Capacitive Sensing Analog Front-End Noise Reduction
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Solution Overview
Problem
Existing capacitance sensing circuitry for touch screens faces challenges in achieving a high signal-to-noise ratio (SNR) due to small capacitance changes caused by user input, which are masked by intrinsic and external noise, limiting resolution and requiring improved noise reduction techniques.
Innovation Solution
The proposed solution involves processing noise in the analog domain by accumulating and filtering it, while amplifying the signal, thereby enhancing the signal-to-noise ratio at the output of the analog accumulator before digital conversion, which maximizes the useful signal and reduces noise limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a unity gain low pass filter is used after the capacitance-to-voltage converter, then noise filtering is provided, but the signal magnitude is minimized to provide headroom for external noise, limiting the resolution
Solution Approach 1:
The system dynamically adjusts the gain of the low pass filter based on the accumulated signal level. When the accumulated signal is small, the filter gain is higher to provide noise headroom. When the accumulated signal is large, the filter gain is reduced to preserve resolution. This dynamic adjustment resolves the contradiction between noise filtering and resolution.
Solution Approach 2:
The patent changes the parameter of filter gain dynamically during operation. The filter gain is adjusted as a function of the accumulated signal level, transforming the static filter into an adaptive one that can optimize both noise filtering and resolution under different operating conditions.
2Use of energy by moving object
If the signal swing is maximized at the output of the low pass filter, then headroom for external noise is provided, but the white noise level must be greater than 1.5 LSB of the analog-to-digital converter
Solution Approach 1:
The system performs preliminary accumulation of the capacitance signal before filtering and ADC conversion. By accumulating multiple samples beforehand, the signal swing is increased naturally, and the signal-to-noise ratio is improved. This preliminary action allows the subsequent filter to work with a larger signal, reducing the impact of white noise relative to the signal level.
Solution Approach 2:
The accumulator continuously sums the capacitance samples over multiple cycles, maintaining a continuously growing signal level. This continuous accumulation ensures that the signal swing remains maximized throughout the measurement period, while the correlated noise averages down, effectively reducing the white noise impact.
3Measurement precision
If high resolution ADCs are used to improve measurement precision, then quantization error is reduced, but power dissipation increases
Solution Approach 1:
The system performs preliminary signal conditioning and accumulation before the ADC conversion. By pre-processing the signal to maximize swing and minimize noise, the effective resolution requirement of the ADC is reduced. This allows the use of lower resolution (and thus lower power) ADCs while maintaining the same effective measurement precision.
Solution Approach 2:
The accumulator and dynamic filter act as intermediary stages between the capacitance sensor and the ADC. These intermediaries perform signal enhancement and noise reduction, effectively pre-conditioning the signal so that a lower resolution ADC can achieve the same effective precision as a higher resolution ADC would provide directly.
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 results in a higher SNR at the input of the analog-to-digital converter, allowing for lower resolution ADCs without quantization error limitations, reducing ADC power dissipation, and improving noise reduction, thereby enhancing the accuracy of capacitance sensing.
Implementation Method 1
the analog accumulator includes a low pass filter having a frequency response to filter wideband noise
Implementation Method 2
an amplifier configured to amplify the accumulated analog value
Data Source
AI summary
Capacitance sensing circuits and methods are provided. The capacitance sensing circuit includes a capacitance-to-voltage converter configured to receive a signal from a capacitance to be sensed and to provide an output signal representative of the capacitance, an output chopper configured to convert the output signal of the capacitance-to-voltage converter to a sensed voltage representative of the capacitance to be sensed, an analog accumulator configured to accumulate sensed voltages during an accumulation period of NA sensing cycles and to provide an accumulated analog value, an amplifier configured to amplify the accumulated analog value, and an analog-to-digital converter configured to convert the amplified accumulated analog value to a digital value representative of the capacitance to be sensed. The analog accumulator may include a low pass filter having a frequency response to filter wideband noise.