GS3490 Datasheet, PDF (14/22 Pages) Semtech Corporation – Configurable Adaptive Cable Equalizer/Cable Driver

4.1.4 Carrier Detect (CD)
The Carrier Detect output pin (CD) indicates the presence of a signal at the input of the
GS3490. When CD is LOW, the device has detected an input on its SDI/SDI pins. When
CD is HIGH, the device has not detected an input signal.
When EQ_DISABLE is HIGH, the CD output will still function to facilitate the detection of
a serial input data signal. As a result, when in Equalizer Mode, auto power-down can be
implemented by directly tying CD to EQ_DISABLE. When connected, the GS3490 will
automatically go into lower power mode when there is a loss of Serial Digital Input
signal.
Note 1: CD will only operate correctly for data rates greater than 19Mb/s.
Note 2: If EQ_SQ_ADJ is being used to limit the maximum gain of the device, and the
maximum cable length is exceeded when EQ_BYPASS pin is set LOW, the CD pin will be
set HIGH even if a carrier is present.
Note 3: If the CD pin is connected to the EQ_DISABLE pin, EQ_SQ_ADJ must be either
left open, or connected to ground.
4.1.5 Squelch Adjust (EQ_SQ_ADJ)
The GS3490 features a Squelch Adjust (EQ_SQ_ADJ) threshold.
This feature can be useful in applications where there are multiple input channels using
the GS3490, since the maximum gain can be limited to avoid crosstalk.
The EQ_SQ_ADJ pin acts to change the threshold of the Carrier Detect (CD) pin, through
voltage level variances. When the input signal drops below a certain threshold, the CD
pin will be driven HIGH, indicating that there is not a valid input signal. In applications
where squelch adjust is not required, the EQ_SQ_ADJ pin can be left unconnected.
This feature has been designed for use in applications, where signal crosstalk and circuit
noise cause the equalizer to output erroneous data when no input signal is present. The
use of a Carrier Detect function with a fixed internal reference does not solve this
problem, since the signal-to-noise ratio on the circuit board could be significantly less
than the default signal detection level set by the on-chip reference.
Note: When using EQ_SQ_ADJ to limit the maximum gain of the GS3490, auto
power-down is not supported and so CD should not be connected to EQ_DISABLE.
4.1.6 Differential Digital Data Outputs (DDO/DDO)
The digital data output signals have a nominal output voltage swing of either 850mV_ppd
or 425mV_ppd, as set by the EQ_OP_CTL pin.
When EQ_DISABLE is HIGH, the differential digital outputs are high impedance and will
be pulled HIGH by the on-chip termination.
Table 4-2 shows the typical output common mode voltage levels (V_CMDDO) related to
the two output swing options and the type of output transmission termination as
shown in Figure 4-1 and Figure 4-2.
GS3490
Final Data Sheet
GENDOC-058568
Rev. 1
April 2014
www.semtech.com
14 of 22
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