Datasheet

ADV7125
Rev. C | Page 12 of 16
Table 7. Typical Video Output Truth Table (R
SET
= 530 Ω, R
LOAD
= 37.5 Ω)
Video Output Level IOG (mA)
IOG
(mA)
IOR/IOB (mA)
IOR
/
IOB
(mA)
SYNC
BLANK
DAC Input Data
White Level 26.0 0 18.67 0 1 1 0xFFH
Video Video + 7.2 18.67 − Video Video 18.67 − Video 1 1 Data
Video to BLANK
Video 18.67 − Video Video 18.67 − Video 0 1 Data
Black Level 7.2 18.67 0 18.67 1 1 0x00H
Black to BLANK
0 18.67 0 18.67 0 1 0x00H
BLANK Level
7.2 18.67 0 18.67 1 0 0xXXH (don’t care)
SYNC Level
0 18.67 0 18.67 0 0 0xXXH (don’t care)
VIDEO SYNCHRONIZATION AND CONTROL
The ADV7125 has a single composite sync (
SYNC
) input
control. Many graphics processors and CRT controllers have the
ability to generate horizontal sync (HSYNC), vertical sync
(VSYNC), and composite
SYNC
.
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.
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.
REFERENCE INPUT
The ADV7125 contains an on-board voltage reference. The V
REF
pin should be connected as shown in Figure 10.
A resistance, R
SET
, connected between the R
SET
pin and GND,
determines the amplitude of the output video level according to
Equation 1 and Equation 2 for the ADV7125.
IOG (mA) = 11,444.8 × V
REF
(V)/R
SET
(Ω) (1)
IOR, IOB (mA) = 7989.6 × V
REF
(V)/R
SET
(Ω) (2)
Equation 1 applies to the ADV7125 only, when
SYNC
is being
used. If
SYNC
is not being encoded onto the green channel,
Equation 1 is similar to Equation 2.
Using a variable value of R
SET
allows for accurate adjustment of
the analog output video levels. Use of a fixed 560  R
SET
resistor
yields the analog output levels quoted in the Specifications section.
These values typically correspond to the RS-343A video wave-
form values, as shown in Figure 5.
DACS
The ADV7125 contains three matched 8-bit DACs. The DACs
are designed using an advanced, high speed, segmented architec-
ture. 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. Because 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.
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  load, such
as a doubly terminated 75  coaxial cable. Figure 6 shows the
required configuration for each of the three RGB outputs
connected into a doubly terminated 75  load. This arrangement
develops RS-343A video output voltage levels across a 75 
monitor.
A suggested method of driving RS-170 video levels into a 75 
monitor is shown in Figure 7. The output current levels of the
DACs remain unchanged, but the source termination resistance,
Z
S
, on each of the three DACs is increased from 75  to 150 .
IOR, IOG, IOB
Z
S
= 75Ω
(SOURCE
TERMINATION)
TERMINATION REPEATED THREE TIMES
FOR RED, GREEN, AND BLUE DACs
Z
L
= 75Ω
(MONITOR)
Z
0
= 75Ω
(CABLE)
DACs
03097-006
Figure 6. Analog Output Termination for RS-343A
IOR, IOG, IOB
Z
S
= 150Ω
(SOURCE
TERMINATION)
TERMINATION REPEATED THREE TIMES
FOR RED, GREEN, AND BLUE DACs
Z
L
= 75Ω
(MONITOR)
Z
0
= 75Ω
(CABLE)
DACs
03097-007
Figure 7. Analog Output Termination for RS-170
More detailed information regarding load terminations for
various output configurations, including RS-343A and RS-170,
is available in the AN-205 Application Note, Video Formats and
Required Load Terminations, available from Analog Devices at
www.analog.com.