AD9854AST datasheet(41/52 Pages) AD

41 / 52 page

AD9854

Rev. E | Page 41 of 52

multipliers, the inverse sinc filter, the Q DAC, and the on-board

comparator are disabled.

FREQUENCY (MHz)

1400

1200

1000

800

600

400

200

100

140

180

220

260

300

ALL CIRCUITS ENABLED

BASIC CONFIGURATION

NOTES

THIS GRAPH ASSUMES THAT THE AD9854 DEVICE IS SOLDERED

TO A MULTILAYER PCB PER THE RECOMMENDED BEST

MANUFACTURING PRACTICES AND PROCEDURES FOR THE

GIVEN PACKAGE TYPE.

Figure 62. Current Consumption vs. Clock Frequency

Figure 63 shows the approximate current consumed by each of

four functions.

FREQUENCY (MHz)

100

140

180

220

260

300

450

400

350

300

250

200

150

100

Q DAC

500

INVERSE SINC FILTER

OUTPUT SCALING

MULTIPLIERS

COMPARATOR

NOTES

THIS GRAPH ASSUMES THAT THE AD9854 DEVICE IS

SOLDERED TO A MULTILAYER PCB PER THE RECOMMENDED

BEST MANUFACTURING PRACTICES AND PROCEDURES FOR

THE GIVEN PACKAGE TYPE.

Figure 63. Current Consumption by Function vs. Clock Frequency

EVALUATION OF OPERATING CONDITIONS

The first step in applying the AD9854 is to select the internal

clock frequency. Clock frequency selections greater than

200 MHz require the use of the thermally enhanced package

(AD9854ASVZ); other clock frequencies may allow the use of the

standard plastic surface-mount package, but more information is

needed to make that determination.

The second evaluation step is to determine the maximum

required operating temperature for the AD9854 in a given

application. Subtract this value from 150°C, which is the

maximum junction temperature allowed for the AD9854. For

the extended industrial temperature range, the maximum

operating temperature is 85°C, which results in a difference of

65°C. This is the maximum temperature gradient that the

device can experience due to power dissipation.

The third evaluation step is to divide the maximum temper-

ature gradient by the thermal impedance to determine the

maximum power dissipation allowed for the application.

For example, 65°C divided by the thermal impedance of the

package being used yields the total power dissipation limit (4.06

W for the ASVZ models and 1.71 W for the ASTZ models).

Therefore, for a 3.3 V nominal power supply voltage, the

current consumed by the device with full operating conditions

must not exceed 515 mA for the standard plastic package and

1242 mA for the thermally enhanced package. The total set of

enabled functions and operating conditions of the AD9854

application must support these current consumption limits.

To determine the suitability of a given AD9854 application

in terms of the power dissipation requirements use Figure 62

and Figure 63. These graphs assume that the AD9854 device is

soldered to a multilayer PCB per the recommended best

manufacturing practices and procedures for the given package

type. This ensures that the specified thermal impedance

specifications are achieved.

THERMALLY ENHANCED PACKAGE

MOUNTING GUIDELINES

Refer to the AN-772 Application Note for details on mounting

devices with an exposed paddle.

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