LMH1981 Evaluation Board
Instruction Manual
National Semiconductor
Application Note 1599
February 11, 2009
General Description
The LMH1981 Evaluation Board can be used to evaluate the
LMH1981 Multi-Format Sync Separator and as a reference
for designing the PCB layout. Refer to the LMH1981
datasheet for more information on PCB layout considerations.
Power Supply
The board should be powered with a clean supply voltage of
3.3V to 5.0V using the banana jacks V
CC
(J2) and GND (J3).
The supply should be well-regulated within ±5% variation of
the voltage range and should not be shared directly with other
digital circuitry.
Video Input
A high-quality DC-coupled video source should be connected
to the video input BNC (J1), which is terminated on-board via
a 75
Ω
load resistor. For AC-coupled video sources, it may be
necessary to reduce the value of the input coupling capacitor
(C4) as described in the datasheet; otherwise, sync loss may
occur during significant changes in video average picture lev-
el (e.g. random white-to-black field transitions). It is recom-
mended to drive the LMH1981 input by a professional-grade
DC-coupled video reference.
Because the input can accept either SD or HD video inputs,
the PCB footprints for the chroma filter components were not
populated. For SD composite video inputs, it may be neces-
sary to use a RC low-pass filter to attenuate the chroma
component so it does not extend below the 50% sync level
and also to improve overall signal-to-noise ratio. The RC filter
cutoff frequency is typically set between 0.5 MHz and 2 MHz,
which corresponds to chroma attenuation between 17 dB and
6 dB for a 3.58 MHz subcarrier (NTSC). For HD video inputs,
it is suggested to bypass any composite video filtering, as it
may reduce the bandwidth of the HD tri-level sync signal and
thus increase timing jitter on the HSync output.
Test Points
Test points are provided to probe the input and output signals
using oscilloscope probes with high input impedance and low
capacitance.
Board Schematic
30012601
© 2009 National Semiconductor Corporation
300126
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LMH1981 Evaluation Board Instruction Manual
AN-1599
