© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FMS6145 • Rev. 2.0.6

7

Application Information

Application Circuits

The FMS6145 Low Cost Video Filter (LCVF) provides

6dB gain from input to output. In addition, the input is

slightly offset to optimize the output driver performance.

The offset is held to the minimum required value to

decrease the standing DC-current into the load. Typical

voltage levels are shown in the diagram below:

There is a 280mV offset from the DC input level to the

0.0 -> 0.02V

0.3 -> 0.32V

0.65 -> 0.67V

1.0 -> 1.02V

0.28V

0.88V

1.58V

2.28V

Driven by:

DC-Coupled DAC Outputs

AC-Coupled and Clamped

Y, CV, R, G, B

0.15V

0.5V

0.85V

0.58V

1.28V

1.98V

Driven by:

AC-Coupled and Biased

U, V, Pb, Pr, C

Figure 7. Typical Voltage Levels

The FMS6145 provides an internal diode clamp to sup-

port AC-coupled input signals. If the input signal does not

go below ground, the input clamp does not operate. This

allows DAC outputs to directly drive the FMS6145 without

an AC coupling capacitor. When the input is AC coupled,

the diode clamp sets the sync tip (or lowest voltage) just

below ground. The worst-case sync tip compression due

to the clamp cannot exceed 7mV. The input level set by

the clamp, combined with the internal DC offset, keeps

the output within its acceptable range.

For symmetric signals like Chroma, U, V, Pb, and Pr, the

average DC bias is fairly constant and the inputs can be

AC coupled with the addition of a pull-up resistor to set

the DC input voltage. DAC outputs can also drive these

same signals without theAC coupling capacitor.Aconceptual

illustration of the input clamp circuit is shown in Figure 8.

YOUT

Video Cables

Video Cables

75

Ω

75

Ω

75

Ω

75

Ω

Driver

YIN

LOAD1

LOAD2

(optional)

0.65V

Figure 8. Input Clamp Circuit

I/O Configurations

For a DC-coupled DAC drive with DC-coupled outputs,

use the configuration in Figure 9.

DVD or

STB

SoC

DAC

Output

75

Ω

LCVF

Clamp

Inactive

0V - 1.4V

Figure 9. DC-Coupled Inputs and Outputs

Alternatively, if the DAC’s average DC output level causes

the signal to exceed the range of 0V to 1.4V, it can be AC

coupled as shown in FIgure 10.

DVD or

STB

SoC

DAC

Output

LCVF

Clamp

Active

0.1μ

0V - 1.4V

75Ω

Figure 10. AC-Coupled Inputs,

DC-Coupled Outputs

When the FMS6145 is driven by an unknown external

source or a SCART switch with its own clamping circuitry,

the inputs should be AC coupled as shown in Figure 11.

75Ω

LCVF

Clamp

Active

0.1μ

External video

source must

be AC coupled

0V - 1.4V

75Ω

Figure 11. SCART with DC-Coupled Outputs