AD8016

Data Sheet

Rev. C | Page 14 of 20

THEORY OF OPERATION

The AD8016 is a current feedback amplifier with high

(500 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,

dVOUT/dIIN or TZ. The open-loop transimpedance is analogous

to the open-loop voltage gain of a voltage feedback amplifier.

Figure 42 shows a simplified model of a current feedback ampli-

fier. Because 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

F

IN

Z

Z

IN

OUT

R

R

G

S

T

S

T

G

V

V

+

×

+

×

=

)

(

)

(

where:

G

F

R

R

G

+

= 1

Ω

≈

=

25

1

m

IN

g

R

Recognizing that G × RIN << RF for low gains, the familiar result

of constant bandwidth with gain for current feedback amplifiers

is evident, the 3 dB point being set when |TZ| = RF. Of course,

for a real amplifier there are additional poles that contribute

excess phase and there is a value for RF below which the ampli-

fier is unstable. Tolerance for peaking and desired flatness

determines the optimum RF in each application.

Figure 42. Simplified Block Diagram

The AD8016 is the first current feedback amplifier capable of

delivering 400 mA of output current while swinging to within

2 V of either power supply rail. This enables full CO ADSL

performance on only 12 V rails, an immediate 20% power

saving. The AD8016 is also unique in that it has a power

management system included on-chip. It features four user

programmable power levels (all of which provide a low output

impedance of the driver), as well as the provision for complete

shutdown (high impedance state). Also featured is a thermal

shutdown with alarm signal.

POWER SUPPLY AND DECOUPLING

The AD8016 should be powered with a good quality (that is,

low noise) dual supply of ±12 V for the best distortion and

multitone power ratio (MTPR) performance. Careful attention

must be paid to decoupling the power supply pins. A 10 μF

capacitor located in near proximity to the AD8016 is required

to provide good decoupling for lower frequency signals. In

addition, 0.1 μF decoupling capacitors should be located as

close to each of the four power supply pins as is physically

possible. All ground pins should be connected to a common

low impedance ground plane.

FEEDBACK RESISTOR SELECTION

In current feedback amplifiers, selection of feedback and gain

resistors has an impact on the MTPR performance, bandwidth,

and gain flatness. Take care in selecting these resistors so that

optimum performance is achieved. Table 6 below shows the

recommended resistor values for use in a variety of gain

settings. These values are suggested as a good starting point

when designing for any application.

Table 6. Resistor Selection Guide

Gain

RF (Ω)

RG (Ω)

+1

1000

∞

−1

500

500

+2

650

650

+5

750

187

+10

1000

111

BIAS PIN AND PWDN FEATURES

The AD8016 is designed to cover both central office (CO)

and customer premise equipment (CPE) ends of an xDSL

application. It offers full versatility in setting quiescent bias

levels for the particular application from full on to reduced

bias (in three steps) to full off (via BIAS pin). This versatility

gives the modem designer the flexibility to maximize efficiency

while maintaining reasonable levels of MTPR performance.

Optimizing driver efficiency while delivering the required DMT

power is accomplished with the AD8016 through the use of on-

chip power management features. Two digitally programmable

logic pins, PWDN1 and PWDN0, may be used to select four

different bias levels: 100%, 60%, 40%, and 25% of full quiescent

power (see Table 7).

Table 7. PWDN Code Selection Guide

PWDN1

Code

PWDN0

Code

Quiescent Bias Level

1

1

100% (full on)

1

0

60%

0

1

40%

0

0

25% (low ZOUT but not off)

X

X

Full off (high ZOUT via 250 μA pulled out of

BIAS pin)

TZ

RIN

IIN

+

+

–

RF

VOUT

RN

RG

VIN