channel is used and IOR is terminated with a doubly terminated
75 Ω load (37.5 Ω), IOB and IOG should be terminated with
37.5 Ω resistors (See Figure 5).
Figure 5. Input and Output Connections for
Standalone Grayscale or Composite Video
Video Output Buffers
The ADV7125 is specified to drive transmission line loads, as
are most monitors rated. The analog output configurations to
drive such loads are described in the Analog Outputs section
and are illustrated in Figure 6. However, in some applications,
it may be required to drive long transmission line cable lengths.
Cable lengths greater than 10 meters can attenuate and distort
high frequency analog output pulses. The inclusion 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 applica-
tions. These include the AD84x series of monolithic op amps.
In very high frequency applications (80 MHz), the AD8061 is
recommended. More information on line driver buffering
circuits is given in the relevant op amp data sheets.
Use of buffer amplifiers also allows implementation of other video
standards besides RS-343A and RS-170. Altering the gain com-
ponents of the buffer circuit will result in any desired video level.
IOR, IOG, IOB
ZS = 75⍀
ZO = 75⍀
GAIN (G) = 1 + Z1
ZL = 75⍀
Figure 6. AD848 As an Output Buffer
PC Board Layout Considerations
The ADV7125 is optimally designed for lowest noise performance,
both radiated and conducted noise. To complement the excel-
lent noise performance of the ADV7125, it is imperative that
great care be given to the PC board layout. Figure 7 shows a
recommended connection diagram for the ADV7125.
The layout should be optimized for lowest noise on the ADV7125
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 by minimized to
minimize inductive ringing.
The ADV7125 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 7. This bead should be located as close as possible
(within three inches) to the ADV7125.
The analog ground plane should encompass all ADV7125
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 PC board layout should have two distinct power planes,
one for analog circuitry and one for digital circuitry. The analog
power plane should encompass the ADV7125 (VAA) and all
associated analog circuitry. This power plane should be con-
nected to the regular PCB power plane (VCC) at a single point
through a ferrite bead, as illustrated in Figure 6. This bead
should be located within three inches of the ADV7125.
The PCB power plane should provide power to all digital logic
on the PC board, and the analog power plane should provide
power to all ADV7125 power pins, voltage reference circuitry,
and any output amplifiers.
The PCB power and ground planes should not overlay portions
of the analog power plane. Keeping the PCB power and ground
planes from overlaying the analog power plane will contribute to
a reduction in plane-to-plane noise coupling.
Noise on the analog power plane can be further reduced by the
use of multiple decoupling capacitors (see Figure 7).
Optimum performance is achieved by the use of 0.1 µF ceramic
capacitors. Each of the two groups of VAA should be individually
decoupled to ground. This should be done by placing the capaci-
tors as close as possible to the device with the capacitor leads as
short as possible, thus minimizing lead inductance.
It is important to note that while the ADV7125 contains circuitry
to reject power supply noise, this rejection decreases with fre-
quency. If a high frequency switching power supply is used, the
designer should pay close attention to reducing power supply
noise. A dc power supply filter (Murata BNX002) will provide
EMI suppression between the switching power supply and the
main PCB. Alternatively, consideration could be given to using
a three-terminal voltage regulator.
Digital Signal Interconnect
The digital signal lines to the ADV7125 should be isolated as
much as possible from the analog outputs and other analog
circuitry. Digital signal lines should not overlay the analog
Due to the high clock rates used, long clock lines to the ADV7125
should be avoided to minimize noise pickup.
Any active pull-up termination resistors for the digital inputs
should be connected to the regular PCB power plane (VCC) and
not the analog power plane.