Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2000-03-31
2001-05-22
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C348S308000
Reexamination Certificate
active
06236347
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to digital-to-analog converters (DACs), and more particularly to multiple-mode DACs with variable precision and range.
BACKGROUND OF THE INVENTION
Digital signals produced by computer systems are often converted to analog voltages to drive user interfaces such as cathode-ray tube (CRT) displays. Many kinds of digital-to-analog converters (DACs) are known and used. One common type drives a variable current through a fixed resistance to produce a variable voltage across the resistor. The current and thus the voltage are varied by switching on or off many current sources. The precision of the DEC is determined by the size of the smallest current source that can be switched.
FIG. 1
illustrates a common 8-bit DEC. An 8-bit digital value is encoded with a binary value from 0 to 255, representing a digitized voltage. Decoder
12
fully decodes the 8-bit binary value into 255 signals that are output to bus
16
. These 255 signals control 255 switchable current sources. As more of the current sources are switched on, the current output on line
14
increases, increasing the voltage across a resistance. For example, when the 8-bit binary value is 00000000, all current sources are off. When the binary value is 00000001, only one of the current sources, source
28
, is on. The current through line
14
is i. When the binary value is 00000010, two of the current sources, sources
28
,
22
, are on. The current through line
14
is 2i. For 00000011, three sources
28
,
22
,
20
are on, and the current output is 3i. The maximum current, 255i, is output when all 255 sources
28
,
22
,
20
. . .
24
,
26
are turned on.
Such a DEC is known as an 8-bit DAC, since it converts an 8-bit binary input into a current that varies in steps of {fraction (1/255)}
th
of the output range. The current sources must be carefully matched to produce the same output current i, which is a very small current. Otherwise, the output can be non-linear with distortions.
Sometimes a higher precision is required, such as for a higher-resolution or high-color display. The current and voltage range of the DAC can be doubled by doubling the number of current sources.
FIG. 2
shows a common 9-bit DAC. A 9-bit binary value is decoded into 511 signals by decoder
30
. The 511 signals from decoder
30
are output on bus
38
to 511 current sources.
The 511 signals from bus
38
control 511 current sources
28
,
22
,
20
, . . .
24
,
26
, . . .
34
,
36
. Each current source adds a current i to output line
14
. The total current output on line
14
varies from zero to 511i in increments of i.
The higher precision 9-bit DAC requires twice as many current sources as does the 8-bit DAC, even though the input value increased by just one binary bit. Complexity of the DAC increases significantly when precision is increased by just one binary input bit.
Variable-Precision DAC Needed—
FIG. 3
FIG. 3
highlights an application that could benefit from a variable-precision DAC. As features are added to personal computers (PCs), the hardware must be able to meet new demands. One feature being added to PCs is the ability to drive television monitors as well as CRT monitors. CRT monitors use computer-display standards such as VGA and SVGA. Television monitors use entirely different standards, such as the National Television Standards Committee (NTSC) format or the Phase-Alternating-Line (PAL) format.
Not only are different horizontal and vertical frequencies used for SVGA and NTSC, but voltage ranges also differ. NTSC requires a wider voltage range than SVGA.
Pixels generated by the PC may be displayed on either SVGA monitor
102
or NTSC TV monitor
104
. While SVGA monitor
102
requires a voltage range of V, NTSC TV monitor
104
requires a larger voltage range, up to 2V. Even when the NTSC voltage range is less than 2V, but above V, an additional (9
th
) input bit is used for NTSC pixels.
An 8-bit DAC can be used to drive SVGA monitor
102
, while a separate 9-bit DAC is used to drive NTSC TV monitor
104
. Although this is the most simple approach, the redundancy in DACs is undesirable. Instead, a variable DAC
100
is desirable. Such a variable DAC
100
would operate as an 8-bit DAC outputting a voltage range V when driving SVGA monitor
102
, but change modes to operate as a 9-bit DAC outputting a voltage range 2V when driving NTSC TV monitor
104
.
Some programmable or weighted DACs are known. See for example U.S. Pat. No. 5,570,090 by Cummins, assigned to Analog Devices Inc., and U.S. Pat. No. 4,482,887 by Crauwels, assigned to IBM Corp.
What is desired is a variable or programmable DAC. It is desired to operate the DAC with an 8-bit input for driving a SVGA monitor, but operate the DAC with a 9-bit input for driving a NTSC TV monitor. It is desired to operate the DAC with a wider output-voltage range for NTSC mode than for SVGA mode. A multi-mode DAC is desired.
SUMMARY OF THE INVENTION
A multi-mode digital-to-analog converter (DAC) has a bias-voltage generator for generating a bias voltage and a plurality of current sources. Each produces a current controlled by the bias voltage and outputs part of a base current. The base current is a current from zero to a maximum current with 2
N
−1 current increments, where N is a number of input bits to the multi-mode DAC in a lower-precision mode.
A least-significant-bit (LSB) current source produces a small current controlled by the bias voltage. It outputs the small current for adding to the base current. The LSB current source outputs the small current during a higher-precision mode in response to a least-significant bit of the input bits to the multi-mode DAC. The LSB current source does not output the small current during the lower-precision mode.
The small current is less than the current increments from the plurality of current sources. Thus the LSB current source outputs the small current for the higher-precision mode but not for the lower-precision mode.
In further aspects the plurality of current sources output a maximum current for the higher-precision mode that is double a maximum current output for the lower-precision mode. Thus currents are doubled for the higher-precision mode. The bias-voltage generator adjusts the bias voltage for the higher-precision mode by lowering the bias voltage. The bias voltage is thus changed when switching from the lower-precision mode to the higher-precision mode.
In still further aspects a digital input contains the input bits. The digital input includes N input bits for the lower-precision mode but N+1 bits for the higher-precision mode. N is 8 for the lower-precision mode but 9 digital bits are input to the multi-mode DAC for conversion to an analog voltage for the higher-precision mode. Thus the multi-mode DAC is an 8/9 bit DAC.
In other aspects a SVGA output drives a SVGA monitor with analog voltages converted from the N input bits when using the lower-precision mode. A NTSC output drives a NTSC TV monitor with the analog voltages converted from N+1 input bits when using the higher-precision mode. Thus the SVGA monitor is driven during the lower-precision mode but the NTSC TV monitor is driven by the multi-mode DAC during the higher-precision mode.
REFERENCES:
patent: 4016555 (1977-04-01), Tyrrel
patent: 4644325 (1987-02-01), Miller
patent: 5592166 (1997-01-01), Wincn
patent: 5600321 (1997-02-01), Wincn
patent: 5612696 (1997-03-01), Kim
patent: 5805095 (1998-09-01), Humphreys et al.
patent: 5831566 (1998-11-01), Ginetti
patent: 5870049 (1999-02-01), Huang et al.
Auvinen Stuart T.
Jean-Pierre Peguy
NeoMagic Corp.
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