LMH0387SL Datasheet, PDF (13/23 Pages) Texas Instruments – LMH0387 3 Gbps HD/SD SDI Configurable I/O Adaptive Cable Equalizer / Cable Driver

LMH0387
www.ti.com
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”
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.
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.
For correct equalizer operation with the default SMPTE 800 mV_P-P launch amplitude and no external
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”,
the equalizer operates normally and expects a launch amplitude of 800 mV_P-P. Bit 7 may be set to “1” to optimize
the equalizer for input signals with 6 dB of attenuation (400 mV_P-P).
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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