10.2 High Speed Outputs (SDO/SDO and SCO/SCO)
When trace lengths become greater than 1in, controlled
impedance traces should be used. The recommended
interface for differential signals is shown in Figure 18. In this
SDO/SDO and SCO/SCO are current mode outputs that
require external pullup resistors (see Figure 20). To
calculate the output sink current use the following
relationship:
case,
a parallel resistor (RLOAD) is placed near the
GS9035C inputs to terminate the controlled impedance
trace. The value of RLOAD should be twice the value of the
characteristic impedance of the trace. Both traces should
Output Sink Current = Output Signal Swing / Pullup Resistor
be in
a symmetric arrangement and same physical
A diode can be placed between Vcc and the pullup resis
tors to reduce the common mode voltage by approximately
0.7 volts. When the output traces are longer than 1in,
controlled impedance traces should be used. The pullup
resistors should be placed at the end of the output traces
as they terminate the trace in its characteristic impedance
(75Ω).
transmission line dimensions since common-mode signals
or common-mode noise is not terminated. In addition,
series resistors, RSOURCE, can be placed near the driving
chip to serve as source terminations. They should be equal
to the value of the trace impedance. Assuming 800mV
output swings at the driver, RLOAD =100Ω, RSOURCE =50Ω
and ZO = 50Ω.
V
CC
R
SOURCE
Z
O
DDI
DDI
R
GS9035C
LOAD
R
SOURCE
75Ω
75Ω
75Ω
75Ω
SDO
SDO
Z
O
GS9035C
SCO
SCO
Fig. 18 Recommended Interface for Differential Signals
Figure 19 shows the recommended interface when the
GS9035C is driven single-endedly. In this case, the input
must be AC-coupled and a matching resistor (ZO) must be
used.
V
CC
Fig. 20 High Speed Outputs with External Pullups
DDI
Z
GS9035C
O
DDI
Fig. 19 Recommended Interface for Single-Ended Driver
12 of 14
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