FMS6346_08 Datasheet, PDF (6/10 Pages) Fairchild Semiconductor – Six Channel, 6th-Order SD/HD Video Filter Driver

Applications Information
Functional Description
The FMS6346 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 de-
crease the standing DC current into the load. Typical volt-
age levels are shown in Figure 11.
1.0 -> 1.02V
0.65 -> 0.67V
0.3 -> 0.32V
0.0 -> 0.02V
V_IN
2.28V
1.58V
0.88V
0.28V
V_OUT
Driven by:
DC-Coupled DAC Outputs
AC-Coupled and Clamped
Y, CV, R, G, B
There is a 280mV offset from the DC input level to the
DC output level. V_OUT = 2 * V_IN + 280mV.
0.65V
YIN
YOUT
Driver
800kΩ
Figure 12. Input Clamp Circuit
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs, use
the configuration shown in Figure 13.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75Ω
Clamp
Inactive
0.85V
0.5V
0.15V
V_IN
1.98V
1.28V
0.58V
V_OUT
Driven by:
AC-Coupled and Biased
U, V, Pb, Pr, C
Figure 11. Typical Voltage Levels
The FMS6346 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 FMS6346 without
an AC coupling capacitor. The worst-case sync tip com-
pression due to the clamp does not exceed 7mV. The in-
put level set by the clamp, combined with the internal DC
offset, keeps the output within its acceptable range. When
the input is AC-coupled, the diode clamp sets the sync tip
(or lowest voltage) just below ground.
For symmetric signals like C, U, V, Cb, Cr, 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 con-
ceptual illustration of the input clamp circuit is shown in
Figure 12.
Figure 13. DC-Coupled Inputs and Outputs
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 14.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
0.1μF
LCVF
75Ω
Clamp
Active
Figure 14. AC-Coupled Inputs, DC-Coupled Outputs
When the 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 15.
0V - 1.4V
External video
source must
be AC coupled
0.1μF
75Ω
LCVF
75Ω
Clamp
Active
Figure 15. SCART Configuration with DC-Coupled
Outputs
© 2006 Fairchild Semiconductor
FMS6346 Rev. 1.0.4
6
www.fairchildsemi.com