AD9854 Analog Devices, AD9854 Datasheet - Page 33

• Manufacturer Part Number: AD9854
• Description: CMOS 300 MHz Quadrature Complete-DDS
• Manufacturer: Analog Devices

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.
Figure 57b shows the approximate current consumed by each of
four functions.
REV. 0
Figure 57b. Current Consumption by Function vs. Clock
Frequency
Figure 57a. Current Consumption vs. Clock Frequency
1400
1200
1000
800
600
400
200
500
450
400
350
300
250
200
150
100
50
0
0
20
1
ALL CIRCUITS ENABLED
60
2
100
3
Q DAC
FREQUENCY – MHz
FREQUENCY – MHz
140
4
INVERSE SINC FILTER
OUTPUT SCALING
180
5
BASIC CONFIGURATION
MULTIPLIERS
220
6
COMPARATOR
260
7
300
8
–33–
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.
AD9854