Datasheet

AD9640
Rev. B | Page 30 of 52
POWER DISSIPATION AND STANDBY MODE
As shown in Figure 63, the power dissipated by the AD9640
is proportional to its sample rate. In CMOS output mode,
the digital power dissipation is determined primarily by the
strength of the digital drivers and the load on each output bit.
The maximum DRVDD current (I
DRVDD
) can be calculated as
I
DRVDD
= V
DRVDD
× C
LOAD
× f
CLK
× N
where N is the number of output bits (30 in the case of the AD9640
with the FD bits disabled). This maximum current occurs when
every output bit switches on every clock cycle, that is, a full-
scale square wave at the Nyquist frequency of f
CLK
/2. In practice,
the DRVDD current is established by the average number of
output bits switching, which is determined by the sample rate
and the characteristics of the analog input signal.
Reducing the capacitive load presented to the output drivers can
minimize digital power consumption. The data in Figure 63 was
taken with the same operating conditions as the Typical
Performance Characteristics, with a 5 pF load on each output
driver.
0 150125
1.25
0.75
1.0
0
06547-076
ENCODE FREQUENCY (MHz)
TOTAL POWER (W)
SUPPLY CURRENT (A)
0.5
0.25
0.5
0.4
0.3
0.2
0.1
0
25 50 75 100
I
AVDD
TOTAL POWER
I
DRVDD
I
DVDD
Figure 63. AD9640-150 Power and Current vs. Clock Frequency
0 125
1.25
0.75
1.0
0
06547-075
ENCODE FREQUENCY (MHz)
TOTAL POWER (W)
SUPPLY CURRENT (A)
0.5
0.25
0.5
0.4
0.3
0.2
0.1
0
25 50 75 100
I
AVDD
TOTAL POWER
I
DVDD
I
DRVDD
Figure 64. AD9640-125 Power and Current vs. Clock Frequency
0
1
0
ENCODE FREQUENCY (MHz)
TOTAL POWER (W)
0.75
0.25
0.5
25 50 75 100
06547-074
SUPPLY CURRENT (A)
0.4
0.3
0.2
0.1
0
I
AVDD
TOTAL POWER
I
DRVDD
I
DVDD
Figure 65. AD9640-105 Power and Current vs. Clock Frequency
08
0.75
0
06547-073
ENCODE FREQUENCY (MHz)
TOTAL POWER (W)
SUPPLY CURRENT (A)
0
0.5
0.25
0.3
0.2
0.1
0
20 40 60
I
AVDD
TOTAL POWER
I
DRVDD
I
DVDD
Figure 66. AD9640-80 Power and Current vs. Clock Frequency
By asserting PDWN (either through the SPI port or by asserting
the PDWN pin high), the AD9640 is placed in power-down
mode. In this state, the ADC typically dissipates 2.5 mW.
During power-down, the output drivers are placed in a high
impedance state. Asserting the PDWN pin low returns the
AD9640 to its normal operational mode. Note that PDWN is
referenced to the digital supplies (DRVDD) and should not
exceed that supply voltage.
Low power dissipation in power-down mode is achieved by
shutting down the reference, reference buffer, biasing networks,
and clock. Internal capacitors are discharged when entering power-
down mode and then must be recharged when returning to normal
operation. As a result, wake-up time is related to the time spent
in power-down mode, and shorter power-down cycles result in
proportionally shorter wake-up times.
When using the SPI port interface, the user can place the ADC
in power-down mode or standby mode. Standby mode allows
the user to keep the internal reference circuitry powered when
faster wake-up times are required. See the Memory Map Register
Description section for more details.