AD7911/AD7921
Rev. A | Page 18 of 28
MODES OF OPERATION
The two modes of operation of the AD7911/AD7921 are
normal mode and power-down mode. The mode of operation is
selected by controlling the logic state of the
CS
signal. The point
at which
CS
is pulled high after the conversion has been initi-
ated determines whether the AD7911/AD7921 enter power-
down mode. Similarly, if already in power-down mode,
CS
can
control whether the device returns to normal operation or
remains in power-down mode.
Power-down mode is designed to provide flexible power
management options and to optimize the ratio of power
dissipation to throughput rate for different application
requirements.
NORMAL MODE
Normal mode is intended for the fastest throughput rate
performance. The user does not have to worry about any
power-up time, because the AD7911/AD7921 remain fully
powered all the time. Figure 26 shows the operation of the
AD7911/AD7921 in this mode.
The conversion is initiated on the falling edge of
CS
, as
described in the section. To ensure that the part
remains fully powered up at all times,
Serial Interface
CS
must remain low until
at least 10 SCLK falling edges have elapsed after the falling edge
of
CS
. If
CS
is brought high any time after the 10th SCLK falling
edge but before the end of t
CONVERT
, the part remains powered-
up, but the conversion is terminated and DOUT goes back into
three-state. For the AD7911/AD7921, a minimum of 14 and
16 serial clock cycles, respectively, are needed to complete the
conversion and access the complete conversion result.
CS
can idle high until the next conversion or can idle low until
CS
returns high sometime prior to the next conversion
(effectively idling
CS
low). Once a data transfer is complete
(DOUT has returned to three-state), another conversion can be
initiated after the quiet time, t
QUIET
, has elapsed by bringing
CS
low again.
POWER-DOWN MODE
Power-down mode is intended for use in applications where
slower throughput rates are required. Either the ADC is
powered down between each conversion, or a series of
conversions can be performed at a high throughput rate and
then the ADC is powered down for a relatively long duration
between these bursts of several conversions. When the
AD7911/AD7921 are in power-down mode, all analog circuitry
is powered down.
To enter power-down mode, the conversion process must be
interrupted by bringing
CS
high any time after the second
falling edge of SCLK and before the 10th falling edge of SCLK,
as shown in . Once Figure 27
CS
has been brought high in this
window of SCLKs, then the part enters power-down mode, the
conversion that was initiated by the falling edge of
CS
is
terminated, and DOUT goes back into three-state. If
CS
is
brought high before the second SCLK falling edge, then the part
remains in normal mode and does not power down. This helps
to avoid accidental power-down due to glitches on the
CS
line.
To exit this mode of operation and power the AD7911/AD7921
up again, a dummy conversion is performed. On the falling
edge of
CS
, the device begins to power up and continues to
power up as long as
CS
is held low until after the falling edge of
the 10th SCLK. The device is fully powered up once 16 SCLKs
have elapsed and valid data results from the next conversion, as
shown in . If Figure 28
CS
is brought high before the 10th falling
edge of SCLK, then the AD7911/AD7921 go back into power-
down mode. This helps to avoid accidental power-up due to
glitches on the
CS
line or an inadvertent burst of 8 SCLK cycles
while
CS
is low. Therefore, although the device might begin to
power up on the falling edge of
CS
, it powers down again on the
rising edge of
CS
, as long as this occurs before the 10th SCLK
falling edge.
04350-0-025
CS
1 10 12 14 16 1 10 12 14 16
AD7911/AD7921
SCLK
DIN
DOUT
CHANNEL FOR NEXT CONVERSION CHANNEL FOR NEXT CONVERSION
CONVERSION RESULT CONVERSION RESULT
Figure 26. Normal Mode Operation
