AD9854SLASHPCB datasheet(33/44 Pages) AD

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AD9854

–33–

REV. 0

FREQUENCY – MHz

1400

1200

1000

800

600

400

200

100

140

180

220

260

300

ALL CIRCUITS ENABLED

BASIC CONFIGURATION

Figure 57a. Current Consumption vs. Clock Frequency

Figure 57a shows the supply current consumed by the AD9854

over a range of frequencies for two possible configurations: all

circuits enabled means the output scaling multipliers, the inverse

sinc filter, the Q DAC, and the on-board comparator are all en-

abled. Basic configuration means the output scaling multipliers,

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

are all disabled.

FREQUENCY – MHz

450

400

350

300

250

200

150

2345678

100

Q DAC

500

INVERSE SINC FILTER

OUTPUT SCALING

MULTIPLIERS

COMPARATOR

Figure 57b. Current Consumption by Function vs. Clock

Frequency

Figure 57b shows the approximate current consumed by each of

four functions.

EVALUATION OF OPERATING CONDITIONS

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

clock frequency. Clock frequency selections above 200 MHz

will require the thermally-enhanced package (AD9854ASQ);

clock frequency selections of 200 MHz and below may allow

the user to use the standard plastic surface-mount package, but

more information will be needed to make that determination.

The second step is to determine the maximum required operating

temperature for the AD9854 in the given application. Subtract

this value from 150

°C, which is the maximum junction tem-

perature allowed for the AD9854. For the extended industrial

temperature range of 85

°C, the result will be 65°C. This is the

maximum rise in temperature that the junction may experience

due to power dissipation.

The third step is to divide this maximum rise number by the

thermal impedance, to arrive at the maximum power dissipation

allowed for the application. For the example so far, 65

°C divided

by both versions of the AD9854 package’s thermal impedances

of 38

°C/W and 16°C/W, yields a total power dissipation limit

of 1.7 W and 4.1 W (respectively). This means that for a 3.3 V

nominal power supply voltage, the current consumed by the device

under full operating conditions must not exceed 515 mA in the

standard plastic package and 1242 mA in the thermally-enhanced

package. The total set of enabled functions and operating condi-

tions of the AD9854 application must support these current

consumption limits.

Figures 57a and Figure 57b may be used to determine the

suitability of a given AD9854 application vs. power dissipation

requirements. These graphs assume that the AD9854 device will

be 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 spec-

iﬁcations will be achieved.

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