FMS6144A Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – Low-Cost, Four-Channel, 6th-Order SD Video Filter Driver

Application Information
Application Circuits
The FMS6144A 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
decrease the standing DC current into the load. Typical
voltage levels are shown in Figure 13:
1.0 -> 1.02V
0.65 -> 0.67V
0.3 -> 0.32V
0.0 -> 0.02V
V_IN
0.65 V
YIN
75Ω ^{Video Cables}
75Ω
LOAD2
(optional)
Driver
YOUT _75Ω
Video Cables
75Ω
LOAD1
Figure 14. Input Clamp Circuit
I/O Configurations
For a DC-coupled DAC drive with DC-coupled outputs,
use the configurationin Figure 15.
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.
0.85V
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 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.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75W
Clamp
Inactive
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
0V - 1.4V
0.1μ
LCVF
75Ω
Clamp
Active
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.
0V - 1.4V
External video
source must
be AC coupled
0.1μ
75Ω
LCVF
75Ω
Clamp
Active
Figure 17. SCART with DC-Coupled Outputs
© 2009 Fairchild Semiconductor Corporation
FMS6144A • Rev. 1.0.1
9
www.fairchildsemi.com