AD8123ACPZ Analog Devices Inc, AD8123ACPZ Datasheet - Page 13

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AD8123ACPZ

Manufacturer Part Number
AD8123ACPZ
Description
Triple Equalizer
Manufacturer
Analog Devices Inc
Datasheet

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USING THE AD8123 WITH COAXIAL CABLE
The V
types of cable, including coaxial cable. Figure 18 presents the
recommended settings for V
AD8123 is used with good quality 75 Ω video cable. Figure 24
shows how to derive V
cable application where V
The op amp in the circuit that develops V
insert the offset of −0.62 V with a gain from V
is close to unity. A passive offset circuit would require an offset
injection voltage that is much larger in magnitude than the
available −5 V supply. Clearly, the V
also be developed independently.
The AD8123 differential input can accept signals carried over
unbalanced cable, as shown in Figure 25, for an unbalanced
75 Ω coaxial cable termination.
DRIVING 75 Ω VIDEO CABLE WITH THE AD8123
When the RGB outputs must drive a 75 Ω line rather than a
high impedance load, an additional gain of two is required to
make up for the double termination loss (75 Ω source and load
terminations). There are two options available for this.
One option is to place the additional gain of 2 at the drive end
by using the
The
and thereby provides the required additional gain of 2 without
having to add additional amplifiers to the signal chain. The
AD8148
sync-on-common-mode is not required, it can be deactivated
on the
Figure 24. Deriving V
AD8148
POLE
AD8148
+5V
also contains sync-on-common-mode encoding. If
control allows the AD8123 to be used with other
V
PEAK
20kΩ
AD8148
INPUT FROM
has a fixed gain of 4 instead of the usual gain of 2
10kΩ
75Ω CABLE
by connecting its SYNC LEVEL input to ground.
POLE
Figure 25. Terminating a 75 Ω Cable
and V
5.11kΩ
1.16kΩ
1.24kΩ
triple differential driver to drive the cable.
POLE
GAIN
20Ω
PEAK
and V
–5V
from V
PEAK
47.5kΩ
75Ω
originates from a low-Z source.
24.3kΩ
, V
PEAK
GAIN
POLE
with Low-Z Source for Coaxial Cable
GAIN
V
V
V
from V
POLE
GAIN
PEAK
, and V
AD8123
INPUT STAGE
control voltage can
≈ 1.06 × V
≈ 0.76 × V
GAIN
PEAK
GAIN
is required to
PEAK
PEAK
in a coaxial
PEAK
when the
to V
– 0.62V
– 0.41V
GAIN
that
Rev. A | Page 13 of 16
The other option is to include a triple gain-of-2 buffer, such as the
ADA4862-3, on the AD8123 RGB outputs, as shown in Figure 26
for one channel (power supplies not shown). The
provides the gain of 2 that compensates for the double-
termination loss.
DRIVING A CAPACITIVE LOAD
When driving a high impedance capacitive input, it is necessary
to place a small series resistor between each of the three AD8123
video outputs and the load to buffer the input capacitance of the
device being driven. Clearly, the resistor value must be small
enough to preserve the required bandwidth.
FILTERING THE RGB OUTPUTS
In some cases, it is desirable to place low-pass filters on the
AD8123 video outputs to reduce high frequency noise. A 3-pole
Butterworth filter with cutoff frequency in the neighborhood of
140 MHz is sufficient in most applications. Figure 27 and Figure 28
present filters for the high impedance load case (driving a delay
line, crosspoint switch, ADA4862-3) and the double-termination
case (75 Ω source and load resistances), respectively. In the high
impedance load case, the load capacitance must be absorbed in
the capacitor that is placed across the load. For example, in
Figure 27, if the high-Z load were the input to an ADA4862-3,
which has an input capacitance of 2 pF, the filter capacitor value
in parallel with the input would be 15 pF to obtain 17 pF.
These filters are by no means the only choices but are presented
here as examples. In the high-Z load case, it is important to
keep the filter source resistance large enough to buffer the
capacitive loading presented by the first capacitor in the filter.
Figure 27. 140 MHz Low-Pass Filter on AD8123 Output Feeding High-Z Load
FROM AD8123
ONE VIDEO
OUTPUT
AD8123
OUTPUT
Figure 28. 135 MHz Low-Pass Filter on AD8123 Output Feeding
*INPUT CAPACITANCE OF LOAD MUST BE
OUTPUT
ABSORBED INTO THIS VALUE.
AD8123
Figure 26. Using ADA4862-3 on AD8123 Outputs
ONE CHANNEL OF ADA4862-3
75 Ω
500 Ω
Doubly Terminated Load
100 Ω
15pF
180nH
500 Ω
5.6pF
150nH
15pF
75 Ω
17pF*
HIGH-Z
Z
0
= 75Ω
Z
0
= 75Ω
ADA4862-3
AD8123
75Ω
75Ω

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