Capacitive Sensor Readout Circuit for EMI Error Cancellation
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Solution Overview
Problem
Existing capacitive sensor readout circuits face challenges in reducing electromagnetic interference (EMI) effects, particularly in terms of alternating current (AC) and direct current (DC) interference, which can impact the accuracy and reliability of sensor measurements.
Innovation Solution
The proposed circuit operates a capacitive sensor in three distinct modes: one for offset correction and precharging, another for high impedance to allow EMI-induced DC errors to couple in, and a third for integrating these errors with opposite signs to compensate for them. This approach utilizes a double integration method to reduce both AC and DC effects of EMI and noise contributions from the circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If chopping is used to reduce electromagnetic interference, then AC EMI is distributed across the sampling frequency bandwidth, but interference at the chopping frequency is sampled back to DC and impacts the baseband
Solution Approach 1:
The patent implements a periodic three-mode operation cycle that alternates between feedback mode, high-impedance mode, and reference mode. This periodic switching at a frequency different from the chopping frequency prevents EMI aliasing into the baseband by ensuring that DC errors couple during the high-impedance phase and are subsequently integrated with opposite signs during reference voltage application, thereby canceling them out while maintaining continuous signal processing.
2Measurement precision
If a feedback loop is used to provide voltage to the capacitive bridge, then the sensor can be read out, but EMI-induced DC errors are introduced into the measurement
Solution Approach 1:
The patent converts the harmful EMI-induced DC errors into a beneficial cancellation mechanism. During the high-impedance mode, EMI errors are allowed to couple into the feedback node and are integrated by the integrator. In the subsequent reference mode, the same integrator integrates the errors with opposite signs when reference voltages are applied, thereby canceling the DC errors while preserving the sensor signal.
Solution Approach 2:
The patent dynamically changes the impedance parameter of the feedback node by switching between low-impedance (during feedback and reference modes) and high-impedance states. This parameter change enables the system to selectively couple EMI-induced DC errors during the high-impedance phase while maintaining normal feedback operation during other phases, allowing error separation and subsequent cancellation through differential integration.
3Productivity
If the sensor output is continuously connected to the feedback circuit, then the sensor operates continuously, but noise and EMI are continuously coupled into the circuit
Solution Approach 1:
The patent implements periodic disconnection of the sensor output from the feedback circuit by switching the sensor output to high impedance during dedicated time intervals. This periodic disconnection prevents continuous noise and EMI coupling while maintaining continuous sensor operation through the alternating three-mode cycle, ensuring that the sensor signal is processed continuously but error coupling is minimized through temporal separation.
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 significantly improves electromagnetic compatibility (EMC) and noise performance by effectively compensating for EMI-induced DC errors, thereby enhancing the accuracy and reliability of capacitive sensor measurements.
Implementation Method 1
An integrator integrates the current over a time course and outputs an output voltage resulting therefrom
Implementation Method 2
The circuit uses a double integration method to significantly reduce both the DC effects of EMI and the noise contribution of the circuit
Implementation Method 3
Capacitive sensors can be read using appropriate readout circuits
Data Source
AI summary
A circuit for operating a capacitive sensor which can be operated in first to third modes. A GM stage receives a sensor voltage applied to a sensor output of the capacitive sensor and outputs a current. An integrator integrates the current over a time course and outputs an output voltage resulting therefrom. The circuit provides the output voltage to an analog-to-digital converter. In the first and second modes, the output voltage is provided to the capacitive sensor as a feedback voltage. In the first mode, an offset correction is further performed in the GM stage and the integrator. In the second mode, the sensor output of the capacitive sensor is switched to high impedance. In the third mode, a reference voltage is provided to the capacitive sensor.