LMH0303 Datasheet, PDF (16/27 Pages) National Semiconductor (TI) – 3 Gbps HD/SD SDI Cable Driver with Cable Detect

LMH0303
SNLS285H – APRIL 2008 – REVISED MAY 2016
8 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LMH0303 is a single-channel SDI cable driver that supports different application spaces. The following
sections describe the typical use cases and common implementation practices.
8.1.1 General Guidance for All Applications
The LMH0303 supports two modes of configuration: Pin control or SMBus mode. Once one of these two control
mechanism is chosen, pay attention to the PCB layout for the high-speed signals (see Layout Guidelines). The
SMPTE specifications also define the use of AC-coupling capacitors for transporting uncompressed serial data
streams with heavy low-frequency content. This specification requires the use of a 4.7-µF AC-coupling capacitor
to avoid low frequency DC wander. The 75-Ω signal is also required to meet certain rise and fall timing to
facilitate highest eye opening for the receiving device.
SMPTE specifies the requirements for the Serial Digital Interface to transport digital video at SD, HD, 3 Gb/s, and
higher data rates over coaxial cables. One of the requirements is meeting the required return loss. This
requirement specifies how closely the port resembles 75-Ω impedance across a specified frequency band.
Output return loss is dependent on board design. The LMH0303 supports these requirements. To gain additional
return loss margin, return loss network in Figure 5 can be optimized.
8.1.2 Cable Fault Detection Operation
The termination fault detection of the LMH0303 indicates when no cable is connected to the output (near end).
The termination fault detection works by detecting reflections on the output. The device measures the peak-to-
peak output voltage. The output amplitude is normally 800 mV_p-p. No termination results in 2x the output voltage
(1600 mV_p-p) due to the 100% reflection. At the lowest threshold settings, the LMH0303 detects fault even with
the amplitude down near 800 mV_p-p (perfect termination). At the highest threshold settings, the LMH0303 will
detect no fault even as the amplitude approaches 1600 mV_p-p (completely unterminated). With the default register
settings (HDTFThresh = 9) and with a 3G input, the cable driver will detect the unloaded condition if it sees an
amplitude greater than about 450 mV_p-p below the maximum (that is, the output amplitude, including the
reflection, is about 1.15 V_p-p). Each step of the HD threshold register changes this decision voltage by
approximately 100 mV and this can range from 50 to 200 mV. These results are dependent on the board layout
and passive components.
Since the termination fault detection threshold is dependent on the PCB layout, the threshold may need to be
fine tuned for each design. To help in setting the termination fault threshold, the termination fault counters
(registers 0x0A and 0x0B) gauges how the cable driver is interpreting the output termination. The termination
fault counter counts the termination faults seen at the LMH0303 output. It counts up or down: up one tick when a
termination fault is detected, and down one tick when a proper termination is detected. The counter ranges from
0 to 31 (decimal). When there is no termination fault, the value will be near 0. When the count hits 31, the
termination fault indicator is asserted. These registers can be used to fine tune the termination fault threshold
setting. If there are many termination fault counts when the output is properly terminated, then the termination
fault threshold should be increased. If the register is not showing consistent counts of 31 when the output is
unterminated, then the termination fault threshold should be decreased.
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