LMH0387

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SNLS315G – APRIL 2010 – REVISED APRIL 2013

OUTPUT DRIVER ADJUSTMENTS (REGISTER 01h)

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

register 01h.

Output Swing

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

Offset Voltage

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 (REGISTER 02h)

The LMH0387 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.

attenuation, the equalizer launch amplitude fine tuning must be set to the “nominal” setting via the SPI.

To do this, write 30h (“00110000” binary) to SPI register 02h.

Coarse Control

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

Fine Control

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

register 02h. These bits may be used to tweak the input gain stage -2% to +60% around the coarse control

setting. For typical equalizer operation, bits [6:3] of SPI register 02h should be changed from the default setting

of “0000” to the nominal setting of “0110”.

CABLE LENGTH INDICATOR (CLI) (REGISTER 03h)

The Cable Length Indicator (CLI) provides an indication of the length of cable attached to the equalizer 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 5 shows typical CLI values vs. Belden

1694A cable length. CLI is valid for Belden 1694A cable lengths of 0-120m at 2.97 Gbps, 0-200m at 1.485 Gbps,

and 0-400m at 270 Mbps.

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