FMS6690 Rev. 2A

5

www.fairchildsemi.com

Applications Information

Functional Description

The FMS6690 Low Cost Video Filter (LCVF) provides 6dB gain

(9dB optional, contact factory for further information) from input

to output. In addition, the input will be 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.

Figure 1. Typical Voltage Levels

The FMS6690 provides an internal diode clamp to support AC-

coupled input signals. If the input signal does not go below

ground, the input clamp will not operate. This allows DAC out-

puts to directly drive the FMS6690 without an AC coupling

capacitor. The worst-case sync tip compression due to the

clamp will not exceed 7mV. The input level set by the clamp

combined with the internal DC offset will keep the output within

its acceptable range. When the input is AC-coupled, the diode

clamp will set the sync tip (or lowest voltage) just below ground.

For symmetric signals like C, U, V, Cb, Cr, Pb and Pr, the aver-

age 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 below:

Figure 2. Input Clamp Circuit

I/O Configurations

For DC-coupled DAC drive with DC-coupled outputs, use this

configuration:

Figure 3. 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:

When the FMS6690 is driven by an unknown external source or

a SCART switch with its own clamping circuitry, the inputs

should be AC-coupled as follows:

Figure 5. SCART Configuration with DC-coupled

Outputs

There will be a 280mV offset from the DC input level to

the DC output level.

Vout = 2 * Vin + 280mV

0.0 -> 0.02V

0.3 -> 0.32V

0.65 -> 0.67V

1.0 -> 1.02V

Vin

0.15V

0.5V

0.85V

Vin

0.28V

0.88V

1.58V

2.28V

Driven by:

DC-Coupled DAC Outputs or

AC-Coupled and Clamped

Y, R, G, B, CV

Vout

0.58V

1.28V

1.98V

Driven by:

DC-Coupled DAC Outputs

AC-Coupled and Biased

U, V, Pb, Pr, C

Vout

YOUT

Driver

YIN

0.65V

800k

Ω

DVD or

STB

SoC

DAC

Output

75

LCVF

Clamp

Inactive

0V - 1.4V

DVD or

STB

SoC

DAC

Output

75

LCVF

Clamp

Active

0.1u

0V - 1.4V

75

LCVF

Clamp

Active

75

0.1u

External Video

source must

be AC-coupled.

0V - 1.4V