AD816

REV. B

–12–

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.

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

µF) will be required to provide the best

settling time and lowest distortion. A parallel combination of

10.0

µF and 0.1 µF is recommended. Under some low frequency

applications, a bypass capacitance of greater than 10

µF may be

necessary. Due to the large load currents delivered by the AD816,

special consideration must be given to careful bypassing. The

ground returns on both supply bypass capacitors as well as signal

common must be “star” connected as shown in Figure 44.

(OPTIONAL)

+OUT

–OUT

+IN

–IN

DRIVER A

DRIVER B

IN

IN

RECEIVER A

RECEIVER B

OUT

OUT

Figure 44. Signal Ground Connected in “Star”

Configuration

POWER CONSIDERATIONS

The 500 mA drive capability of the AD816 driver enables it to

drive a 50

Ω load at 40 V p-p when it is configured as a dif-

5 watts. To ensure reliability, the junction temperature of the

AD816 should be maintained at less than 175

°C. For this rea-

son, the AD816 will require some form of heat sinking in most

applications. The thermal diagram of Figure 45 gives the basic

components of

θ

Equation 1

DIE MOUNT)

θ

TO CASE)

θ

CASE

θ

JC

θ

CA

θ

JA

WHERE:

θ

θ

Figure 45. A Breakdown of Various Package Thermal

Resistances

Figure 46 gives the relationship between output voltage swing

This data is given for both sine wave and square wave (worst

case) conditions. It should be noted that these graphs are for

mostly resistive (phase <

±10°) loads. When the power dissipation

requirements are known, Equation 1 and the graph on Figure 47

can be used to choose an appropriate heat sinking configuration.

4

3

10

20

30

40

2

1

f = 1kHz

SQUARE WAVE

SINE WAVE

Figure 46. Total Power Dissipation vs Differential Driver

Output Voltage