www.fairchildsemi.com 6 FMS6246 Rev. 1B
FMS6246 Six Channel, 6th Order SD/PS Video Filter Driver
Applications Information
Functional Description
The FMS6246 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 9. Typical Voltage Levels
The FMS6246 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 FMS6246 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 10. Input Clamp Circuit
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs, use this
configuration:
Figure 11. 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:
Figure 12. AC-coupled Inputs, DC-coupled Outputs
When the FMS6246 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 13. SCART Configuration with DC-coupled
Outputs
There will be a 280mV offset from the DC input level to the
DC output level. V
OUT
= 2 * V
IN
+ 280mV
0.0 -> 0.02V
0.3 -> 0.32V
0.65 -> 0.67V
1.0 -> 1.02V
V
IN
0.28V
0.88V
1.58V
2.28V
Driven by:
DC-Coupled DAC Outputs
AC-Coupled and Clamped
Y, CV, R, G, B
V
OUT
0.15V
0.5V
0.85V
V
IN
0.58V
1.28V
1.98V
Driven by:
AC-Coupled and Biased
U, V, Pb, Pr, C
V
OUT
DVD or
STB
SoC
DAC
Output
75Ω
LCVF
Clamp
Inactive
0V - 1.4V
DVD or
STB
SoC
DAC
Output
LCVF
Clamp
Active
0.1u
0V - 1.4V
75Ω
75Ω
LCVF
Clamp
Active
0.1u
External Video
source must
be AC-coupled.
0V - 1.4V
75Ω
Y
OUT
Driver
Y
IN
0.65V
800kΩ
