Datasheet

0 50 100 150 200 250 300 350 400
BELDEN 1694A CABLE LENGTH (m)
2.0
2.2
2.4
2.6
2.8
3.0
3.2
CD
THRESH
(V)
LMH0387
www.ti.com SNLS315G –APRIL 2010–REVISED APRIL 2013
The default differential output swing (V
SSP-P
) is 700 mV
P-P
. The differential output swing may be adjusted via the
SPI in 100 mV increments from 400 mV
P-P
to 800 mV
P-P
(see OUTPUT DRIVER ADJUSTMENTS (REGISTER
01h) in the Input Mode (Equalizer) SPI Register Access section).
The LMH0387 equalizer output should be DC coupled to the input of the receiving device as long as the common
mode ranges of both devices are compatible. 100Ω differential transmission lines should be used to connect
between the LMH0387 outputs and the input of the receiving device where possible.
The LMH0387 allows flexibility when interfacing to low voltage crosspoint switches (i.e. 1.8V) and other devices
with limited input ranges. The LMH0387 equalizer outputs can be DC coupled to these devices in most cases,
avoiding the need to AC couple.
The LMH0387 may be AC coupled to the receiving device when necessary. For example, the LMH0387
equalizer outputs are not strictly compatible with 3.3V CML and thus should not be connected via 50Ω resistors
to 3.3V. If the input common mode range of the receiving device is not compatible with the output common mode
range of the LMH0387, then AC coupling is required. Following the AC coupling capacitors, the signal may have
to be biased at the input of the receiving device.
CARRIER DETECT (CD)
Carrier detect CD indicates if a valid signal is present at the LMH0387 BNC_IO pin. If CD
THRESH
is used, the
carrier detect threshold will be altered accordingly. CD provides a high voltage when no signal is present at the
LMH0387 BNC_IO pin. CD is low when a valid input signal is detected.
CARRIER DETECT THRESHOLD (CD
THRESH
)
The CD
THRESH
pin sets the threshold for the carrier detect. The carrier detect threshold is set by applying a
voltage inversely proportional to the length of cable to equalize before loss of carrier is triggered. The applied
voltage must be greater than the CD
THRESH
floating voltage (typically 1.3V) in order to change the CD threshold.
As the applied CD
THRESH
voltage is increased, the amount of cable that will be equalized before carrier detect is
de-asserted is decreased. CD
THRESH
may be left unconnected or connected to ground for normal CD operation.
Figure 4 shows the minimum CD
THRESH
input voltage required to force carrier detect to inactive vs. Belden 1694A
cable length. The results shown are 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.
Figure 4. CD
THRESH
vs. Belden 1694A Cable Length
AUTO SLEEP
The LMH0387 equalizer is set for auto sleep operation by default. The equalizer portion of the LMH0387 powers
down when no input signal is detected on the BNC_IO pin. The equalizer powers on again once an input signal
is detected. The auto sleep functionality can be changed to force sleep or turned off completely via the SPI
registers.
Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LMH0387