AD9639 Analog Devices, Inc., AD9639 Datasheet - Page 25

• Manufacturer Part Number: AD9639
• Description: Quad, 12-bit, 170 Msps/210 Msps Serial Output 1.8 V Adc
• Manufacturer: Analog Devices, Inc.

Digital Outputs and Timing
The AD9639 has differential digital outputs that power up
by default. The driver current is derived on chip and sets the
output current at each output equal to a nominal 8 mA. Each
output presents a 100 Ω dynamic internal termination to reduce
unwanted reflections.
A 100 Ω differential termination resistor should be placed at
each receiver input to result in a nominal 400 mV peak-to-peak
swing at the receiver. Alternatively, single-ended 50 Ω termina-
tion can be used. When single-ended termination is used, the
termination voltage should be DRVDD/2; otherwise, ac coupling
capacitors can be used to terminate to any single-ended voltage.
The AD9639 digital outputs can interface with custom ASICs
and FPGA receivers, providing superior switching performance
in noisy environments. Single point-to-point network topologies
are recommended with a single differential 100 Ω termination
resistor placed as close to the receiver logic as possible. The
common mode of the digital output automatically biases itself
to half the supply of the receiver (that is, the common-mode
voltage is 0.9 V for a receiver supply of 1.8 V) if dc-coupled
connecting is used. For receiver logic that is not within the
bounds of the DRVDD supply, an ac-coupled connection
should be used. Simply place a 0.1 μF capacitor on each output
pin and derive a 100 Ω differential termination close to the
receiver side.
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If there is no far-end receiver termination or if there is poor
differential trace routing, timing errors may result. To avoid
such timing errors, it is recommended that the trace length be
less than 6 inches and that the differential output traces be close
together and at equal lengths.
OUTPUT SWING = 400mV p-p
OUTPUT SWING = 400mV p-p
DOUT + x
DOUT – x
DRVDD
Figure 57. DC-Coupled Digital Output Termination Example
Figure 58. AC-Coupled Digital Output Termination Example
DOUT + x
DOUT – x
DRVDD
0.1µF
0.1µF
DIFFERENTIAL
DIFFERENTIAL
TRACE PAIR
TRACE PAIR
100Ω
100Ω
100Ω
100Ω
V
OR
RXCM
RECEIVER
V
V
CM
CM
RECEIVER
= Rx V
= DRVDD/2
AD9639
CM