Capacitor Sensing System With Kelvin And AC Coupling
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Solution Overview
Problem
Capacitive sensors face challenges in accuracy due to large variations in capacitor values and total resistance from switches and long routing in the external capacitor charging path, leading to errors in sensing physical quantities like touch.
Innovation Solution
The implementation of Kelvin sensing and AC coupling in the front-end of the capacitive sensing system improves accuracy and flexibility by isolating error sources, using a current digital to analog converter (DAC) and comparators to adjust the charging current based on voltage comparisons between external and reference capacitors, and employing AC coupling capacitors to match internal capacitances and reject common-mode errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional charging path is used to charge the external capacitor, then the device complexity is reduced, but the measurement precision deteriorates due to large variations in capacitor values and total resistance from switches and long routing
Solution Approach 1:
The charging path is segmented into two separate paths: a first path for charging the external capacitor and a second path for sensing the voltage. This segmentation allows the sensing path to be isolated from the high-current charging path, reducing the impact of resistance variations and capacitor value variations on measurement precision.
Solution Approach 2:
An AC coupling capacitor is introduced as an intermediary element in the sensing path. This capacitor blocks DC offset voltages and common-mode errors while allowing AC signal transmission, thereby improving capacitance sensing accuracy without requiring a completely redesigned charging path.
2Measurement precision
If Kelvin sensing and AC coupling are implemented in the front-end, then the measurement precision improves by isolating error sources, but the device complexity increases due to additional components and circuit structures
Solution Approach 1:
The front-end circuit is segmented into distinct functional blocks: a charging circuit for applying test signals, a sensing circuit for measuring voltage, and a processing circuit for calculating capacitance. This modular segmentation improves measurement precision by isolating error sources in each block while making the overall complexity manageable through structured design.
Solution Approach 2:
AC coupling capacitors are used as intermediary elements between the charging circuit and the sensing circuit. These capacitors block DC offset and common-mode errors from propagating to the sensing stage, thereby improving measurement precision without requiring complete redesign of the front-end architecture.
3Ease of operation
If the charging current path includes long routing and switches, then the ease of operation is improved for flexible capacitor selection, but the measurement precision deteriorates due to total resistance effects
Solution Approach 1:
The circuit is segmented into a high-current charging path that can use long routing and switches for flexible capacitor selection, and a low-current sensing path that uses minimal routing and no switches to achieve high voltage sensing accuracy. This segmentation allows each path to be optimized for its specific function without compromising the other.
Solution Approach 2:
AC coupling capacitors serve as intermediaries that transfer the voltage signal from the charging path to the sensing path while blocking the effects of resistance in the charging path. This allows flexible capacitor selection in the charging path without degrading voltage sensing accuracy in the sensing path.
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 the accuracy and flexibility of capacitive sensing systems by reducing errors and improving the matching of internal capacitances, thereby enhancing the rejection of common-mode errors and supply perturbations, leading to more precise determination of capacitance changes.
Implementation Method 1
a pad for coupling to an external capacitor. A first current source is coupled to the pad through a first path and supplies current to charge the external capacitor
Implementation Method 2
The comparator compares a first voltage at the pad corresponding to a voltage across the external capacitor and a predetermined voltage and supplies a first compare indication indicative of the comparison
Data Source
AI summary
A capacitor sense system includes a pad for coupling to an external capacitor. A current digital to analog converter (DAC) supplies current to charge the external capacitor. A reference capacitor is charged by a current source. A first comparator compares a voltage across the external capacitor sensed at the pad to a reference voltage and generates a first comparison. A second comparator compares a voltage across a reference capacitor to the reference voltage and generates a second comparison. The stored first and second comparisons are used to control the current DAC. First and second AC coupling capacitors are coupled respectively between the pad and the first comparator and between the reference capacitor and the second comparator. Sensing at the pad allows more accuracy and the AC coupling capacitors provide better matching and allow for different DC biases to be set for the external capacitor and the first comparator.