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-

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

Figure 32. Circuit for Driving a Large Capacitive Load

100

90

10

0%

500ns

OUTPUT ERROR:

0.02%/DIV

OUTPUT:

5V/DIV

11

6

4

5

10

–

+

AD841

2.2µF

0.1µF

2.2µF

0.1µF

499Ω

1kΩ

15pF

1kΩ

INPUT

51Ω