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FMS6144A • Rev. 1.0.2

9

Application Information

Application Circuits

The FMS6144A VoltagePlus™ video filter 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 Figure 13:



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 13. Typical Voltage Levels

The FMS6144A provides an internal diode clamp to

support 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

FMS6144A 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 the AC coupling capacitor. A

conceptual illustration of the input clamp circuit is shown

in Figure 14.

YOUT

Video Cables

Video Cables

75Ω

75Ω

75Ω

75Ω

Driver

YIN

LOAD1

LOAD2

(optional)

0.65V

Figure 14. Input Clamp Circuit

I/O Configurations

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

use the configurationin Figure 15.

DVD or

STB

SoC

DAC

Output

75W

LCVF

Clamp

Inactive

0V - 1.4V

Figure 15. 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 follows:

DVD or

STB

SoC

DAC

Output

LCVF

Clamp

Active

0.1μ

0V - 1.4V

75Ω

Figure 16. AC-Coupled Inputs, DC-Coupled Outputs

When FMS6144A is driven by an unknown external

source or a SCART switch with its own clamping circuitry,

the inputs should be AC coupled like Figure 17.

75Ω

LCVF

Clamp

Active

0.1μ

External video

source must

be AC coupled

0V - 1.4V

75Ω

Figure 17. SCART with DC-Coupled Outputs