FMS6141_06 Datasheet, PDF (5/11 Pages) Fairchild Semiconductor – Low-Cost, Single-Channel 4th-Order Standard Definition Video Filter Driver

Application Information
Input Considerations
The FMS6141 Low-Cost Video Filter provides 6dB (2X)
gain from input to output. The device provides an
internal diode clamp to support AC-coupled input
signals. In this configuration, a 0.1µF 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 lowest voltage) to just
below ground. The input level set by the clamp,
combined with the internal DC offset, keeps the output
signal within an acceptable range. This clamp feature
also allows the FMS6141’s input to be directly driven
(DC-coupled) by a ground referenced DAC output.
Figure 4 shows typical DC voltage levels for the input
and output signals when driven by a DC-coupled DAC
output or an AC-coupled and clamped Y, CV signal.
1.0 -> 1.02V
0.65 -> 0.67V
0.3 -> 0.32V
0.0 -> 0.02V
2.28V
1.58V
0.88V
Vin
0.28V
Vout
Figure 4. Typical DC Voltage Levels
Output Considerations
The FMS6141’s output is DC offset from the input by
280mv; therefore V_OUT = 2 • V_IN+280mv. This offset is
required to obtain optimal performance from the output
driver and is held at the minimum value to decrease the
standing DC current into the load. Since the FMS6141
has a 2x (6dB) gain, the output is typically connected via
a 75Ω-series, back-matching resistor followed by the
75Ω video cable. Due to the inherent divide-by-two of
this configuration, the blanking level at the load of the
video signal is always less than 1V. When AC-coupling
the output, ensure that the coupling capacitor of choice
passes the lowest 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’s input impedance and
the leakage current of the input being driven. To obtain
the highest quality output video signal, the series
termination resistor must be placed as close to the
device’s output pin as possible. This greatly reduces
parasitic capacitance and inductance effects on the
FMS6141’s output driver. The distance from the device’s
pin to the series termination resistor should be no
greater than 0.1 inches, as shown in Figure 5.
Figure 5. Resistor Placement
© 2006 Fairchild Semiconductor Corporation
FMS6141 Rev. 1.0.1
5
www.fairchildsemi.com