AD7918_15 datasheet(22/28 Pages) AD

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AD7908/AD7918/AD7928

Data Sheet

Rev. E | Page 22 of 28

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

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.

SCLK

DOUT

DIN

PART IS IN

FULL SHUTDOWN

PART BEGINS TO POWER UP ON

CS RISING EDGE AS PM1 = PM0 = 1

THE PART IS FULLY POWERED UP

ONCE

tPOWER 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

t12

Figure 22. Full Shutdown Mode Operation

SCLK

DOUT

DIN

DUMMY CONVERSION

CHANNEL IDENTIFIER

BITS + CONVERSION RESULT

CHANNEL IDENTIFIER

BITS + CONVERSION RESULT

INVALID DATA

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

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

Figure 23. Auto Shutdown Mode Operation

SCLK

DOUT

DIN

DUMMY CONVERSION

INVALID DATA

KEEP DIN LINE TIED HIGH FOR FIRST TWO DUMMY CONVERSIONS

CONTROL REGISTER IS LOADED ON THE FIRST

12 CLOCK EDGES

DUMMY CONVERSION

INVALID DATA

DATA IN TO CONTROL REGISTER

CORRECT VALUE IN CONTROL

NEXT CONVERSION, USER CAN

WRITE TO SHADOW REGISTER

IN NEXT CONVERSION

Figure 24. Placing AD7928 into the Required Operating Mode After Supplies are Applied

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