ADV476KN50 Просмотр технического описания (PDF) - Analog Devices

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ADV476KN50

CMOS Monolithic 256x18 Color Palette RAM-DAC

Analog Devices 

ADV476

Figure 5. RGB Video Output Waveform

Table V. Video Output Truth Table

Description

RED, GREEN,

DAC

BLUE, (mA)1 BLANK Input Data

WHITE LEVEL

VIDEO

BLACK LEVEL

BLANK LEVEL

19.05

Video

0

0

1

FFH

1

DATA

1

00H

0

xxH

NOTE

1Typical with full Scale RED, GREEN, BLUE = 19.05 mA. IREF = 8.88 mA.

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:

I REF

=

VO (FULL SCALE

2.15 × RL

)

where RL = 37.5 Ω (for doubly terminated 75 Ω load)

= 75 Ω (for singly terminated 75 Ω load)

and VO = 0.714 V (RS-343A video levels)

= 1.0 V (RS-170 video levels).

In a standard application which requires RS-343A video levels

to be driven into a doubly terminated 75 Ω load (RL = 37.5 Ω),

the necessary reference input current is:

IREF = 8.88 mA.

To drive the same levels into a singly terminated 75 Ω load

(RL = 75 Ω), the reference current is:

IREF = 4.44 mA.

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.

Figure 6. Current Reference Design Using an LM334

Current Source

–8–

REV. B

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