
FMS6141
—
Low-Cost,
Single-Channel
4
th
-Order
Standard
Definition
Video
Filter
Driver
2006 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FMS6141 Rev. 1.0.3
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 F 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.
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 will be DC offset from the input
by 150 mv therefore VOUT = 2*VIN 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 series back-matching resistor
followed by the 75 video cable. Because of the
inherent divide by two of this configuration, the blanking
level at the load of the video signal is always less then
1 V. When AC-coupling the output ensure that the
coupling capacitor of choice will pass 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 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