Capacitive Sensor Interface Circuit Using Periodic Voltage Switching
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Solution Overview
Problem
Existing electronic interface circuits for capacitive sensors face challenges in achieving accurate and efficient measurement of physical parameters like acceleration, force, and pressure due to non-linearities and voltage deviations, which affect signal-to-noise ratio and measurement speed, especially at low voltage levels.
Innovation Solution
The proposed method involves a new electrode excitation scheme that applies significant voltage variations between measurement phases, using a digital-to-analog converter with a resistive divider and multiplexers to generate reference voltages, allowing for improved signal-to-noise ratio and rapid stabilization of digital measurement signals without increasing current draw or supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional electrode excitation with fixed voltage levels is used, then the electronic circuit operates simply, but the signal-to-noise ratio is poor and measurement sensitivity is low
Solution Approach 1:
The patent implements periodic switching between two excitation voltage levels (first voltage level and second voltage level) applied to the fixed electrodes during different phases of the measurement cycle. This periodic action creates time-varying electrostatic forces that enable differential measurement and significantly improve signal-to-noise ratio compared to static excitation schemes.
Solution Approach 2:
The patent changes the voltage parameter of the fixed electrodes dynamically during measurement phases. By switching between distinct voltage levels (e.g., VDD and VSS, or different reference voltages), the system creates measurable variations in capacitance that enhance detection sensitivity while maintaining circuit simplicity through digital control.
2Measurement precision
If high supply voltage is used to improve signal-to-noise ratio, then measurement sensitivity increases, but power consumption increases
Solution Approach 1:
Instead of maintaining continuously high voltage, the patent uses periodic switching between voltage levels during measurement phases. The duty cycle can be optimized to achieve sufficient signal-to-noise ratio while minimizing average power consumption. The differential measurement approach extracts maximum signal information from the voltage transitions.
Solution Approach 2:
The measurement system uses the voltage switching itself to generate the measurement signal through capacitance variation. The electrostatic force changes during voltage transitions directly produce the measurable effect, eliminating the need for separate high-power signal generation circuits.
3Measurement precision
If complex electronic interface circuits are used to compensate for non-linearities and voltage deviations, then measurement accuracy improves, but device size and power consumption increase
Solution Approach 1:
The patent extracts and eliminates the source of error by using symmetric differential measurement. By measuring the difference between two capacitors subjected to opposite voltage variations, common-mode non-linearities and voltage deviations are naturally rejected, eliminating the need for complex compensation circuits.
Solution Approach 2:
The patent introduces controlled asymmetry in the excitation scheme where the first and second voltage levels are applied differently to the two fixed electrodes during measurement phases. This asymmetric excitation pattern, combined with differential reading, creates a measurement signal that is inherently insensitive to symmetric errors like substrate potential effects and component mismatches.
4Productivity
If traditional measurement methods are used, then the circuit operates simply, but measurement speed is slow and stabilization time is long
Solution Approach 1:
The patent uses periodic voltage switching at optimized frequencies to accelerate measurement convergence. The alternating excitation phases rapidly drive the system toward equilibrium, reducing stabilization time. The periodic action also enables frequency-domain signal processing that can extract measurements more quickly from noisy data.
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 enhances measurement sensitivity and signal-to-noise ratio by up to 1.65 times compared to traditional methods, while reducing the size and power consumption of integrated components and eliminating voltage deviations, enabling quick and accurate digital output.
Implementation Method 1
the fixed electrodes of two capacitors or pairs of capacitors are biased or cyclically excited by voltages of opposite polarity with respect to a resting reference voltage
Implementation Method 2
The capacitive sensor is made up of at least two capacitors mounted in differential. A common electrode of the capacitors is capable of moving between two fixed electrodes under the action, for example, of a force to modify the capacitive value of each capacitor.
Data Source
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AI summary
The method allows a physical parameter to be measured by an electronic circuit (1) connected to a capacitive sensor (2) with two capacitors (C1, C2). The electronic circuit includes an amplifier (4) connected to the common electrode (CM) of the capacitors, a logic unit (5) for digital processing of the amplifier information and the provision of a digital measurement signal, a digital-to-analog converter (7) to provide a measurement voltage based on the digital measurement signal, a switching unit (3) to alternately provide to the first and second fixed electrodes of the capacitors, the measurement voltage, and a regulated voltage (VREG) of a negative bias or a low voltage (VSS) of a positive bias from a supply voltage source.For positive bias, the first phase (P1) consists of biasing the first fixed electrode with the measurement voltage (VDAC_Pp) defined by the first binary word (DACbus) and the reference voltage (Vref), and the second fixed electrode with the low voltage (VSS), and a second phase (P2) consists of biasing the second fixed electrode with the measurement voltage (VDAC_Pn) defined by the second binary word and the reference voltage (Vref), the second binary word being defined as the inverse of the first binary word with respect to a reference binary word.