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LMH1981MTEVAL 产品设计参考手册 - TI（德州仪器）
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LMH1981MTEVAL数据手册
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User's Guide
SNLA094C–February 2007–Revised April 2013
AN-1599 LMH1981 Evaluation Board Instruction Manual
1 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. For more information on PCB layout considerations, see the
LMH1981 Multi-Format Video Sync Separator Data Sheet (SNLS214).
2 Power Supply
The board should be powered with a clean supply voltage of 3.3 V to 5.0 V 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.
3 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 data sheet; otherwise, sync loss
may occur during significant changes in video average picture level (random white-to-black field
transitions). It is recommended 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 necessary 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.
4 Test Points
Test points are provided to probe the input and output signals using oscilloscope probes with high input
impedance and low capacitance.
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SNLA094C–February 2007–Revised April 2013 AN-1599 LMH1981 Evaluation Board Instruction Manual
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