© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FMS6141 Rev. 1.0.0

5

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.

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’s output is DC offset from the input by

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