REV. 0

ADV7125

–9–

CIRCUIT DESCRIPTION AND OPERATION

The ADV7125 contains three 8-bit DACs, with three input

channels, each containing an 8-bit register. Also integrated on

board the part is a reference amplifier. CRT control functions

BLANK and SYNC are integrated on board the ADV7125.

Digital Inputs

Twenty-four bits of pixel data (color information) R0–R7, G0–G7,

and B0–B7 are latched into the device on the rising edge of

each clock cycle. This data is presented to the three 8-bit DACs

and then converted to three analog (RGB) output waveforms

(See Figure 2).

CLOCK

DATA

ANALOG OUTPUTS

(IOR, IOR, IOB

IOR, IOG, IOB)

DIGITAL INPUTS

(R7–R0, G7–G0, B7–B0,

SYNC, BLANK)

Figure 2. Video Data Input/Output

The ADV7125 has two additional control signals that are latched

to the analog video outputs in a similar fashion.

BLANK and

SYNC are each latched on the rising edge of CLOCK to maintain

synchronization with the pixel data stream.

The

BLANK and SYNC functions allow for the encoding of

these video synchronization signals onto the RGB video output.

This is done by adding appropriately weighted current sources

to the analog outputs, as determined by the logic levels on the

BLANK and SYNC digital inputs. Figure 3 shows the analog

output, RGB video waveform of the ADV7125. The influence of

SYNC and BLANK on the analog video waveform is illustrated.

Table I details the resultant effect on the analog outputs of

BLANK and SYNC.

All these digital inputs are specified to accept TTL logic levels.

Clock Input

The CLOCK input of the ADV7125 is typically the pixel clock

rate of the system. It is also known as the dot rate. The dot rate,

and thus the required CLOCK frequency, will be determined by

the on-screen resolution, according to the following equation:

Dot Rate = (Horiz Res)

× (Vert Res) × (Refresh Rate)/(Retrace Factor)

Horiz Res = Number of Pixels/Line

Vert Res = Number of Lines/Frame

Refresh Rate = Horizontal Scan Rate. This is the rate at which

the screen must be refreshed, typically 60 Hz for a noninterlaced

system or 30 Hz for an interlaced system.

Retrace Factor = Total Blank Time Factor. This takes into account

that the display is blanked for a certain fraction of the total

duration of each frame (e.g., 0.8).

RED, BLUE

GREEN

mA

V

mA

V

18.62

0.7

26.67

1.000

00

8.62

0.3

00

WHITE LEVEL

BLANK LEVEL

SYNC LEVEL

100 IRE

43 IRE

NOTES

1. OUTPUTS CONNECTED TO A DOUBLY TERMINATED 75

LOAD.

3. RS-343A LEVELS AND TOLERANCES ASSUMED ON ALL LEVELS.

Figure 3. RGB Video Output Waveform

Ω, R

Ω)

Description

IOG (mA)

IOG (mA)

IOR/IOB

IOR/IOB

SYNC

BLANK

DAC Input Data

WHITE LEVEL

26.67

0

18.62

0

1

1

FFH

VIDEO

Video + 8.05

18.62 – Video

Video

18.62 – Video

1

1

Data

VIDEO to

BLANK

Video

18.62 – Video

Video

18.62 – Video

0

1

Data

BLACK LEVEL

8.05

18.62

0

18.62

1

1

00H

BLACK to

BLANK

0

18.62

0

18.62

0

1

00H

BLANK LEVEL

8.05

18.62

0

18.62

1

0

xxH

SYNC LEVEL

0

18.62

0

18.62

0

0

xxH