AD8151AST datasheet(18/36 Pages) AD

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AD8151

–18–

REV. 0

VCC

VEE

DISABLE

IOUT

OUTyyP

OUTyyN

VCC – 2V

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 k

Ω 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

VCOM

OUTyyN

OUTyyP

AD8151

DCOM

VCC

VCOM

OUTyyN

OUTyyP

AD8151

Figure 12. Output Pull-Up Networks: a) ECL 100 k

Ω,

b) ECL 10 k

Ω

The output levels are simply:

VOH = VCOM

VOL = VCOM – IOUTRL

VSWING = VOH – VOL = IOUTRL

VCOM = VCC – IOUTRCOM (100 k

Ω Mode)

VCOM = VCC – V (DCOM) (10 k

Ω 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.

VCC

RCOM

VCOM

OUTyyN

OUTyyP

AD8151

OUTyyN

OUTyyP

AD8151

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

VEE

RSET

REF

VCC

IOUT/20

1.2V

Figure 14. Simpliﬁed Reference Circuit

The nominal output current is given by the following expression:

OUT

SET









The minimum set resistor is RSET,MIN = 960

Ω resulting in

IOUT,MAX = 25 mA. The maximum set resistor is RSET,MAX =

4.8 k

Ω resulting in I

OUT,MIN = 5 mA. Nominal 800 mV differen-

tial output swing can be achieved in a 50

Ω load using RSET =

1.5 k

Ω (IOUT = 16 mA), or in a doubly-terminated 75 Ω load

using RSET = 1.13 k

Ω (I

OUT = 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.

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