adv101 Analog Devices, Inc., adv101 Datasheet - Page 6

• Manufacturer Part Number: adv101
• Description: Cmos 80 Mhz, Triple 8-bit Video Dac
• Manufacturer: Analog Devices, Inc.

ADV101
TERMINOLOGY
Blanking Level
The level separating the SYNC portion from the video portion
of the waveform. Usually referred to as the front porch or back
porch. At 0 IRE units, it is the level which will shut off the pic-
ture tube, resulting in the blackest possible picture.
Color Video (RGB)
This usually refers to the technique of combining the three pri-
mary colors of red, green and blue to produce color pictures
within the usual spectrum. In RGB monitors, three DACs are
required, one for each color.
Sync Signal (SYNC)
The position of the composite video signal which synchronizes
the scanning process.
Gray Scale
The discrete levels of video signal between reference black and
reference white levels. An 8-bit DAC contains 256 different lev-
els, while a 6-bit DAC contains 64.
CIRCUIT DESCRIPTION AND OPERATION
The ADV101 contains three 8-bit D/A converters, with three
input channels, each containing an 8-bit register. Also inte-
grated on board the part is a reference amplifier and CRT con-
trol functions BLANK, SYNC and REF WHITE.
Digital Inputs
24-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
is then converted to three analog (RGB) output waveforms.
(See Figure 2.)
Three other digital control signals are latched to the analog
video outputs in a similar fashion. BLANK, SYNC and REF
WHITE 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 ADV101. The influence of
SYNC and BLANK on the analog video waveform is illustrated.
(R0–R7, G0–G7, B0–B7;
ANALOG OUTPUTS
(IOR, IOG, IOB, I
DIGITAL INPUTS
SYNC, BLANK,
REFWHITE)
CLOCK
SYNC
Figure 2. Video Data Input/Output
)
–6–
Raster Scan
The most basic method of sweeping a CRT one line at a time to
generate and display images.
Reference Black Level
The maximum negative polarity amplitude of the video signal.
Reference White Level
The maximum positive polarity amplitude of the video signal.
Sync Level
The peak level of the SYNC signal.
Video Signal
That portion of the composite video signal which varies in gray
scale levels between reference white and reference black. Also re-
ferred to as the picture signal, this is the portion which may be
visually observed.
The REF WHITE control input drives the RGB video outputs
to the white level. This function could be used to overlay a cur-
sor or crosshair onto the RGB video output.
Table I details the resultant effect on the analog outputs of
BLANK, SYNC and REF WHITE.
All these digital inputs are specified to accept TTL logic levels
Clock Input
The CLOCK input of the ADV101 is typically the pixel clock
rate of the system. It is also known as the dot rate. The dot rate
and hence the required CLOCK frequency, will be determined
by the on-screen resolution, according to the following equation.
Horiz Res
Vert Res
Refresh Rate
Retrace Factor
DATA
Dot Rate = (Horiz Res)
Dot Rate =
(Retrace Factor)
= Number of pixels/line
= Number of lines/frame
= Horizontal Scan Rate. This is the rate at
= Total blank time factor. This takes into ac-
which the screen must be refreshed, typi-
cally 60 Hz for a noninterlaced system or
30 Hz for an interlaced system.
count that the display is blanked for a certain
fraction of the total duration of each frame
(e.g., 0.8).
(Vert Res)
(Refresh Rate)/
REV. B