linearity, then measurement precision is improved, but circuit complexity increases
Solution Approach 1:
The GM stage, integrator, and hold circuit are merged into a single integrated circuit block that performs multiple functions (transconductance conversion, integration, signal holding) within one unified structure, reducing overall system complexity while achieving low noise and high linearity
Solution Approach 2:
The hold circuit provides feedback voltage to the capacitive sensor, creating a feedback mechanism that stabilizes the sensor operation and improves linearity without requiring a full force-feedback architecture, thus reducing complexity while maintaining precision
3Reliability
If alternating operational modes with hold circuit are used to maintain stability against vibrations and electromagnetic interference, then reliability is improved, but device complexity increases
Solution Approach 1:
The hold circuit captures and maintains the integrated voltage value before subsequent processing or conversion, preparing a stable reference value in advance that is immune to vibrations and electromagnetic interference occurring during later operation phases
Solution Approach 2:
The circuit dynamically switches between different operational modes (first mode for reset/discharge, second mode for measurement) with the second input contact of the GM stage switching between floating state and ground coupling, adapting its configuration to optimize stability against environmental disturbances
4Measurement precision
If differential architecture with boxcar sampling is implemented to reduce noise, then measurement precision is improved, but area of the circuit increases
Solution Approach 1:
The integrated circuit performs multiple functions including GM stage operation, boxcar sampling, integration, signal holding, and feedback provision within a single multi-functional block, achieving differential noise rejection without requiring separate dedicated circuits for each function, thus reducing overall circuit area
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 design achieves high linearity, stability, low noise sensitivity, and compact footprint, suitable for automotive applications, while reducing circuit complexity and energy consumption.
Implementation Method 1
The GM stage is configured to receive a sensor voltage from the capacitive sensor at the first input contact of the GM stage and to convert the sensor voltage into a sensor current
Implementation Method 2
convert the sensor voltage into a sensor current in order to charge a first boxing capacitance with the sensor current in the second time interval
Implementation Method 3
The integrator is configured to integrate a voltage applied across the first boxing capacitance over its time course in the second time interval
Implementation Method 4
The hold voltage is configured to tap the integrator's output voltage during the second time interval and maintain it as a hold voltage at an output of the hold circuit
Implementation Method 5
a circuit for operating a capacitive sensor
Data Source
Figure 1
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AI summary
The present invention relates to a circuit (1) for operating a capacitive sensor (10), which is designed to be operated alternatingly in a first mode over a first interval (Φ1) and in a second mode over a second interval (Φ2). Said circuit comprises: a gm stage (2) with a first input contact (20A) and a second input contact (20B), which is configured to receive a sensor voltage of the capacitive sensor (10) at the first input contact (20A) of the gm stage (2) and to convert the sensor voltage into a sensor current, to charge a first boxing capacitance (Cbox1) with the sensor current in the second interval (Φ2); an integrator (3), which is configured, in the second interval (Φ2), to integrate a voltage applied via the first boxing capacitance (Cbox1) over the time curve thereof and to output an output voltage (Voint) resulting therefrom at a first output (35A, 35B) of the integrator (3); and a hold circuit (4), which is configured to tap the output voltage (Voint) from the integrator (3) in the second interval (Φ2) and to maintain same at an output (46A, 46B) of the hold circuit (4) as a hold voltage (Vo_zm). The circuit (1) is configured to provide the hold voltage (Vo_zm) to the capacitive sensor (10) as a feedback voltage and to an analogue-digital converter (11) as an input voltage to be converted in the first and/or in the second interval (Φ1, Φ2) and to discharge the first boxing capacitance (Cbox1) in the first interval (Φ1).