Datasheet

AD9974
Rev. A | Page 36 of 52
STANDBY MODE OPERATION
The AD9974 contains two standby modes to optimize the
overall power dissipation in a particular application. Bit 1 and
Bit 0 of Address 0x00 control the power-down state of the device.
STANDBY[1:0] = 00 = normal operation (full power)
STANDBY[1:0] = 01 = reference standby mode
STANDBY[1:0] = 10 or 11 = total shutdown mode
(lowest power)
Tabl e 20 summarizes the operation of each power-down mode.
The OUT_CONTROL register takes priority over the reference
standby mode in determining the digital output states, but total
shutdown mode takes priority over OUT_CONTROL. Total
shutdown mode has the lowest power consumption.
When returning from total shutdown mode to normal operation,
the timing core must be reset at least 100 μs after the STANDBY
register is written to.
There is an additional register to disable the internal voltage
reference buffer (Address 0x00[2]) independently. By default
the buffer is disabled, but it must be enabled for normal operation.
CLI FREQUENCY CHANGE
If the input clock, CLI, is interrupted or changes to a different
frequency, the timing core must be reset for proper operation.
After the CLI clock has settled to the new frequency, or the
previous frequency has resumed, write 0 and then 1 to the
TGCORE_RST register (Address 0x14). This guarantees proper
timing core operation.
Table 20. Standby Mode Operation
I/O Block Total Shutdown (Default)
1, 2
OUT_CONTROL = Low
2
Reference Standby
AFE Off No change Only REFT, REFB on
Timing Core Off No change On
H1 High-Z Low Low (4.3 mA)
H2 High-Z High High (4.3 mA)
H3 High-Z Low Low (4.3 mA)
H4 High-Z High High (4.3 mA)
HL High-Z Low Low (4.3 mA)
RG High-Z Low Low (4.3 mA)
DOUT Low
3
Low Low
1
To exit total shutdown, write 00 to STANDBY (Address 0x00, Bits[1:0]), then reset the timing core after 100 μs to guarantee proper settling.
2
Total shutdown mode takes priority over OUT_CONTROL for determining the output polarities.
3
The status of the DOUT pins is unknown at power-up. Low status is guaranteed in total shutdown mode after the power-up sequence is completed.