Browse Prior Art Database

Comparator Reference Circuit

IP.com Disclosure Number: IPCOM000101043D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 95K

Publishing Venue

IBM

Related People

Haigh, DC: AUTHOR [+2]

Abstract

Described is a way of obtaining a sufficiently accurate comparator reference voltage without the expense of a dedicated voltage reference device, for use in video subsystems in personal computers. The disclosed circuit permits improvements to: tolerance of the comparator reference voltage; accuracy of the nominal comparator reference voltage and accuracy of the nominal digital-to-analog converter (DAC) reference voltage. Benefits are at the cost of one extra resistor.

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

Comparator Reference Circuit

       Described is a way of obtaining a sufficiently accurate
comparator reference voltage without the expense of a dedicated
voltage reference device, for use in video subsystems in personal
computers.  The disclosed circuit permits improvements to: tolerance
of the comparator reference voltage; accuracy of the nominal
comparator reference voltage and accuracy of the nominal
digital-to-analog converter (DAC) reference voltage. Benefits are at
the cost of one extra resistor.

      Video subsystems used in personal computer systems have
analogue comparators on the video outputs which compare the voltage
on these outputs with a fixed voltage, called the comparator
reference voltage which is usually half the full-scale video output
voltage.  The state of the comparators may be  read by the software
for diagnostic purposes and to ascertain whether the attached monitor
is mono or colour.

      In prior art, two ways of generating the comparator reference
voltage have been used.  On some planars, the voltage generated by a
dedicated voltage reference device is reduced by a potential divider
to the desired voltage.  On other adapter cards, a potential divider
reduces the analogue  volt supply to the desired voltage.  The
first way provides a more precise voltage, but is more expensive than
the second way, which does not use a dedicated voltage reference
device.  The accuracy of the second way is sufficient  when the
offset voltage of the comparators is small enough; a sufficiently low
offset voltage is achievable with bipolar comparators.

      Consider a video subsystem where the video comparators are
integrated with a serializer/palette/DAC chip, which uses CMOS
circuitry.  Analogue comparators built from CMOS devices exhibit a
larger offset voltage than bipolar comparators, so that the accuracy
of the comparator reference voltage provided by the simpler and
cheaper method of the prior art is not adequate.  DACs used in prior
art need a reference current to define the DAC output current. In the
considered video subsystem, DACs use a reference voltage and a
resistor.  The simplest way to generate an accurate comparator
reference voltage would be to reduce this DAC reference voltage with
a potential divi...