AD812

–13–

REV. B

Power Supply Bypassing

Adequate power supply bypassing can be very important when

optimizing the performance of high speed circuits. Inductance

in the supply leads can (for example) contribute to resonant

circuits that produce peaking in the amplifier’s response. In

addition, if large current transients must be delivered to a load,

then large (greater than 1

µF) bypass capacitors are required to

produce the best settling time and lowest distortion. Although

0.1

µF capacitors may be adequate in some applications, more

elaborate bypassing is required in other cases.

When multiple bypass capacitors are connected in parallel, it is

important to be sure that the capacitors themselves do not form

resonant circuits. A small (say 5

Ω) resistor may be required in

series with one of the capacitors to minimize this possibility.

As discussed below, power supply bypassing can have a signifi-

cant impact on crosstalk performance.

Achieving Low Crosstalk

Measured crosstalk from the output of amplifier 2 to the input

of amplifier 1 of the AD812 is shown in Figure 40. The crosstalk

from the output of amplifier 1 to the input of amplifier 2 is a few

dB better than this due to the additional distance between criti-

cal signal nodes.

A carefully laid-out PC board should be able to achieve the level

of crosstalk shown in the figure. The most significant contribu-

tors to difficulty in achieving low crosstalk are inadequate power

supply bypassing, overlapped input and/or output signal paths,

and capacitive coupling between critical nodes.

The bypass capacitors must be connected to the ground plane at

a point close to and between the ground reference points for the

two loads. (The bypass of the negative power supply is particu-

larly important in this regard.) There are two amplifiers in the

package, and low impedance signal return paths must be pro-

vided for each load. (Using a parallel combination of 1

µF,

0.1

µF, and 0.01 µF bypass capacitors will help to achieve opti-

mal crosstalk.)

–10

–60

–110

1M

100M

10M

–70

–80

–90

–100

–50

–40

–30

–20

100k

FREQUENCY – Hz

Figure 40. Crosstalk vs. Frequency

The input and output signal return paths must also be kept from

overlapping. Since ground connections are not of perfectly zero

impedance, current in one ground return path can produce a

voltage drop in another ground return path if they are allowed

to overlap.

Electric field coupling external to (and across) the package can

be reduced by arranging for a narrow strip of ground plane to be

run between the pins (parallel to the pin rows). Doing this on

both sides of the board can reduce the high frequency crosstalk

by about 5 dB or 6 dB.

Driving Capacitive Loads

When used with the appropriate output series resistor, any load

capacitance can be driven without peaking or oscillation. In

most cases, less than 50

Ω is all that is needed to achieve an

extremely flat frequency response. As illustrated in Figure 44,

the AD812 can be very attractive for driving largely capacitive

loads. In this case, the AD812’s high output short circuit

current allows for a 150 V/

µs slew rate when driving a 510 pF

capacitor.

AD812

8

4

0.1 F

1.0 F

0.1 F

1.0 F

Figure 41. Circuit for Driving a Capacitive Load

1

10

1000

100

FREQUENCY – MHz

6

9

3

0

–3

12

–6

G = +2

Figure 42. Response to a Small Load Capacitor at

±5 V