AD8018ARU-EVAL Analog Devices Inc, AD8018ARU-EVAL Datasheet - Page 8

• Manufacturer Part Number: AD8018ARU-EVAL
• Description: BOARD EVAL FOR AD8018
• Manufacturer: Analog Devices Inc

Specifications of AD8018ARU-EVAL

• Rohs Status: RoHS non-compliant
• Main Purpose: Interface, Line Driver, xDSL
• Utilized Ic / Part: AD8018ARU
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / Rohs Status: Not Compliant

AD8018
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, V
current noise. The low impedance current feedback summing
junction is at the negative input, V
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. R
proportional to the transconductance of the amplifier’s input stage,
g
where:
Recognizing that G
when T
depends primarily on the feedback resistor. There is a value of
R
fier will have additional poles that will contribute excess phase
shift. The optimum value for R
amount of peaking tolerable in the application.
mi
F
. Circuit analysis of the pictured follower with gain yields:
below which the amplifier will be unstable, as an actual ampli-
Z(S)
= R
V
OUT
F
, one can see that the amplifier’s bandwidth
/
V
IN
R
T
R
IN
Z S
IN
G
( )
G
< R
P
1
, to provide low input current and
1
/
T
F
1
g
, and that the –3 dB point is set
Z S
mi
F
( )
R R
S T
depends on the gain and the
N
F
C R
R
. The output stage is another
/
125
T
R
(
T
G
F
Z S
T
( )
–100
–110
)
–10
–20
–30
–40
–50
–60
–70
–80
–90
G R
SIDE B DRIVEN
V
G = 2
V
IN
S
IN
=
R
= 2V p-p
IN
100k
L
2.5
= 5
is inversely
SIDE A DRIVEN
1M
R
FREQUENCY – Hz
L
R
SIDE A DRIVEN
= 5
L
= 100
FEEDBACK RESISTOR SELECTION
In current feedback amplifiers, selection of the feedback and gain
resistors will impact 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.
V
P
10M
R
SIDE B DRIVEN
L
V
= 100
+
–
IN
100M
V
N
R
G
G = 1
R
1G
IN
I
IN
I
R
T
F
= I
IN
C
T
R
T
V
O
V
V
O
OUT
+
–
BIAS