AD8391ARZ Analog Devices Inc, AD8391ARZ Datasheet - Page 10

• Manufacturer Part Number: AD8391ARZ
• Description: IC LINE DRVR XDSL PWR DWN 8SOIC
• Manufacturer: Analog Devices Inc
• Type: Driverr

Specifications of AD8391ARZ

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 3 V ~ 12 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• For Use With: AD8391AR-EVAL - BOARD EVAL FOR AD8391
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD8391
GENERAL INFORMATION
Theory of Operation
The AD8391 is a dual current feedback amplifier with high
output current capability. It is fabricated on Analog Devices’
proprietary eXtra Fast Complementary Bipolar Process (XFCB) that
enables the construction of PNP and NPN transistors with f
greater than 3 GHz. The process uses dielectrically isolated
transistors to eliminate the parasitic and latch-up problems caused
by junction isolation. These features enable the construction of
high frequency, low distortion amplifiers.
The AD8391 has a unique pin out. The two noninverting inputs
of the amplifier are connected to the V
biased by two 5 k resistors forming a voltage divider between
+V
10 pF internal capacitor from V
are available at Pin 1 and Pin 8, allowing the gain of the amplifiers to
be set with external resistors. See Page 1 for a connection diagram
of the AD8391.
A simplified schematic of an amplifier is shown in Figure 3.
Emitter followers buffer the positive input, V
input current and current noise. The low impedance current
feedback summing junction is at the negative input, V
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 bandwidth and distortion perfor-
mance are relatively insensitive to its closed-loop signal gain,
which is a distinct advantage over a voltage-feedback architecture.
Figure 4 shows a simplified model of a current feedback amplifier.
The feedback signal is an error current that flows into the inverting
node. R
the amplifier’s input stage, g
follower with gain circuit yields:
Recognizing that G R
when Tz(s) = R
depends primarily on the feedback resistor. There is a value of
R
will have additional poles that will contribute excess phase shift.
The optimum value for R
of peaking tolerable in the application. For more information
about current feedback amplifiers, see ADI’s high speed design
techniques at www.analog.com/technology/amplifiersLinear/
designTools/evaluationBoards/pdf/1.pdf.
F
S
below which the amplifier will be unstable, as the amplifier
and –V
IN
is inversely proportional to the transconductance of
S
. V
MID
F
, one can see that the amplifier’s bandwidth
V
V
OUT
is accessible through Pin 7. There is also a
IN
Tz s
R
IN
IN
F
Tz s
<< R
G
mi
depends on the gain and the amount
. Circuit analysis of the pictured
where:
1
g
1
1
F
G
mi
MID
, and that the –3 dB point is set
sC R
R
R
R
R
F
to –V
Tz s
T
F
125
G
F
(
MID
G
T
S.
)
pin, which is internally
The two inverting pins
R
P
IN
, to provide low
N
. The
T
’s
–10–
Feedback Resistor Selection
In current feedback amplifiers, selection of the feedback and
gain resistors will impact distortion, 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 circuits in TPC 1 and TPC 19. These values are only
intended to be a starting point when designing for any application.
V
P
Figure 4. Model of Current Feedback Amplifier
V
+
–
IN
V
Gain
–1
–2
–3
–4
–5
N
Figure 3. Simplified Schematic
R
Table I. Resistor Selection Guide
G
G = 1
R
IN
I
IN
R
909
909
909
909
909
F
( )
I
R
T
F
= I
IN
C
T
R
T
R
909
453
303
227
178
G
( )
V
O
V
V
OUT
+
–
O
REV. A
BIAS