Data Sheet AD841
Rev. C | Page 11 of 16
Figure 31 shows the long-term stability of the settling charac-
teristics of the AD841 output after a 10 V step. There is no
evidence of settling tails after the initial transient recovery
time. The use of a junction isolated process, together with
careful layout, avoids these problems by minimizing the effects
of transistor isolation capacitance discharge and thermally
induced shifts in circuit operating points. These problems
do not occur even under high output current conditions.
Figure 31. AD841 Settling Demonstrating No Settling Tails
GROUNDING AND BYPASSING
In designing practical circuits with the AD841, the user must
remember that whenever high frequencies are involved, some
special precautions are in order. Circuits must be built with
short interconnect leads. Large ground planes should be used
whenever possible to provide a low resistance, low inductance
circuit path, as well as minimizing the effects of high frequency
coupling. Avoid sockets because the increased interlead
capacitance can degrade bandwidth.
Feedback resistors should be of low enough value to assure that
the time constant formed with the circuit capacitances will not
limit the amplifier performance. Resistor values of less than
5 kΩ are recommended. If a larger resistor must be used, a
small (<10 pF) feedback capacitor in parallel with the feed-
back resistor, R
F
, may be used to compensate for these stray
capacitances and optimize the dynamic performance of the
amplifier in the particular application.
Bypass power supply leads to ground as close as possible to
the amplifier pins. A 2.2 µF capacitor in parallel with a 0.1 µF
ceramic disk capacitor is recommended.
CAPACITIVE LOAD DRIVING ABILITY
Like all wideband amplifiers, the AD841 is sensitive to capaci-
tive loading. The AD841 is designed to drive capacitive loads
of up to 20 pF without degradation of its rated performance.
Capacitive loads of greater than 20 pF will decrease the dynamic
performance of the part although instability should not occur
unless the load exceeds 100 pF (for a unity-gain follower). A
resistor in series with the output can be used to decouple larger
capacitive loads.
Figure 32 shows a typical configuration for driving a large
capacitive load. The 51 Ω output resistor effectively isolates the
high frequency feedback from the load and stabilizes the circuit.
Low frequency feedback is returned to the amplifier summing
junction via the low-pass filter formed by the 51 Ω resistor and
the load capacitance, C
L
.
Figure 32. Circuit for Driving a Large Capacitive Load
100
90
10
0%
500ns
OUTPUT ERROR:
0.02%/DIV
OUTPUT:
5V/DIV
11340-031
11
6
4
5
10
–
+
AD841
+V
S
–V
S
2.2µF
0.1µF
2.2µF
0.1µF
499Ω
1kΩ
15pF
1kΩ
INPUT
51Ω
C
L
R
L
V
OUT
11340-032
