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# New Circuit to Measure the Resistance And Capacitance of Liquid Crystal Cells

IP.com Disclosure Number: IPCOM000100402D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 103K

IBM

## Related People

Lien, SCA: AUTHOR [+2]

## Abstract

Disclosed is a new circuit for measuring the capacitance and resistance of a liquid crystal cell in simulated active matrix LCD operations.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

New Circuit to Measure the Resistance And Capacitance of Liquid Crystal Cells

Disclosed is a new circuit for measuring the capacitance
and resistance of a liquid crystal cell in simulated active matrix
LCD operations.

In an active matrix LCD (*) the active matrix element acts as a
switching device to charge each pixel to a desired voltage level
during the line scanning time, and then the charge in the pixel is
held during the holding period of time.  Since the active matrix
element usually has a very low off-state leakage current (10 E-12 amp
or less), the ability to hold the pixel charge depends mainly on the
dielectric constant and resistivity of the liquid crystal filled
inside the display.

In order to simulate the active matrix LCD operations and also
to measure the capacitance and resistance of a liquid crystal cell, a
circuit, shown in the figure, has been designed and tested.  In the
simulated active matrix LCD operations, the LCD cell is repeatedly
charged and discharged in positive and negative polarities.  Since
the electrical characteristics of the cell are similar to those of a
capacitor, the cell voltage can drift due to repeatedly unbalanced
charge and discharge cycles in both polarities. The design principle
of the circuit is to adjust the amplitude of the negative charge
cycle according to the average DC voltage of the cell so that the DC
voltage can be maintained near zero volt.  From the predetermined
positive charge current Icharge the output V+ of buffer A1 is
selected as: V+ = Icharge  . Rfdk                   (1)

The negative charge current is determined by the output of
amplifier A2.  This amplifier inverts the sum of V+, output from A1,
and WV, feedback from A3.  Two analog switches and two resistors
combine the outputs of A1 and A2 into a square wave.  The output of a
flip-flop is used as the clock signal of the switches and sets the
frequency of the square wave.  The flip-flop divides the frequency of
a square wave, from an external function generator, by two.  A
current source, which consists of the amplifier A4 and feedback
resistor Rfdk, is used to charge the LCD cell.  The charge time is
controlled by the pulses, which are fed to the analog switch S3 f...