AD8001A Analog Devices, AD8001A Datasheet - Page 10

• Manufacturer Part Number: AD8001A
• Description: 800 Mhz, 50 MW Current Feedback Amplifier
• Manufacturer: Analog Devices

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 ex-
pressed as transimpedance, V
transimpedance 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 40, basic
analysis yields the following result.
Recognizing that G
the first order that bandwidth for this amplifier is independent
of gain (G). This simple analysis in conjunction with Figure 41
can, in fact, predict the behavior of the AD8001 over a wide
range of conditions.
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
ure 42 illustrates this problem. Here the fine scale (0.1 dB/div)
flatness is plotted vs feedback resistance. These plots were taken
using an evaluation card which is available to customers so that
these results may readily be duplicated (see Evaluation Board
section).
IN
Z
V
G
is proportional to 1/g
V
IN
O
g
V
R2
M
IN
1
, where the g
G
R
R
R
IN
1
2
T
<< R1 for low gains, it can be seen to
Figure 40.
Z
( )
R
S
R
O
IN
IN
R1
/ I
M
T
in question is the trans-
–IN
M
G
Z
1
, the equivalent voltage
, or T
( )
/
S
g
M
R
F
IN
can be difficult. Fig-
Z
. The open-loop
50
F
V
R
. In practice
OUT
1
–10–
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 associ-
ated parasitic capacitance and inductance. Surface mount resis-
tors were used for the bulk of the characterization for this data
sheet. It is not recommended that leaded components be used
with the AD8001.
100k
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
10k
100
0.1
1M
1k
10
0
100k
Figure 41. Transimpedance vs. Frequency
1M
Figure 42. 0.1 dB Flatness vs. Frequency
G = +2
1M
FREQUENCY – Hz
FREQUENCY – Hz
10M
10M
R
F
R
= 698
F
= 750
100M
100M
R
649
F
=
1G
REV. C