ADV7125
Therefore, if we have a graphics system with a 1024 × 1024
resolution, a noninterlaced 60 Hz refresh rate, and a retrace
factor of 0.8, then:
Analog Outputs
The ADV7125 has three analog outputs, corresponding to the
red, green, and blue video signals.
The red, green, and blue analog outputs of the ADV7125 are
high impedance current sources. Each one of these three RGB
current outputs is capable of directly driving a 37.5 Ω load,
such as a doubly terminated 75 Ω coaxial cable. Figure 4a
shows the required configuration for each of the three RGB
outputs connected into a doubly terminated 75 Ω load. This
arrangement develops RS-343A video output voltage levels
across a 75 Ω monitor.
Dot Rate = 1024 ×1024 × 60 / 0.8
= 78.6 MHz
The required CLOCK frequency is thus 78.6 MHz.
All video data and control inputs are latched into the ADV7125
on the rising edge of CLOCK, as previously described in the
Digital Inputs section. It is recommended that the CLOCK
input to the ADV7125 be driven by a TTL buffer (e.g., 74F244).
A suggested method of driving RS-170 video levels into a 75 Ω
monitor is shown in Figure 4b. The output current levels of the
DACs remain unchanged, but the source termination resistance,
ZS, on each of the three DACs is increased from 75 Ω to 150 Ω.
Video Synchronization and Control
The ADV7125 has a single composite sync (SYNC) input con-
trol. Many graphics processors and CRT controllers have the
ability to generate horizontal sync (HSYNC), vertical sync
(VSYNC), and composite SYNC.
IOR, IOG, IOB
Z
= 75⍀
O
In a graphics system that does not automatically generate a
composite SYNC signal, the inclusion of some additional logic
circuitry enables the generation of a composite SYNC signal.
DACs
(CABLE)
Z
= 75⍀
S
Z
= 75⍀
L
(SOURCE
(MONITOR)
TERMINATION)
The sync current is internally connected directly to the IOG
output, thus encoding video synchronization information onto
the green video channel. If it is not required to encode sync
information onto the ADV7125, the SYNC input should be tied
to logic low.
TERMINATION REPEATED THREE TIMES
FOR RED, GREEN, AND BLUE DACs
Figure 4a. Analog Output Termination for RS-343A
Reference Input
The ADV7125 contains an on-board voltage reference. The
IOR, IOG, IOB
Z
= 75⍀
O
DACs
V
REF pin is normally terminated to VAA through a 0.1 µF capaci-
(CABLE)
Z
= 150⍀
(SOURCE
TERMINATION)
S
tor. Alternatively, the part could, if required, be overdriven by
Z = 75⍀
L
(MONITOR)
an external 1.23 V reference (AD1580).
A resistance, RSET, connected between the RSET pin and GND
determines the amplitude of the output video level according to
Equations 1 and 2 for the ADV7125:
TERMINATION REPEATED THREE TIMES
FOR RED, GREEN, AND BLUE DACs
Figure 4b. Analog Output Termination for RS-170
IOG * mA = 11,444.8 ×V
V / R
( )
Ω
( )
(1)
(
)
REF
SET
More detailed information regarding load terminations for various
output configurations, including RS-343A and RS-170, is avail-
able in an application note entitled, Video Formats and Required
Load Terminations available from Analog Devices,
IOR, IOB mA = 7,989.6 ×V
V / R
Ω
(2)
(
)
( )
( )
REF
SET
*Applies to the ADV7125 only when SYNC is being used. If SYNC is not being
encoded onto the green channel, Equation 1 will be similar to Equation 2.
(www.analog.com/library/applicationNotes/video/AN205.pdf).
Using a variable value of RSET allows for accurate adjustment of
the analog output video levels. Use of a fixed 560 Ω RSET resistor
yields the analog output levels quoted in the specification page.
These values typically correspond to the RS-343A video wave-
form values as shown in Figure 3.
Figure 3 shows the video waveforms associated with the three RGB
outputs driving the doubly terminated 75 Ω load of Figure 4a. As
well as the gray scale levels (black level to white level), the diagram
also shows the contributions of SYNC and BLANK for the
ADV7125. These control inputs add appropriately weighted cur-
rents to the analog outputs, producing the specific output level
requirements for video applications. Table I details how the SYNC
and BLANK inputs modify the output levels.
DACs
The ADV7125 contains three matched 8-bit DACs. The DACs
are designed using an advanced, high speed, segmented archi-
tecture. The bit currents corresponding to each digital input are
routed to either the analog output (bit = “1”) or GND (bit = “0”)
by a sophisticated decoding scheme. As all this circuitry is on
one monolithic device, matching between the three DACs is
optimized. As well as matching, the use of identical current sources
in a monolithic design guarantees monotonicity and low glitch.
The on-board operational amplifier stabilizes the full-scale
output current against temperature and power supply variations.
Grayscale Operation
The ADV7125 can be used for standalone, grayscale (mono-
chrome) or composite video applications (i.e., only one channel
used for video information). Any one of the three channels, red,
green, or blue, can be used to input the digital video data. The
two unused video data channels should be tied to logical zero.
The unused analog outputs should be terminated with the same
load as that for the used channel. In other words, if the red
–10–
REV. 0
ADI [ ADI ]
