FMS6246 Fairchild Semiconductor, FMS6246 Datasheet - Page 5

• Manufacturer Part Number: FMS6246
• Description: 6th Order SD/PS Video Filter Driver
• Manufacturer: Fairchild Semiconductor

FMS6246 Rev. 1A
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.
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
0.65 -> 0.67V
2.28V
1.58V
0.88V
0.28V
0.85V
0.15V
1.98V
1.28V
0.58V
1.0 -> 1.02V
0.3 -> 0.32V
0.0 -> 0.02V
0.5V
There will be a 280mV offset from the DC input level to
the DC output level.
Vout = 2 * Vin + 280mV
Figure 1. Typical Voltage Levels
Vout
Vout
Vin
Vin
Driven by:
DC-Coupled DAC Outputs or
AC-Coupled and Clamped
Y, R, G, B, CV
Driven by:
DC-Coupled DAC Outputs
AC-Coupled and Biased
U, V, Pb, Pr, C
5
coupling capacitor. A conceptual illustration of the input clamp
circuit is shown below:
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs, use this
configuration:
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 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 4.
External Video
source must
be AC-coupled.
Figure 5.
DVD or
Output
DAC
STB
SoC
DVD or
Output
Figure 3. DC-coupled Inputs and Outputs
DAC
0.65V
STB
SoC
Y IN
Figure 2. Input Clamp Circuit
AC-coupled Inputs, DC-coupled Outputs
SCART Configuration with DC-coupled
800kΩ
0V - 1.4V
0.1u
75
0V - 1.4V
0.1u
Outputs
0V - 1.4V
Inactive
Clamp
LCVF
Clamp
Driver
Active
LCVF
Clamp
Active
LCVF
Y OUT
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