AD7908 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'AD7908' PDF : 24 Pages View PDF
AD7908/AD7918/AD7928
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.
PART IS IN
FULL SHUTDOWN
CS
1
SCLK
PART BEGINS TO POWER UP ON
CS RISING EDGE AS PM1 = PM0 = 1
t12
THE PART IS FULLY POWERED UP
ONCE tPOWER UP HAS ELAPSED
14
16
1
14
16
DOUT
DIN
CS
SCLK
DOUT
DIN
CHANNEL IDENTIFIER BITS + CONVERSION RESULT
DATA IN TO CONTROL REGISTER
DATA IN TO CONTROL/SHADOW REGISTER
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
Figure 22. Full Shutdown Mode Operation
PART ENTERS SHUTDOWN ON CS
RISING EDGE AS PM1 = 0, PM0 = 1
PART BEGINS TO POWER
UP ON CS FALLING EDGE
PART IS FULLY
POWERED UP
PART ENTERS SHUTDOWN
ON CS RISING EDGE
AS PM1 = 0, PM0 = 1
DUMMY CONVERSION
1
12
16
1
12
16
1
12
16
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
DATA IN TO CONTROL/SHADOW REGISTER
CONTROL REGISTER IS LOADED ON THE
FIRST 12 CLOCKS, PM1 = 0, PM0 = 1
INVALID DATA
CONTROL REGISTER CONTENTS SHOULD
NOT CHANGE. WRITE BIT = 0
Figure 23. Auto Shutdown Mode Operation
CHANNEL IDENTIFIER
BITS + CONVERSION RESULT
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
Rev. D | Page 22 of 32
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