adv476 Analog Devices, Inc., adv476 Datasheet - Page 8

• Manufacturer Part Number: adv476
• Description: Cmos Monolithic 256x18 Color Palette Ram-dac
• Manufacturer: Analog Devices, Inc.

ADV476
Reference Input
The ADV476 requires an active current reference to enable the
DACs provide stable and accurate video output levels. The rela-
tionship between the output voltage and the required input ref-
erence current is given by:
where
and
In a standard application which requires RS-343A video levels
to be driven into a doubly terminated 75
the necessary reference input current is:
To drive the same levels into a singly terminated 75
(R
A suggested current reference design for the doubly terminated
case, with RS-343A video levels and based on the LM334, a
three-terminal adjustable current source, is shown in Figure 6.
L
= 75 ), the reference current is:
R
VO = 0.714 V (RS-343A video levels)
L
= 37.5
= 75
= 1.0 V
I
REF
I
REF
I
VO ( FULL SCALE )
(for doubly terminated 75
(for singly terminated 75
(RS-170 video levels).
REF
= 8.88 mA.
= 4.44 mA.
2.15 R
Description
WHITE LEVEL
VIDEO
BLACK LEVEL
BLANK LEVEL
NOTE
1
Typical with full Scale RED, GREEN, BLUE = 19.05 mA. I
L
load (R
Figure 5. RGB Video Output Waveform
Table V. Video Output Truth Table
L
= 37.5 ),
load)
load
load)
RED, GREEN,
BLUE, (mA)
19.05
Video
0
0
–8–
1
Figure 6. Current Reference Design Using an LM334
Current Source
BLANK
1
1
1
0
REF
Input Data
DAC
FFH
DATA
00H
xxH
= 8.88 mA.
REV. B