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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

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

Term ination Resistor