Segmented Resistor String DAC for LCD Gray Scale Generation
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Solution Overview
Problem
Conventional digital-to-analog converters (DACs) for liquid crystal displays face challenges with exponentially increasing resistor and switch counts, leading to space occupation issues and accuracy problems, especially near reference voltage levels, due to parasitic wiring resistance and nonlinear gray scale requirements.
Innovation Solution
A DAC design with a resistor string divided into upper, lower, and midrange voltage sections, using selectors to reduce circuit elements and a midrange voltage generator to improve accuracy, allowing for gamma corrections within the linear part of the gamma curve, thereby reducing parasitic loads and increasing output levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the number of output levels is increased to provide more vivid color display, then the display quality is improved, but the number of resistors and switches increases exponentially occupying more area
Solution Approach 1:
The resistor string is divided into multiple segments corresponding to different gray scale ranges (first range, second range, third range). Each segment is independently controlled by dedicated switches, allowing the circuit to achieve high-resolution output levels without requiring a single exponentially large resistor string and switch matrix.
2Area of stationary object
If the number of channels is increased to drive large area displays, then the display area is improved, but the parasitic wiring resistance and circuit elements concentrate at single points causing voltage distortion
Solution Approach 1:
The resistor string and switching circuit are segmented into multiple independent sections, each serving a specific gray scale range. This distribution of circuit elements across different physical locations prevents concentration of parasitic elements at single points, thereby reducing voltage distortion even when driving hundreds of channels for large display areas.
3Manufacturing precision
If gamma correction is applied near upper and lower reference voltage levels, then the accuracy is improved, but the nonlinear gray scale requires complex correction circuits
Solution Approach 1:
Different gray scale ranges are assigned different characteristics: the first and third ranges (near reference voltage levels) are designed with nonlinear spacing to provide gamma correction, while the second range (mid-gray levels) uses linear spacing. This local differentiation allows accurate voltage generation across the full range without requiring complex global correction circuits.
Data Source
AI summary
An analog-to-digital converter has a resistor string that generates a series of voltages that are equally spaced in the middle range of the series and unequally spaced at the upper and lower ends. An upper selector selects voltages at the upper end. A lower selector selects voltages at the lower end. A pair of midrange selectors select a pair of adjacent voltages in the middle range. A midrange voltage generator generates further voltages equally spaced between the two selected midrange voltages. An output selector selects one of the further voltages. The selectors are controlled by various bits of a digital input signal. The voltage selected by the upper selector, lower selector, or output selector becomes an analog output signal. This analog-to-digital converter has comparatively few resistors and transistors and can generate accurate voltages for driving a gray-scale display.