Capacitive Sensor EMI Robustness via Chopping
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Solution Overview
Problem
Capacitive sensors in inertial systems face significant electromagnetic interference (EMI) due to capacitive coupling with bond wires and nearby electronics, which can obliterate desired signals, especially in high-density electronic environments, and existing solutions like shielding and differential approaches have limitations such as bulkiness and difficulty in maintaining precise capacitance matching.
Innovation Solution
A capacitive sensor system incorporating a differential amplifier and a chopping system with a high and low state, where the chopping system switches the connections between the capacitive core outputs and the differential amplifier inputs, applying step voltages to effectively smear EMI noise across a wide frequency range or away from a specific band, thereby reducing interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If shielding the sensor with metal is used to block external electric fields, then electromagnetic interference protection is improved, but device size and cost increase
Solution Approach 1:
The patent changes the operating parameters of the capacitive sensor by dynamically switching between two capacitance states (C1 and C2) using a chopping signal. This parameter modulation allows the sensor to operate in a regime where EMI appears as a common-mode signal that can be rejected by the differential amplifier, eliminating the need for bulky metal shielding while maintaining EMI protection.
Solution Approach 2:
The patent replaces the mechanical/physical shielding approach (metal Faraday cage) with an electronic signal processing approach. By using differential signaling and chopping techniques, the system achieves EMI rejection through electrical means rather than physical barriers, significantly reducing device volume.
2Object-affected harmful factors
If a differential approach is used to subtract electromagnetic disturbance, then EMI rejection is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent introduces dynamic switching between two capacitance states (C1 and C2) using a chopping signal that alternates at a specific frequency. This dynamic operation transforms the static differential measurement into a time-varying signal where EMI appears as a common-mode component. The differential amplifier naturally rejects this common-mode EMI signal, achieving robust EMI rejection without requiring extremely precise capacitance matching during manufacturing.
3Object-affected harmful factors
If differential signaling is used to cancel common mode signals, then EMI rejection is improved, but reliability decreases due to bond wire asymmetry
Solution Approach 1:
The patent employs periodic chopping action that switches the capacitive sensor between two states (C1 and C2) at a defined frequency. This periodic modulation causes the EMI-induced common-mode signal to also modulate at the same frequency. The differential amplifier configured with appropriate filtering can then reject this periodic common-mode interference, maintaining reliability even when bond wires experience physical disturbance or asymmetry.
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 approach significantly reduces electromagnetic interference by distributing noise across a wide frequency range, improving robustness by up to 40 dB, as demonstrated, and is applicable to various capacitive sensors like gyroscopes and accelerometers, overcoming the limitations of traditional shielding and differential methods.
Implementation Method 1
electromagnetic disturbance or interference (EMI) occurs primarily due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc.
Implementation Method 2
electromagnetic disturbance or interference (EMI) occurs primarily due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc.
Data Source
Figure 1~2
Figure 3
Figure 4A1~4C2
AI summary
A capacitive sensor system and method resistant to electromagnetic interference is disclosed. The system includes a capacitive core, differential amplifier with inverting and non- inverting inputs, capacitive paths, and chopping system. Core can include inputs and outputs coupled to variable capacitors, and common nodes coupling variable capacitors. Capacitive paths couple core outputs to amplifier inputs. When chopping system is high, one polarity voltage is applied to core inputs, a first core output is coupled to the inverting input and a second core output is coupled to the non-inverting input. When the chopping system is low, opposite polarity voltage is applied to core inputs, and core output to amplifier input couplings are flipped. Capacitive paths can include bond wires. Chopping system can be varied between high and low at frequencies that smear noise away from a frequency band of interest, or that smear noise substantially evenly across a wide frequency range.