GS2985 datasheet(23/45 Pages) SEMTECH

23 / 45 page

GS2985 Multi-Rate SDI Reclocker with Equalization &

De-emphasis

Data Sheet

36663 - 5

July 2012

23 of 45

Proprietary & Confidential

4.8.1 Lock Detect and Asynchronous Lock

The reference crystal is used to assist the PLL in achieving a short lock time. The lock detection

algorithm is a continuous process, which begins at device power up or after a system reset, and

continues until the device is powered down.

The asynchronous lock time is defined as the time it takes the device to lock when a video signal

is first applied to the serial digital inputs, or when the digital video signal rate changes.

The synchronous lock time is defined as the time it takes the device to lock to a signal which has

been momentarily interrupted.

4.9 Serial Data Output

The GS2985 features two current-mode differential output drivers, each capable of driving a

maximum of 800mVpp, differential, into an external 100Ω differential load.

Each of the GS2985's output buffers include two on-chip, 50Ω termination resistors.

4.9.1 Output Signal Interface Levels

The serial digital outputs of the GS2985 are compatible when DC-coupled with all Gennum

serial digital interface products that feature a differential LVPECL or CML receiver designed for

SDI applications and operate from 3.3V or 2.5V supplies. This includes but is not limited to:

GS2978, GS2988, and GS2989 cable drivers.

4.9.2 Adjustable Output Swing

It is possible, via the host interface, to force the output swing to 400mVpp or 800mVpp

differential, when the outputs are terminated with 50Ω loads.

The default output swing upon power-up is 400mVpp differential.

4.9.3 Output De-emphasis

The GS2985 features adjustable output de-emphasis to compensate for PCB trace dielectric

losses.

The output de-emphasis has eight settings, evenly distributed from a minimum of 0dB (output

de-emphasis OFF) to a peak de-emphasis setting that is optimized for compensating the

high-frequency losses associated with approximately 20 inches of 5-mil stripline in FR4

material. These settings are accessible via the serial host interface.

The action of the de-emphasis settings is to attenuate the trailing edge of the output data

waveform relative to the output swings set through the host interface.

Each serial digital output DDOn, DDOn includes a DEn_EN pin to turn its output de-emphasis

on or off. De-emphasis is also turned OFF when in Bypass mode.

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