AD813

REV. B

–14–

Printed Circuit Board Layout Guidelines

As with all wideband amplifiers, printed circuit board parasitics

can affect the overall closed-loop performance. Most important

for controlling the 0.1 dB bandwidth are stray capacitances at

the output and inverting input nodes. Increasing the space be-

tween signal lines and ground plane will minimize the coupling.

Also, signal lines connecting the feedback and gain resistors

should be kept short enough that their associated inductance

does not cause high frequency gain errors.

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 AD813 is shown in Figure 40. All other

crosstalk combinations, (from the output of one amplifier to the

input of another), are a few dB better than this due to the addi-

tional distance between critical signal nodes.

100k

1M

100M

10M

FREQUENCY – Hz

–50

–30

–40

–60

–70

–80

–90

–20

–10

–100

–110

Figure 40. Worst Case Crosstalk vs. Frequency

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

loads. (The bypass of the negative power supply is particularly

important in this regard.) This requires careful planning as

there are three amplifiers in the package, and low impedance

signal return paths must be provided for each load. (Using a

parallel combination of 1

µF, 0.1 µF, and 0.01 µF bypass ca-

pacitors will help to achieve optimal crosstalk.)

The input and output signal return paths (to the bypass caps)

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 re-

turn 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 AD813 can be very attractive for driving large capacitive

loads. In this case, the AD813’s high output short circuit cur-

rent allows for a 150 V/

µs slew rate when driving a 510 pF

capacitor.

AD813

4

11

0.1 F

1.0 F

0.1 F

1.0 F

Figure 41. Circuit for Driving a Capacitive Load