AD8392ARE Просмотр технического описания (PDF) - Analog Devices

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AD8392ARE

Low Power, High Output Current, Quad Op Amp, Dual-Channel ADSL/ADSL2+ Line Driver

Analog Devices 

AD8392

THEORY OF OPERATION

The AD8392 is a current feedback amplifier with high

(400 mA) output current capability. With a current feedback

amplifier, the current into the inverting input is the feedback

signal, and the open-loop behavior is that of a transimpedance,

dVO/dIIN or TZ.

Of course, for a real amplifier there are additional poles that

contribute excess phase, and there is a value for RF below which

the amplifier is unstable. Tolerance for peaking and desired

flatness determines the optimum RF in each application.

RF

The open-loop transimpedance is analogous to the open-loop

voltage gain of a voltage feedback amplifier. Figure 33 shows a

RG

simplified model of a current feedback amplifier. Since RIN is

proportional to 1/gm, the equivalent voltage gain is just TZ × gm,

where gm is the transconductance of the input stage. Basic

analysis of the follower with gain circuit yields

VO

VIN

=

G

×

TZ (S)

TZ (S)

+ G × RIN

+

RF

E where:

T G =1+ RF

RG

E R IN

=

1

gm

≈ 50 Ω

Since G × RIN << RF for low gains, a current feedback amplifier

has relatively constant bandwidth versus gain, the 3 dB point

OBSOL being set when |TZ| = RF.

RN

VIN

RIN

IIN TZ

VOUT

Figure 33. Simplified Block Diagram

The AD8392 is capable of delivering 400 mA of output current

while swinging to within 2 V of either power supply rail. The

AD8392 also has a power management system included on-chip.

It features four user-programmable power levels (three active

power modes as well as the provision for complete shutdown).

Rev. A | Page 11 of 16

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