Current-Mode CBC Circuit Using Single Capacitor and Current Mirrors
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Solution Overview
Problem
Capacitive sensing systems face challenges in maintaining a stable voltage bias at the receiver electrode due to interfering signals, which can cause the receiver input voltage to exceed rail voltages, leading to missed touch events.
Innovation Solution
A current-mode coarse-baseline-correction (CBC) circuit using a single passive capacitor and multiple current mirrors is implemented to source or sink current, maintaining the receiver voltage at a predetermined bias point, thereby preventing voltage rail saturation and allowing detection of touch events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a traditional CBC circuit with multiple passive capacitors is used, then the receiver voltage can be maintained at a bias point, but the circuit occupies excessive area and reduces dynamic range
Solution Approach 1:
The patent combines multiple passive capacitors into a single passive capacitor by using multiple current mirrors to generate multiple correction currents. The current mirrors replicate and scale a single correction current to provide the necessary charge compensation to multiple receiver electrodes, thereby reducing the total component count and circuit area while maintaining voltage stability at the bias point
Solution Approach 2:
The patent changes the approach from using multiple capacitors with different capacitance values to using a single capacitor with one capacitance value combined with current mirrors that adjust current magnitude through scaling factors. This parameter transformation allows the system to achieve the same voltage correction function with fewer components, improving area efficiency while preserving the ability to maintain receiver voltage at the predetermined bias point
2Reliability
If interfering signals are present at the receiver electrode, then the receiver input voltage exceeds rail voltages, but the current-mode CBC circuit provides headroom to prevent saturation
Solution Approach 1:
The patent applies preliminary anti-action by proactively compensating for voltage deviations at the receiver electrode before they cause saturation. The CBC circuit continuously monitors the receiver voltage and applies correction currents through current mirrors to counteract the effect of interfering signals, maintaining the voltage within the operational range and preventing rail saturation that would otherwise cause missed touch events
Solution Approach 2:
The patent introduces current mirrors as intermediary elements between the single passive capacitor and the multiple receiver electrodes. These current mirrors act as mediators that translate the charge correction from a single capacitor into multiple scaled correction currents, enabling precise voltage control at each receiver electrode and providing headroom against interfering signals
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
The current-mode CBC circuit reduces the size and number of passive components required, providing more headroom for the receiver to detect touch events while minimizing the impact of interfering signals, thus enhancing the dynamic range and accuracy of capacitive sensing.
Implementation Method 1
a current mirror where an input of the current mirror is electrically coupled to the capacitor and an output of the current mirror is coupled to the receiver electrode
Data Source
AI summary
The embodiments herein are generally directed to using a current-mode CBC circuit to maintain a voltage bias setting at a receiver when performing capacitive sensing. To do so, the CBC circuit may compensate for the change in voltage at a receiver by providing a current at the input of the receiver. Instead of using a passive CBC capacitor for each receiver, the input device may use a single CBC capacitor and a plurality of current mirrors to source and sink the current required to correct the input voltage at a plurality of receivers. As a result, the current-mode CBC circuit includes only one passive capacitor (or bank of capacitors) and a plurality of current mirrors which may provide space and cost benefits relative to a CBC circuit that uses a passive capacitor (or bank of capacitors) for each receiver channel.