SPT
8
5/14/97
SPT1018
Figure 4 - CONVert, CONVert Switching Levels
V
IDF
V
ICM MAX
0.0 V
-1.3 V
CONV
CONV
V
ICM MIN
Figure 5A - Standard Load
OUT +
Video Monitor
Inverse
Video
OUT-
RL
75
Ω
75
Ω COAX
RS
75
Ω
RL
75
Ω
75
Ω COAX
RS
75
Ω
SPT1018
Figure 5B - Test Load
Video Out
0 to -1 Volt
OUT +
OUT -
RL
37.5Ω
CL
<5 pF
TYPICAL RGB GRAPHICS SYSTEM
In an RGB graphics system, the color displayed is determined
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 convert-
ers track each other over a wide range of operating condi-
tions. Since the D/A output is proportional to the product of the
reference and digital input code, a common 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 SPT1019 contains an internal precision bandgap refer-
ence 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. (See
the SPT1019 data sheet).
The circuits shown in figure 6 illustrate how a single SPT1019
may be used as a master reference in a system with multiple
DACs (such as RGB). The other DACs are simply slaved from
the SPT1019’s reference output. The SPT1018s shown are
especially well-suited to be slaved to a SPT1019 for a better
TC tracking from DAC-to-DAC, since they are essentially
SPT1019s without the reference. The SPT1018 is pin-com-
patible with the TDC1018, that does not have an internal
reference. Although either the TDC1018 or the SPT1018 may
be slaved from an SPT1019, the higher performance
SPT1018 and the above mentioned DAC-to-DAC TC track-
ing is the best choice for new designs.
No external reference is required for operation of the
SPT1019, as this function is provided internally. The internal
reference is a bandgap type and is suitable for operation over
extended temperature ranges. The SPT1018 must use an
external reference.
Figure 6 - Typical RGB Graphics System
Ref-
Ref+
750
Ω
500
Ω
750
Ω
500
Ω
R
G
1 k
Ω
Ref-
Ref+
B
1 k
Ω
Ref Out
ISet
R2
750
Ω
R1
500
Ω
IRef
ISet
ISet
ISet
SPT1019
(Master)
SPT1018
(Slave)
SPT1018
(Slave)
Figure 7 - Burn-In Circuit
CONV
1 k
Ω
100
Ω
6.5
Ω
-1.2 V
(Max 1.5 mA)
1 k
Ω
24 Pin DIP
All Resistors Are 5% 1/4 W cc
Clock = -0.9 to -1.7 V, 100 kHz
Ref+
CONV
1 k
Ω
VCC
Clock
(Max 60 µA)
-5.9 V
(Max 200 mA)
(Max 50 mA)
Out-
Ref-
VEE
Out+
-1.3 V
(Max 60 µA)
1 k
Ω
6.5
Ω
100
Ω
(Max 50 mA)
(Max 1.5 mA)
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