Datasheet

0 50 100 150 200 250 300 350 400

BELDEN 1694A CABLE LENGTH (m)

CLI (decimal value)

LMH0384

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SNLS308F –APRIL 2009–REVISED APRIL 2013

OUTPUT DRIVER ADJUSTMENTS

The output driver swing (amplitude) and offset voltage (common mode voltage) are adjustable via SPI register

01h.

The output swing is adjustable via bits [7:5] of SPI register 01h. The default value for these register bits is “011”

for a peak to peak differential output voltage of 700 mV

P-P

. The output swing can be adjusted in 100 mV

increments from 400 mV

P-P

to 800 mV

P-P

The offset voltage is adjustable via bits [4:2] of SPI register 01h. The default value for these register bits is “001”

for an output offset of 1.25V. The output common mode voltage may be adjusted in 200 mV increments, from

1.05V to 1.85V. It can also be set to “101” for the maximum offset voltage. At this maximum offset voltage

setting, the outputs are referenced to the positive supply and the offset voltage is around 2.1V.

LAUNCH AMPLITUDE OPTIMIZATION

The LMH0384 can compensate for attenuation of the input signal prior to the equalizer. This compensation is

useful for applications with a passive splitter at the equalizer input or a non-ideal input termination network, and

is controlled by SPI register 02h.

Bit 7 of SPI register 02h is used for coarse control of the launch amplitude setting. At the default setting of “0”,

the LMH0384 operates normally and expects a launch amplitude of 800 mV

P-P

. Bit 7 may be set to “1” to optimize

the LMH0384 for input signals with 6 dB of attenuation (400 mV

P-P

Once the coarse control is set, the LMH0384 input compensation may be further fine tuned by bits [6:3] of SPI

setting.

CABLE LENGTH INDICATOR (CLI)

The Cable Length Indicator (CLI) provides an indication of the length of cable attached to the input. CLI is

accessible via bits [7:3] of SPI register 03h. The 5-bit CLI ranges in decimal value from 0 to 25 (“00000” to

“11001” binary) and increases as the cable length is increased. Figure 7 shows typical CLI values vs. Belden

1694A cable length. CLI is valid for Belden 1694A cable lengths of 0-140m at 2.97 Gbps, 0-200m at 1.485 Gbps,

and 0-400m at 270 Mbps.

Figure 7. CLI vs. Belden 1694A Cable Length

APPLICATION OF CLI: EXTENDING 3G REACH

An application of CLI is to extend the 3G reach in systems which have margin in the jitter budget. This allows for

additional cable reach at 2.97 Gbps at the expense of slightly higher output jitter. The extended 3G reach mode

provides 15m of additional Belden 1694A cable reach, with an increase of output jitter at this longer cable length

of 0.05 to 0.1 UI.

Product Folder Links: LMH0384