LMH0324
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ZHCSIC8B –APRIL 2016–REVISED JUNE 2018
7.3 Feature Description
The LMH0324 consists of several key blocks:
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4-Level Input Configuration Pins
Carrier Detect
Adaptive Cable Equalizer
Launch Amplitude
Input-Output Mux Selection
Output Function Control
Output Driver Amplitude and De-Emphasis Control
7.3.1 4-Level Input Configuration Pins
The 4-level input configuration pins use a resistor divider to provide four logic states for each control pin. There is
an internal 30-kΩ pullup and a 60-kΩ pulldown connected to the control pin that sets the default voltage at 2/3 x
VDDIO. These resistors, together with the external resistor, combine to achieve the desired voltage level. By
using the 1-kΩ pull-down, 20-kΩ pulldown, no connect, and 1-kΩ pullup, the optimal voltage levels for each of the
four input states are achieved as shown in Table 1.
Table 1. 4-Level Control Pin Settings
LEVEL
SETTING
RESULTING PIN VOLTAGE
H
F
R
L
Tie 1 kΩ to VDDIO
Float (leave pin open)
Tie 20 kΩ to VSS
Tie 1 kΩ to VSS
VDDIO
2/3 × VDDIO
1/3 × VDDIO
0
Typical 4-Level Input Thresholds:
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Internal Threshold between L and R = 0.2 × VDDIO
Internal Threshold between R and F = 0.5 × VDDIO
Internal Threshold between F and H = 0.8 × VDDIO
7.3.2 Carrier Detect
An internal Carrier Detector circuit is used to monitor the presence or absence of the input signal. When the input
signal amplitude exceeds the carrier detector’s threshold, adaptation is activated, and CD_N is pulled low at the
end of adaptation. When the input signal amplitude is below the carrier detector’s threshold, input equalization
circuitry is powered down and the CD_N is pulled high to indicate absence of input signal. The LMH0324 high
gain adaptive cable equalizer supports long cable reach. As a result, the carrier detector threshold is sensitive,
and system designers need to pay close attention to the PCB layout to avoid excessive crosstalk from interfering
with the carrier detection.
In the absence of input signal, the LMH0324 automatically goes into Power Save Mode to conserve power
consumption. When valid signal is detected, the LMH0324 automatically exits the Power Save Mode and returns
to the normal operating mode. An LED can be connected to CD_N through a current limiting resistor to provide
visual indication of the carrier detection.
7.3.3 Adaptive Cable Equalizer
IN0+ is the input to the adaptive cable equalizer. It has an on-chip 75-Ω termination to the input common mode
voltage and includes a series return loss compensation network for meeting stringent SMPTE return loss
requirements. It is designed for AC coupling, requiring a 4.7-µF AC coupling capacitor for minimizing base-line
wander due to the rare-occurring pathological bit pattern. The cable equalizer is designed with high gain and low
noise circuitry to compensate for the insertion loss of a coaxial cable, such as Belden 1694A, which is widely
used in broadcast video infrastructures.
Internal control loops are used to monitor the input signal quality and automatically select the optimum
equalization boost and DC offset compensation. The LMH0324 is designed to handle the stringent pathological
pattern defined in the SMPTE RP 198 and SMPTE RP 178 standards.
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