Datasheet

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,
dV
OUT
/dI
IN
or T
Z
. 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 R
IN
is proportional to 1/g
m
, the equivalent voltage
gain is just T
Z
× g
m
, where g
m
is the transconductance of the
input stage. Basic analysis of the follower with gain circuit yields
FIN
Z
Z
IN
OUT
RRGST
S
T
G
V
V
+×+
×=
)(
)(
where:
G
F
R
R
G += 1
Ω≈= 25
1
m
IN
g
R
Recognizing that G × R
IN
<< R
F
for low gains, the familiar result
of constant bandwidth with gain for current feedback amplifiers
is evident, the 3 dB point being set when |T
Z
| = R
F
. Of course,
for a real amplifier there are additional poles that contribute
excess phase and there is a value for R
F
below which the ampli-
fier is unstable. Tolerance for peaking and desired flatness
determines the optimum R
F
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 R
F
(Ω) R
G
(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 Z
OUT
but not off)
X X Full off (high Z
OUT
via 250 μA pulled out of
BIAS pin)
T
Z
R
IN
I
IN
+
+
–
R
F
V
OUT
R
N
R
G
V
IN
01019-042