AD9857 Analog Devices, AD9857 Datasheet - Page 28

• Manufacturer Part Number: AD9857
• Description: CMOS 200 MSPS 14-Bit Quadrature Digital Upconverter
• Manufacturer: Analog Devices

AD9857
Power Management Considerations
The thermal impedance for the AD9857 80-lead LQFP package
is
using this value is calculated using T = P
The AD9857 power dissipation is at or below this value when
the SYSCLK frequency is at 200 MHz or lower with all optional
features enabled. The maximum power dissipation occurs while
operating the AD9857 as a quadrature modulator at the maxi-
mum system clock frequency with TxENABLE in a logic high
state 100% of the time the device is powered. Under these con-
ditions the device operates with all possible circuits enabled at
maximum speed.
Significant power saving may be seen by using a TxENABLE
signal that toggles low during times when the device does not
modulate.
The thermal impedance of the AD9857 package was measured
in a controlled temperature environment at temperatures ranging
from 28 C to 85 C with no air flow. The device under test was
soldered to an AD9857 evaluation board and operated under
conditions that generate maximum power dissipation. The ther-
mal resistance of a package can be thought of as a thermal resistor
that exists between the semiconductor surface and the ambient
air. The thermal impedance of a package is determined by pack-
age material and its physical dimensions. The dissipation of the
heat from the package is directly dependent upon the ambient
air conditions and the physical connection made between the IC
package and the PCB. Adequate dissipation of power from the
AD9857 relies upon all power and ground pins of the device
being soldered directly to copper planes on a PCB.
Many variables contribute to the operating junction temperature
within a device. They include:
1. Package Style
2. Selection Mode of Operation
3. Internal System Clock Speed
4. Supply Voltage
5. Ambient Temperature
JA
= 35 C/W. The maximum allowable power dissipation
P
P
P
1 85
150 85
.
J
T
A
35
–
W
JA
.
The power dissipation of the AD9857 in a given application is
determined by several operating conditions. Some of these con-
ditions, such as supply voltage and clock speed, have a direct
relationship with power dissipation. The most important factors
affecting power dissipation are the following:
Supply Voltage
This affects power dissipation and junction temperature since
power dissipation equals supply voltage multiplied by supply
current. It is recommended that the user design for a 3.3 V
nominal supply voltage in order to manage the affect of supply
voltage on the junction temperature of the AD9857.
Clock Speed
This directly and linearly influences the total power dissipation
of the device and, therefore, junction temperature. As a rule, the
user should always select the lowest internal clock speed possible
to support a given application to minimize power dissipation.
Normally, the usable frequency output bandwidth from a DDS
is limited to 40% of the system clock rate to keep reasonable
requirements on the output low-pass filter. This means that for
the typical DDS application, the system clock frequency should
be 2.5 times the highest output frequency.
Mode of Operation
The AD9857 has three modes of operation that consume sig-
nificantly different amounts of power. When operating in the
Quadrature Modulation Mode, the AD9857 will dissipate about
twice the power as when operating as a single-tone DDS. When
operating as a quadrature modulator, the AD9857 has features
that facilitate power management tactics. For example, the
TxENABLE pin may be used in conjunction with the auto
power-down bit to frame bursts of data and automatically switch
the device into a low-power state when there is no data to
be modulated.
Equivalent I/O Circuits
DAC OUTPUTS
IOUT IOUB
V
DD
V
DD
DIGITAL
OUT
DIGITAL
IN
V
DD