Touch Device Signal Processing Circuit Offset Compensation

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Solution Overview

Problem

The offset voltage of operational amplifiers in touch devices reduces the signal dynamic range, degrading the signal-to-noise ratio and operating performance of touch panels.

Innovation Solution

A signal processing circuit with an integrated programmable gain amplifier (IPGA) that includes an attenuation circuit and a step current circuit, where the step current circuit uses a current mirror with switching devices to provide or draw a step current, reducing the offset current and maintaining signal dynamic range, and the attenuation circuit reduces the gain of the IPGA without increasing the compensation capacitor size.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the offset voltage of the operational amplifier is reduced to increase signal dynamic range, then the signal-to-noise ratio improves, but the manufacturing precision requirements increase

Engineering Contradiction:

Improvesignal dynamic rangeVSAvoidoperational amplifier offset voltage

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

The patent extracts the offset voltage compensation function from the operational amplifier itself and implements it externally through the step current circuit. By measuring the offset voltage separately and compensating it through an external current source, the system achieves offset compensation without requiring ultra-high manufacturing precision of the operational amplifier, thus resolving the contradiction between measurement precision and manufacturing precision.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces an intermediary step current circuit that acts as a mediator between the operational amplifier and the signal processing chain. This intermediary circuit measures the offset voltage and provides compensating current, thereby improving the signal dynamic range without directly modifying the operational amplifier's internal structure, which would require higher manufacturing precision.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If a compensation capacitor is used to reduce offset voltage effects, then the signal dynamic range increases, but the occupied area in the circuit increases

Engineering Contradiction:

Improvesignal dynamic rangeVSAvoidcompensation capacitor area

Core Design Contradiction:

Measurement precisionVSArea of stationary object

Solution Approach 1:

The patent replaces the traditional capacitive compensation method with an active electronic compensation approach using the step current circuit. Instead of using a large compensation capacitor to counteract offset voltage effects, the system uses a controlled current source to actively compensate the offset, thereby achieving the same signal dynamic range improvement without occupying additional circuit area.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the compensation approach from passive capacitive compensation to active current-based compensation. By dynamically adjusting the step current parameter based on the measured offset voltage, the system achieves effective offset compensation without requiring large capacitor values, thus resolving the contradiction between signal dynamic range and occupied area.

Inventive Principle:

Principle #35Parameter changes

3Stability of the object's composition

If the gain of the IPGA is reduced to prevent output saturation, then the signal dynamic range is maintained, but the attenuation circuit complexity increases

Engineering Contradiction:

Improveoutput saturation preventionVSAvoidattenuation circuit

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

The patent merges the attenuation function with the existing IPGA structure by integrating the attenuation circuit within the feedback path of the operational amplifier. This unified approach allows gain control and saturation prevention to be achieved through a single integrated circuit block, minimizing additional complexity while maintaining signal dynamic range stability.

Inventive Principle:

Principle #5Merging (Combining)

Data Source

PatentUS10613686B2Touch device and signal processing circuit as well as operating method thereof

Publication Date: 2020.04.07 PIXART IMAGING INC

AI summary

A signal processing circuit of a touch device including an operational amplifier, a feedback resistor and a step current circuit is provided. The feedback resistor connects between a negative input and an output terminal of the operational amplifier. The step current circuit is coupled to the negative input of the operational amplifier and configured to provide or draw a step current to reduce the current flowing through the feedback resistor so as to compensate the voltage offset of the operational amplifier.