FMS6141 — Low-Cost, Single-Channel 4
th
-Order Standard Definition Video Filter Driver
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5
Application Information
Input Considerations
The FMS6141 Low -Cost Video Filter provides 6 dB (2X)
gain from input to output. The device provides an internal
diode clamp to support AC-coupled input signals. In this
configuration, a 0.1 µF ceramic capacitor is used to AC
couple the input signal. If the input signal does not go
below ground, the clamp is inactive; but if the input signal
goes below ground, the clamp circuitry sets the bottom of
the sync tip (or low est voltage) to just below ground. The
input level set by the clamp, combined w ith the internal DC
offset, keeps the output signal w ithin an acceptable
range. This clamp feature also allow s the FMS6141’s input
to be directly driven (DC-coupled) by a ground referenced
DAC output. Figure 4 show s typical DC voltage levels for
the input and output signals w hen driven by a DC-coupled
DAC output or an AC-coupled and clamped Y, CV signal.
0.28V
0.88V
1.58V
2.28V
0.0 -> 0.02V
0.3 -> 0.32V
0.65 -> 0.67V
1.0 -> 1.02V
Vin Vout
Figure 4. Typical DC Voltage Levels
Output Considerations
The FMS6141 outputs w ill be DC offset from the input by
150 mv therefore V
OUT
= 2*V
IN
DC+150 mv. This offset is
required to obtain optimal performance from the output
driver and is held at the minimum value in order to
decrease the standing DC current into the load. Since the
FMS6141 has a 2x (6 dB) gain, the output is typically
connected via a 75 Ω series back-matching resistor
follow ed by the 75 Ω video cable. Because of the inherent
divide by tw o of this configuration, the blanking level at
the load of the video signal is alw ays less then 1 V. When
AC-coupling the output ensure that the coupling capacitor
of choice w ill pass the low est frequency content in the
video signal and that line time distortion (video tilt) is kept
as low as possible.
The selection of the coupling capacitor is a function of the
subsequent circuit input impedance and the leakage
current of the input being driven. In order to obtain the
highest quality output video signal the series termination
resistor must be placed as close to the device output pin
as possible. This greatly reduces the parasitic
capacitance and inductance effect on the FMS6141
output driver. Recommend distance from device pin to
place series termination resistor should be no greater than
0.1 inches.
Figure 5. Distance from Device Pin to Series
Termination Resistor
