AD8018(Rev0) 查看數據表（PDF） - Analog Devices

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AD8018
(Rev.:Rev0)

5 V, Rail-to-Rail, High-Output Current, xDSL Line Drive Amplifier

Analog Devices 

AD8018

–10

VIN = 2V p-p

–20 G = 2

VS = ؎2.5

–30

RL = 5⍀

SIDE A DRIVEN

–40

RL = 5⍀

SIDE B DRIVEN

–50

–60

–70

RL = 100⍀

SIDE B DRIVEN

–80

–90

–100

RL = 100⍀

SIDE A DRIVEN

–110

100k

1M

10M

100M

1G

FREQUENCY – Hz

TPC 25. Crosstalk vs. Frequency

THEORY OF OPERATION

The AD8018 is composed of two current feedback amplifiers

capable of delivering 400 mA of output current while swinging

to within 0.5 V of either power supply, and maintaining low

distortion. A differential line driver using the AD8018 can provide

CPE performance on a single 5 V supply. This performance is

enabled by Analog Device’s XFCB process and a novel, two-

stage current feedback architecture featuring a patent-pending

rail-to-rail output stage.

A simplified schematic is shown in Figure 4. Emitter followers

buffer the positive input, VP, to provide low input current and

current noise. The low impedance current feedback summing

junction is at the negative input, VN. The output stage is another

high-gain amplifier used as an integrator to provide frequency

compensation. The complementary common-emitter output

provides the extended output swing.

A current feedback amplifier’s dynamic and distortion performance

is relatively insensitive to its closed-loop signal gain, which is

a distinct advantage over a voltage-feedback architecture. Figure

5 shows a simplified model of a current feedback amplifier. The

feedback signal is a current into the inverting node. RIN is inversely

proportional to the transconductance of the amplifier’s input stage,

gmi. Circuit analysis of the pictured follower with gain yields:

VOUT /VIN

=G×

TZ (S )

TZ (S )

+ RF + G × RIN

where:

G = 1 + RF /RG

TZ (S )

=

1+

RT

SCT (RT

)

RIN = 1/ gmi ≅ 125 Ω

Recognizing that G ϫ RIN < RF, and that the –3 dB point is set

when TZ(S) = RF, one can see that the amplifier’s bandwidth

depends primarily on the feedback resistor. There is a value of

RF below which the amplifier will be unstable, as an actual ampli-

fier will have additional poles that will contribute excess phase

shift. The optimum value for RF depends on the gain and the

amount of peaking tolerable in the application.

VP

VN

VO

BIAS

Figure 4. Simplified Schematic

G=1

+

VO

VIN

RIN IIN

IT = IIN CT RT

+

–

VOUT

–

RF

RG

Figure 5. Model of Current Feedback Amplifier

FEEDBACK RESISTOR SELECTION

In current feedback amplifiers, selection of the feedback and gain

resistors will impact on the MTPR performance, bandwidth,

noise, and gain flatness. Care should be exercised in the selection

of these resistors so that the optimum performance is achieved.

Table I shows the recommended resistor values for use in a variety

of gain settings for the test circuit in TPC 1. These values are

intended to be a starting point when designing for any application.

–8–

REV. 0

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