AD8001 Analog Devices, AD8001 Datasheet - Page 10

• Manufacturer Part Number: AD8001
• Description: 800 MHz, 50 mW Current Feedback Amplifier
• Manufacturer: Analog Devices

Specifications of AD8001

• -3db Bandwidth: 880MHz
• Slew Rate: 1kV/µs
• Vos: 2mV
• Ib: 3µA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 2nV/rtHz
• Vcc-vee: 6V to 12V
• Isy Per Amplifier: 5mA
• Packages: DIP,SOIC,SOT

AD8001
THEORY OF OPERATION
A very simple analysis can put the operation of the AD8001, a
current feedback amplifier, in familiar terms. Being a current
feedback amplifier, the AD8001’s open-loop behavior is expressed
as transimpedance, ∆V
ance behaves just as the open-loop voltage gain of a voltage
feedback amplifier, that is, it has a large dc value and decreases
at roughly 6 dB/octave in frequency.
Since the R
gain is just T
conductance of the input stage. This results in a low open-loop
input impedance at the inverting input, a now familiar result.
Using this amplifier as a follower with gain, Figure 4, basic
analysis yields the following result.
Recognizing that G × R
the first order that bandwidth for this amplifier is independent
of gain (G). This simple analysis in conjunction with Figure 5
can, in fact, predict the behavior of the AD8001 over a wide
range of conditions.
100k
10k
100
1M
1k
10
100k
Figure 5. Transimpedance vs. Frequency
IN
Z
is proportional to 1/g
V
G
× g
V
IN
O
Figure 4. Follower with Gain
=
M
V
R2
IN
, where the g
=
1
+
G
1M
O
/∆I
R
R
IN
×
2
1
T
–IN
<< R1 for low gains, it can be seen to
FREQUENCY – Hz
Z
, or T
( )
R
S
R
IN
M
R1
IN
10M
+
in question is the trans-
Z
=
T
. The open-loop transimped-
M
G
Z
1
, the equivalent voltage
( )
/
×
S
g
M
R
IN
≈
100M
+
50 Ω
V
R
OUT
1
1G
–10–
Considering that additional poles contribute excess phase at
high frequencies, there is a minimum feedback resistance below
which peaking or oscillation may result. This fact is used to
determine the optimum feedback resistance, R
parasitic capacitance at Pin 2 will also add phase in the feedback
loop, so picking an optimum value for R
Figure 6 illustrates this problem. Here the fine scale (0.1 dB/
div) flatness is plotted versus feedback resistance. These plots
were taken using an evaluation card which is available to cus-
tomers so that these results may readily be duplicated.
Achieving and maintaining gain flatness of better than 0.1 dB at
frequencies above 10 MHz requires careful consideration of
several issues.
Choice of Feedback and Gain Resistors
Because of the above-mentioned relationship between the band-
width and feedback resistor, the fine scale gain flatness will, to
some extent, vary with feedback resistance. It, therefore, is
recommended that once optimum resistor values have been
determined, 1% tolerance values should be used if it is desired to
maintain flatness over a wide range of production lots. In addition,
resistors of different construction have different associated parasitic
capacitance and inductance. Surface-mount resistors were used
for the bulk of the characterization for this data sheet. It is not
recommended that leaded components be used with the AD8001.
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
0.1
0
1M
Figure 6. 0.1 dB Flatness vs. Frequency
G = +2
FREQUENCY – Hz
10M
R
F
R
= 698
F
= 750
F
can be difficult.
F
. In practice,
100M
R
649
F
=
REV. D