MAX3812
Slew-Rate Selection Switch
The MAX3812 output has a selectable slew-rate in
order to achieve compliance to SMPTE 259M/344M
and SMPTE 292M. Output slew rate is selected with the
SD/HD input (Figure 4). The SD/HD pin must be set
high for SMPTE 259M/344M standards and low for
SMPTE 292M standards.
Differential Data Outputs
The MAX3812 differential data outputs are current-mode
logic, CML (Figure 5). Each side of the differential output
will drive 800mV
P-P
into a doubly terminated 75Ω trans-
mission line (R
SET
= 750Ω).
The cable driver output must have a source impedance
of 75Ω and a return loss of at least 15dB between 5MHz
and 1.485GHz to comply with the SMPTE 259M, SMPTE
344M, and SMPTE 292M standards. In order to meet the
return loss specification, the output network shown in
the Typical Application Circuit is recommended.
The value of the output inductor is typically 6.8nH, but the
value will vary depending on PC board layout. If the out-
puts are AC-coupled, a minimum capacitor value of 4.7µF
should be used to accommodate SMPTE pathological
test signals.
When measuring output return loss, it is necessary to take
the measurement for both a logic-high and logic-low
state. This is due to the output capacitance being slightly
dependent on the logic state. See the Output Return Loss
graph in the Typical Operating Characterisitics.
Output Voltage Swing Control
The external R
SET
resistor connected between V
CC
and
the RSET pin controls the output voltage swing (Figure 5).
Typically the output voltage swing is set to 800mV
P-P
using a value of 750Ω. For output voltage swings other
than 800mV
P-P
, see the graph Output Voltage Swing vs.
R
SET
given in the Typical Operating Characteristics.
Using R
SET
values below 500Ω is not recommended.
The R
SET
resistor should be placed as close as possible
to the RSET pin and the capacitance at this node should
be minimized.
Applications Information
Layout Considerations
Use controlled-impedance transmission lines to interface
with the MAX3812 high-speed inputs and outputs.
Power-supply decoupling capacitors should be placed
as close as possible to the V
CC
connections. To support
a 75Ω transmission-line environment at the outputs, it is
necessary to minimize the parasitic capacitance by
removing the PC board ground plane from under the out-
put pins and output components. Transmission line
stubs associated with BNC connectors should be mini-
mized. Edge-mount BNC connectors are recommended.
Multirate SMPTE SD/HD Cable Driver
with Selectable Slew Rate
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