AD8151AST(Rev0) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8151AST Datasheet PDF : 36 Pages
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AD8151
VCC – 2V
VCC
OUTyyP OUTyyN
DISABLE
IOUT
VEE
VEE
Figure 11. Simplified Output Circuit
To ensure proper operation, all outputs (including unused output)
must be pulled high using external pull-up networks to a level
within the output compliance range. If outputs from multiple
AD8151s are wired together, a single pull-up network may be
used for each output bus. The pull-up network should be chosen
to keep the output voltage levels within the output compliance
range at all times. Recommended pull-up networks to produce
PECL/ECL 100 kΩ and 10 kΩ compatible outputs are shown
in Figure 12. Alternatively, a separate supply can be used to
provide VCOM; making RCOM and DCOM unnecessary.
VCC
RCOM
AD8151 RL
OUTyyN
OUTyyP
VCOM
RL
VCC
DCOM
AD8151 RL
OUTyyN
OUTyyP
VCOM
RL
Figure 12. Output Pull-Up Networks: a) ECL 100 kΩ,
b) ECL 10 kΩ
The output levels are simply:
VOH = VCOM
VOL = VCOM – IOUTRL
VSWING = VOH – VOL = IOUTRL
VCOM = VCC – IOUTRCOM (100 kΩ Mode)
VCOM = VCC – V (DCOM) (10 kΩ Mode)
The common-mode adjustment element (RCOM or DCOM) may
be omitted if the input range of the receiver includes the positive
supply voltage. The bypass capacitors reduce common-mode
perturbations by providing an ac short from the common nodes
(VCOM) to ground.
When busing together the outputs of multiple AD8151s or when
running at high data rates, double termination of its outputs is
recommended to mitigate the impact of reflections due to open
transmission line stubs and the lumped capacitance of the
AD8151 output pins. A possible connection is shown in Figure
13; the bypass capacitors provide an ac short from the common
nodes of the termination resistors to ground. To maintain signal
fidelity at high data rates, the stubs connecting the output pins
to the output transmission lines or load resistors should be as
short as possible.
AD8151
OUTyyN
OUTyyP
AD8151
OUTyyN
OUTyyP
VCC
RCOM
RL
VCOM RL
ZO
ZO
ZO
RL
ZO
RL
RECEIVER
Figure 13. Double Termination of AD8151 Outputs
In this case, the output levels are:
VOH = VCOM – (1/4) IOUTRL
VOL = VCOM – (3/4) IOUTRL
VSWING = VOH – VOL = (1/2) IOUTRL
Output Current Set Pin (REF)
A simplified schematic of the reference circuit is shown in Fig-
ure 14. A single external resistor connected between the REF
pin and VEE determines the output current for all output stages.
This feature allows a choice of pull-up networks and transmission
line characteristic impedances while still achieving a nominal
output swing of 800 mV. At low data rates, substantial power
savings can be achieved by using lower output swings and higher
load resistances.
AD8151
IOUT/20
VCC
1.2V
REF
RSET
VEE
Figure 14. Simplified Reference Circuit
The nominal output current is given by the following expression:
IOUT
=
20
1.2 V
RSET
The minimum set resistor is RSET,MIN = 960 Ω resulting in
IOUT,MAX = 25 mA. The maximum set resistor is RSET,MAX =
4.8 kΩ resulting in IOUT,MIN = 5 mA. Nominal 800 mV differen-
tial output swing can be achieved in a 50 Ω load using RSET =
1.5 kΩ (IOUT = 16 mA), or in a doubly-terminated 75 Ω load
using RSET = 1.13 kΩ (IOUT = 21.3 mA).
To minimize stray capacitance and avoid the pickup of unwanted
signals, the external set resistor should be located close to the
REF pin. Bypassing the set resistor is not recommended.
–18–
REV. 0
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