Datasheet

AD7908/AD7918/AD7928
Rev. D | Page 22 of 32
the operation of the AD7908/AD7918/AD7928 in this mode. In
shutdown mode, all internal circuitry on the AD7908/AD7918/
AD7928 is powered down. The part retains information in the
control register during shutdown. The AD7908/AD7918/
AD7928 remains in shutdown until the next
CS
falling edge it
receives. On this
CS
falling edge, the track-and-hold that was in
hold while the part was in shutdown returns to track. Wakeup
time from auto shutdown is 1 μs, and the user should ensure
that 1 μs has elapsed before attempting a valid conversion.
When running the AD7908/AD7918/AD7928 with a 20 MHz
clock, one dummy cycle should be sufficient to ensure the part
is fully powered up. During this dummy cycle the contents of
the control register should remain unchanged; therefore the
WRITE bit should be 0 on the DIN line. This dummy cycle
effectively halves the throughput rate of the part, with every
other conversion result being valid. In this mode, the power
consumption of the part is greatly reduced with the part
entering shutdown at the end of each conversion. When the
control register is programmed to move into auto shutdown, it
does so at the end of the conversion. The user can move the
ADC in and out of the low power state by controlling the
CS
signal.
Powering Up the AD7908/AD7918/AD7928
When supplies are first applied to the AD7908/AD7918/
AD7928, the ADC can power up in any of the operating modes
of the part. To ensure the part is placed into the required
operating mode, the user should perform a dummy cycle
operation as outlined in Figure 24.
CS
SCLK
DOUT
DIN
1
1614
1
PART IS IN
FULL SHUTDOWN
PA R T B E G I N
S
TO P
O
WER UP ON
CS RISING EDGE AS PM1 = PM0 = 1
THE P
A
RT IS FULL
Y
P
O
WERED UP
ONCE
t
POWER UP
HAS ELAPSED
CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS.
PM1 = 1, PM0 = 1
TO KEEP THE PART IN NORMAL MODE,
LOAD PM1 = PM0 = 1 IN CONTROL REGISTER
CHANNEL IDENTIFIER BITS + CONVERSION RESULT
DATA IN TO CONTROL REGISTER
DATA IN TO CONTROL/SHADOW REGISTER
1614
t
12
0
3089-022
Figure 22. Full Shutdown Mode Operation
1
CS
SCLK
DOUT
DIN
16
1611
16
DUMMY CONVERSION
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
INVALID DATA
DATA IN TO CONTROL/SHADOW REGISTER
DATA IN TO CONTROL/SHADOW REGISTER
TO KEEP PART IN THIS MODE, LOAD PM1 = 0,
PM0 = 1 IN CONTROL REGISTER OR SET WRITE BIT = 0
PART IS FULLY
POWERED UP
1212
12
PART ENTERS SHUTDOWN ON CS
RISING EDGE AS PM1 = 0, PM0 = 1
PART BEGINS TO POWER
UP ON CS FALLING EDGE
PART ENTERS SHUTDOWN
ON CS RISING EDGE
AS PM1 = 0, PM0 = 1
CONTROL REGISTER IS LOADED ON THE
FIRST 12 CLOCKS, PM1 = 0, PM0 = 1
CONTROL REGISTER CONTENTS SHOULD
NOT CHANGE. WRITE BIT = 0
03089-023
Figure 23. Auto Shutdown Mode Operation