BL1511B
APPLICATION NOTE
Functional Description
BL1511B operates from a single 3.3V to 5V supply. In application, BL1511B is a fully integrated
solution for filtering and buffering SDTV signals in front of video decoder or behind video encoder. For
example, BL1511B can replace one passive LC filter and one amplifier driver at CVBS and S-video
output side in set-top box and DVD player, this solution can help you save PCB size and production cost,
it also improves video signal performance comparing with traditional design using discrete components.
BL1511B features a DC-coupled input buffer, 5-pole low-pass filter to eliminate out-of-band noise of
video encoder, and a gain of +6dB in the output amplifier to drive 75ꢀ load. The AC or DC-coupled
input buffer eliminates sync crush, droop, and field tilt. The output of BL1511B also can be DC-coupled
or AC-coupled.
Input Considerations
Besides AC coupling, the BL1511B inputs also can be DC-coupled. In DC coupling application, No
input coupling capacitors are needed because the amplitude of input video signal from DAC includes
ground and extends up to 1.4V, then BL1511B can be directly connected to the output of a
single-supply, current-output DAC without any external bias network. Some time, if DAC’s output level
exceeds the range of 0V to 1.4V, or BL1511B is driven by an unknown external source or a SCART
switch which has its own clamping circuit, AC coupling is needed in such applications.
Output Considerations
The BL1511B outputs can be DC-coupled or AC-coupled. When 0V is input, the BL1511B output
voltage is 395mV typically. In DC coupling design, one 75ꢀ resistor is used to connect BL1511B’s
output pin with external load directly, this serial back-termination resistor is used to match the
impedance of the transmission line between BL1511B and external load to cancel the signal reflection.
The BL1511B outputs can sink and source current allowing the device to be AC-coupled with external
load, in AC coupling, 220µF at least capacitor will be used in order to eliminate field tilt.
Power-Supply Bypassing and Layout
Correct power supply bypassing is very important for optimizing video performance in design. One
0.1µF and one 10µF capacitors are always used to Bypass VCC pin of BL1511B, please place these two
capacitors as close to the BL1511B output pin as possible, a large ground plane is also needed to
ensure optimum performance. The input and output termination resistors should be placed as close to the
related pin of BL1511B as possible to avoid performance degradation. The PCB traces at the output side
should have 75ꢀ characteristic impedance in order to match the 75ꢀ characteristic impedance
cable connecting external load. In design, please keep the board trace at the inputs and outputs of the
BL1511B as short as possible to minimize the parasitic stray capacitance and noise pickup.
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