ADV7127
D/A Converter
Video Output Buffer
The ADV7127 contains a 10-bit D/A converter. The DAC is
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. 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.
The ADV7127 is specified to drive transmission line loads,
which is what most monitors are rated as. The analog output
configurations to drive such loads are described in the Analog
Interface section and illustrated in Figure 23. However, in some
applications it may be required to drive long “transmission line”
cable lengths. Cable lengths greater than 10 meters can attenu-
ate and distort high frequency analog output pulses. The inclu-
sion of output buffers will compensate for some cable distortion.
Buffers with large full power bandwidths and gains between two
and four will be required. These buffers will also need to be able
to supply sufficient current over the complete output voltage
swing. Analog Devices produces a range of suitable op amps for
such applications. These include the AD84x series of monolithic
op amps. In very high frequency applications (80 MHz), the
AD9617 is recommended. More information on line driver
buffering circuits is given in the relevant op amp data sheets.
Analog Output
The analog output of the ADV7127 is a high impedance current
source. The current output is capable of directly driving a
37.5 Ω load, such as a doubly terminated 75 Ω coaxial cable.
Figure 22 shows the required configuration for the output con-
nected into a doubly terminated 75 Ω load. This arrangement
will develop RS-343A video output voltage levels across a 75 Ω
monitor.
Use of buffer amplifiers also allows implementation of other
video standards besides RS-343A and RS-170. Altering the gain
components of the buffer circuit will result in any desired
video level.
I
OUT
Z
= 75⍀
O
DAC
Z
(SOURCE
TERMINATION)
(CABLE)
= 75⍀
S
Z
= 75⍀
Z
Z
1
L
2
(MONITOR)
+V
0.1F
S
Z
= 75⍀
O
I
75⍀
OUT
AD848
Figure 22. Analog Output Termination for RS-343A
(CABLE)
DAC
= 75⍀
(SOURCE
TERMINATION)
Z = 75⍀
L
(MONITOR)
0.1F
A suggested method of driving RS-170 video levels into a 75 Ω
monitor is shown in Figure 23. The output current level of the
DAC remains unchanged, but the source termination resistance,
ZS, on the DAC is increased from 75 Ω to 150 Ω.
Z
S
–V
S
Z
Z
1
2
GAIN (G) = 1 +
Figure 24. AD848 As an Output Buffer
PC Board Layout Considerations
I
OUT
Z
= 75⍀
O
The ADV7127 is optimally designed for lowest noise perfor-
mance, both radiated and conducted noise. To complement the
excellent noise performance of the ADV7127 it is imperative
that great care be given to the PC board layout. Figure 25 shows
a recommended connection diagram for the ADV7127.
DAC
(CABLE)
Z
= 150⍀
S
Z
= 75⍀
L
(SOURCE
TERMINATION)
(MONITOR)
The layout should be optimized for lowest noise on the ADV7127
power and ground lines. This can be achieved by shielding the
digital inputs and providing good decoupling. The lead length
between groups of VAA and GND pins should be minimized to
inductive ringing.
Figure 23. Analog Output Termination for RS-170
More detailed information regarding load terminations for vari-
ous output configurations, including RS-343A and RS-170, is
available in an Application Note entitled “Video Formats &
Required Load Terminations” available from Analog Devices,
publication no. E1228-15-1/89.
Ground Planes
The ADV7127 and associated analog circuitry, should have a
separate ground plane referred to as the analog ground plane.
This ground plane should connect to the regular PCB ground
plane at a single point through a ferrite bead, as illustrated in
Figure 25. This bead should be located as close as possible
(within 3 inches) to the ADV7127.
Figure 21 shows the video waveforms associated with the current
output driving the doubly terminated 75 Ω load of Figure 22.
Gray Scale Operation
The ADV7127 can be used for stand-alone, gray scale (mono-
chrome) or composite video applications (i.e., only one channel
used for video information).
The analog ground plane should encompass all ADV7127
ground pins, voltage reference circuitry, power supply bypass
circuitry, the analog output traces and any output amplifiers.
The regular PCB ground plane area should encompass all the
digital signal traces, excluding the ground pins, leading up to
the ADV7127.
REV. 0
–13–
ROCHESTER [ Rochester Electronics ]
