Datasheet

© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FMS6690 • Rev. 1.0.3 8
FMS6690 — Six Channel, 6
th
Order, SD/PS/HD Video Filter Driver
Applications Information
Functional Description
The FMS6690 Low-Cost Video Filter (LCVF) provides
6dB gain (9dB optional, contact factory for further
information) 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 14.
Figure 14. Typical Voltage Levels
The FMS6690 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
FMS6690 without an AC coupling capacitor. The worst-
case sync tip compression, due to the clamp, does not
exceed 7mV. The input level set by the clamp,
combined with the internal DC offset, keeps the output
within 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 conceptual illustration of the input clamp circuit is
shown in Figure 15.
Y
OUT
800k
Driver
Y
IN
0.65V
Figure 15. Input Clamp Circuit
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs,
use the configuration in Figure 16.
DVD or
STB
So C
DAC
Out pu t
75Ω
LCVF
Cl am p
Inactive
0V - 1.4V
Figure 16. 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 17.
DVD or
STB
So C
DAC
Out pu t
75Ω
LCVF
Cl am p
Act ive
0.1µ
0V - 1.4V
Figure 17. AC-Coupled Inputs, DC-coupled Outputs
When the FMS6690 is driven by an unknown external
source or a SCART with its own clamping circuitry, the
inputs should be AC-coupled, shown in Figure 18.
75Ω
LCVF
Cl am p
Act ive
75Ω
0.1µ
Ex t er n al Vi d eo
source must
be AC-coupled.
0V - 1.4V
Figure 18. SCART with DC-Coupled Outputs