FMS6145_08 Datasheet, PDF (7/10 Pages) Fairchild Semiconductor – Low-Cost Five-Channel 4th-Order Standard Defi nition Video Filter Driver

Application Information
Application Circuits
The FMS6145 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 the diagram below:
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
0.65V
YIN
75Ω
Video Cables
75Ω
LOAD2
(optional)
Driver
YOUT _75Ω
Video Cables
75Ω
LOAD1
Figure 8. Input Clamp Circuit
I/O Configurations
For a DC-coupled DAC drive with DC-coupled outputs,
use the configuration in Figure 9.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75Ω
Clamp
Inactive
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
1.98V
1.28V
0.58V
V_IN
V_OUT
Driven by:
AC-Coupled and Biased
U, V, Pb, Pr, C
Figure 9. 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 shown in FIgure 10.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
0.1μ
LCVF
75Ω
Clamp
Active
Figure 7. Typical Voltage Levels
The FMS6145 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 FMS6145 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 8.
Figure 10. AC-Coupled Inputs,
DC-Coupled Outputs
When the FMS6145 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 11.
0V - 1.4V
External video
source must
be AC coupled
0.1μ
75Ω
LCVF
75Ω
Clamp
Active
Figure 11. SCART with DC-Coupled Outputs
© 2006 Fairchild Semiconductor Corporation
FMS6145 Rev. 2.0.5
7
www.fairchildsemi.com