Figure 8 – Typical RGB Graphics System
SPT5140
(Master)
I
Set
R
SPT5140
(Slave)
I
Set
G
SPT5140
(Slave)
I
Set
B
Ref Out
Ref In
Ref In
R
1
500
Ω
R
2
750
Ω
I
Set
Ref In
I
Ref
500
Ω
750
Ω
I
Set
1kΩ
500
Ω
750
Ω
I
Set
1 kΩ
TYPICAL RGB GRAPHICS SYSTEM
In an RGB graphics system, the color displayed is deter-
mined by the combined intensities of the red, green and
blue (RGB) D/A converter outputs. A change in gain or
offset in any of the RGB outputs will affect the apparent
hue displayed on the CRT screen.
Thus, it is very important that the outputs of the D/A con-
verters track each other over a wide range of operating
conditions. Since the D/A output is proportional to the
product of the reference and digital input code, a com-
mon reference should be used to drive all three D/As in
an RGB system to minimize RGB DAC-to-DAC mismatch
and improve TC tracking.
The SPT5140 contains an internal precision bandgap
reference which completely eliminates the need for an
external reference. The reference can supply up to 50
µA
to an external load, such as two other DAC reference
inputs.
The circuits shown in figure 8 illustrate how a single
SPT5140 may be used as a master reference in a system
with multiple DACs (such as RGB). The other DACs are
simply slaved from the SPT5140’s reference output.
SPT5140
9
3/28/00
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
