Datasheet

0 50 100 150 200 250 300 350 400
1.2
1.4
1.6
1.8
2.0
2.2
MUTE
REF
(V)
BELDEN 1694A CABLE LENGTH (m)
LMH0394
SNLS312L –AUGUST 2010–REVISED APRIL 2013 www.ti.com
DEVICE OPERATION
BLOCK DESCRIPTION
The Equalizer Filter block is a multi-stage adaptive filter. If Bypass is high, the equalizer filter is disabled.
The DC Restoration / Level Control block receives the differential signals from the equalizer filter block. This
block incorporates a self-biasing DC restoration circuit to fully DC restore the signals. If Bypass is high, this
function is disabled.
The signals before and after the DC Restoration / Level Control block are used to generate the Automatic
Equalization Control (AEC) signal. This control signal sets the gain and bandwidth of the equalizer filter.
The Carrier Detect block generates the carrier detect signal based on the SDI input and an adjustment from the
Mute Reference block.
The SPI Control block uses the MOSI, MISO, SCK, and SS signals in SPI mode to control the SPI registers.
SPI_EN selects between SPI mode and pin mode. In pin mode, SPI_EN is driven logic low.
The Output Driver produces SDO and SDO.
MUTE REFERENCE (MUTE
REF
)
The mute reference sets the threshold for CD and (with CD tied to MUTE) determines the amount of cable to
equalize before automatically muting the outputs. This is set by applying a voltage inversely proportional to the
length of cable to equalize. The applied voltage must be greater than the MUTE
REF
floating voltage (typically
1.3V) in order to change the CD threshold. As the applied MUTE
REF
voltage is increased, the amount of cable
that can be equalized before carrier detect is de-asserted and the outputs are muted is decreased. MUTE
REF
may be left unconnected or connected to ground for normal CD operation. Optionally, the LMH0394 allows the
mute reference to be set digitally via SPI register 03h.
Figure 4 shows the minimum MUTE
REF
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 0-200m at 2.97 Gbps, 0-220m at
1.485 Gbps, and 0-400m at 270 Mbps.
Figure 4. MUTE
REF
vs. Belden 1694A Cable Length
CARRIER DETECT (CD) AND MUTE
Carrier detect CD indicates if a valid signal is present at the LMH0394 input. This signal is logical OR operation
of the internal energy detector and MUTE
REF
setting (if used). The internal energy detector detects energy across
different data rates. If MUTE
REF
is used, the carrier detect threshold will be altered accordingly. CD provides a
high voltage when no signal is present at the LMH0394 input. CD is low when a valid input signal is detected.
MUTE can be used to manually mute or enable SDO and SDO. Applying a high input to MUTE will mute the
LMH0394 outputs by forcing the output to a logic 1. Applying a low input will force the outputs to be active.
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