AD8001A Analog Devices, AD8001A Datasheet - Page 11

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

Printed Circuit Board Layout Considerations
As to be expected for a wideband amplifier, PC board parasitics
can affect the overall closed-loop performance. Of concern are
stray capacitances at the output and the inverting input nodes. If
a ground plane is to be used on the same side of the board as
the signal traces, a space (5 mm min) should be left around the
signal lines to minimize coupling. Additionally, signal lines
connecting the feedback and gain resistors should be short
enough so that their associated inductance does not cause high
frequency gain errors. Line lengths on the order of less than
5 mm are recommended. If long runs of coaxial cable are being
driven, dispersion and loss must be considered.
Power Supply Bypassing
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 F) will be required to provide the best
settling time and lowest distortion. A parallel combination of
4.7 F and 0.1 F is recommended. Some brands of electrolytic
capacitors will require a small series damping resistor 4.7
optimum results.
DC Errors and Noise
There are three major noise and offset terms to consider in a
current feedback amplifier. For offset errors refer to the equa-
tion below. For noise error the terms are root-sum-squared to
give a net output error. In the circuit below (Figure 43) they are
input offset (V
noise gain of the circuit (1 + R
(I
input current, which when divided between R
sequently multiplied by the noise gain always appears at the
output as I
low 2 nV/ Hz. At low gains though the inverting input current
noise times R
device matching contribute to better offset and drift specifica-
tions for the AD8001 compared to many other current feedback
amplifiers. The typical performance curves in conjunction with
the equations below can be used to predict the performance of
the AD8001 in any application.
REV. C
V
BN
OUT
R
N
) also multiplied by the noise gain, and the inverting
V
BN
IO
F
Figure 43. Output Offset Voltage
IO
is the dominant noise source. Careful layout and
R
R
R
) which appears at the output multiplied by the
F
N
I
1
. The input voltage noise of the AD8001 is a
R
R
F
I
I
I
BI
BN
I
BN
F
R
/R
F
I
), noninverting input current
R
N
1
F
R
R
and R
V
F
I
OUT
I
I
and sub-
BI
R
for
F
–11–
Driving Capacitive Loads
The AD8001 was designed primarily to drive nonreactive loads.
If driving loads with a capacitive component is desired, best
frequency response is obtained by the addition of a small series
resistance as shown in Figure 44. The accompanying graph
shows the optimum value for R
worth noting that the frequency response of the circuit when
driving large capacitive loads will be dominated by the passive
roll-off of R
Figure 45. Recommended R
40
30
20
10
0
I
0
N
SERIES
Figure 44. Driving Capacitive Loads
and C
5
G = +1
L
.
909
10
C
SERIES
L
– pF
SERIES
15
vs. capacitive load. It is
R
SERIES
vs. Capacitive Load
500
R
L
20
AD8001
C
L
25