obtained from the red input, vertical sync is obtained
from the blue input, and composite sync is obtained from
the green input. Like the MAX9539, the DC-restore func-
tion removes any DC offset in the RGB video inputs and
sets the output black levels to 0V. This happens at the
back porch (trailing edge) of the sync pulse.
Figure 3 illustrates the functionality of the MAX9540. In
this example, the sync signals are of positive polarity.
DC Restore
The MAX9539/MAX9540 DC-restore function removes
the input signal DC level and restores 0V for the black
level of the output video signal. 1nF restore capacitors
are needed for the sample-and-hold circuitry at
REST_R, REST_G, and REST_B. A value less than
0.5nF can cause AC instability in the sample-and-hold
circuitry. A value higher than 2nF increases the settling
time of the sample-and-hold circuitry, shifting the out-
put black level from 0V.
Sync Polarity
Sync polarity refers to the idle state and pulse ampli-
tude of the sync pulse. A sync pulse that idles low and
pulses high is referred to as a positive sync pulse. A
sync pulse that idles high and pulses low is referred to
as a negative sync pulse as seen in Figure 4. To
accommodate positive and negative sync input signals,
the MAX9539/MAX9540 have vertical and horizontal
sync polarity inputs (SP_V and SP_H). Drive SP_V or
SP_H high for positive sync polarity. Drive SP_V or
SP_H low for negative sync polarity. The MAX9540 also
has a composite polarity input (SP_C). Drive SP_C high
for positive sync polarity or drive SP_C low for negative
sync polarity (Table 1).
Layout and Power-Supply Bypassing
The MAX9539/MAX9540 have an extremely high band-
width and require careful board layout. For best perfor-
mance use constant-impedance microstrip or stripline
techniques.
To realize the full AC performance of these high-speed
amplifiers, pay careful attention to power-supply
bypassing and board layout. The PC board should
have at least two layers: a signal and power layer on
one side, and a large, low-impedance ground plane on
the other side. The ground plane should be as free of
voids as possible. With multilayer boards, locate the
ground plane on a layer that incorporates no signal or
power traces.
Observe the following guidelines when designing the
board regardless of whether or not a constant-imped-
ance board is used.
1) Do not use wire-wrap boards or breadboards.
2) Do not use IC sockets; they increase parasitic
capacitance and inductance.
3) Keep lines as short and as straight as possible. Do
not make 90° turns; round all corners.
4) Observe high-frequency bypassing techniques to
maintain the amplifier’s accuracy and stability.
5) Use surface-mount components. They generally
have shorter bodies and lower parasitic reactance,
yielding better high-frequency performance than
through-hole components.
MAX9539/MAX9540
Graphics Video Sync Adder/Extractor
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