Capacitive Sensing Analog Front-End SNR Optimization
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Solution Overview
Problem
Existing capacitance sensing circuitry for touch screens faces limitations in achieving high signal-to-noise ratio (SNR) due to small capacitance changes caused by user input, which are masked by intrinsic and external noise, leading to resolution issues and conflicts between signal magnitude and noise levels.
Innovation Solution
The proposed solution involves processing noise in the analog domain by accumulating and filtering it, while amplifying the signal, thereby enhancing the SNR at the output of the analog accumulator before digital conversion, primarily limiting the SNR by the noise performance of the analog circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a capacitance-to-voltage converter followed by a unity gain low pass filter and analog-to-digital converter is used, then the system can provide noise filtering and digital conversion, but the resolution is limited by the conflict between signal magnitude (minimized for headroom) and white noise level (should be greater than 1.5 LSB)
Solution Approach 1:
The patent changes the gain parameter dynamically by using programmable gain amplifiers (PGAs) that can be adjusted based on the accumulated capacitance value. This allows the system to adapt the amplification level to match the signal magnitude, ensuring that the signal充分利用 the ADC's dynamic range while maintaining adequate noise margin. The gain is adjusted in steps (e.g., 1x, 2x, 4x, 8x) based on the measured capacitance range.
Solution Approach 2:
The system implements dynamic adaptation by continuously monitoring the capacitance measurement and adjusting the PGA gain accordingly. The circuit transitions from a static unity-gain architecture to a dynamic variable-gain architecture that optimizes the signal-to-noise ratio for each measurement cycle based on the actual capacitance value being measured.
2Reliability
If the signal magnitude is minimized to provide headroom for external noise, then the system can accommodate noise variations, but the white noise level becomes insufficient (less than 1.5 LSB of the ADC)
Solution Approach 1:
The patent applies preliminary filtering action by implementing a low-pass filter before the ADC that removes high-frequency noise components from the signal. This preliminary noise reduction allows the system to use higher signal magnitudes without being overwhelmed by external noise, as the filter has already attenuated the noise spectrum above the cutoff frequency.
Solution Approach 2:
The system implements continuous averaging of multiple capacitance measurements to maintain a steady signal level that充分利用s the ADC range while maintaining noise immunity. By continuously accumulating measurements and applying running averages, the system maintains optimal signal levels without requiring large headroom for noise variations.
3Measurement precision
If a high-speed high-resolution ADC is used to improve measurement precision, then the capacitance sensing resolution improves, but the power dissipation and cost increase significantly
Solution Approach 1:
The patent applies partial averaging by accumulating a limited number of capacitance measurements (e.g., 4, 8, or 16 samples) rather than using excessive averaging. This partial action provides sufficient noise reduction to enable the use of lower-resolution, lower-power ADCs while maintaining adequate measurement precision for the application requirements.
Solution Approach 2:
The system replaces the need for high-speed ADC conversion with analog-domain signal processing techniques including programmable gain amplification and low-pass filtering. This substitution allows slower, lower-power ADCs to achieve equivalent or better effective resolution by performing preprocessing in the analog domain before digital conversion.
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 maximizes the signal swing at the input of the analog-to-digital converter, reducing the requirements for ADC speed and power dissipation, allowing for lower resolution ADCs without SNR limitations, and improves SNR by minimizing intrinsic noise, especially in differential circuit configurations.
Implementation Method 1
The low pass filter has a frequency response to filter wideband noise
Implementation Method 2
an amplifier to amplify the accumulated analog value
Implementation Method 3
an analog accumulator to accumulate sensed voltages during an accumulation period of NA sensing cycles
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.