AD7923 datasheet(19/25 Pages) AD

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AD7923

Data Sheet

Rev. D | Page 18 of 24

Only a portion of the cycle time is taken up by the conversion

time and the dummy transfer for wakeup. In this mode, the

power con-sumption of the part is greatly reduced because the

part enters 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 AD7923

When supplies are first applied to the AD7923, the ADC can

power up in any of the operating modes of the part. To ensure

that the part is placed into the required operating mode, the

user should perform a dummy cycle operation, as outlined in

Figure 23 through Figure 25.

The dummy conversion operation must be performed to place

the part into the desired mode of operation. To ensure that the

part is in normal mode, this dummy cycle operation can be

performed with the DIN line tied high, that is, PM1, PM0 = 1, 1

(depending on other required settings in the control register),

but the minimum power-up time of 1 µs must be allowed from

the rising edge of CS, where the control register is updated,

before attempting the first valid conversion. This is to allow for

the possibility that the part initially powered up in shutdown.

If the desired mode of operation is full shutdown, then again

only one dummy cycle is required after supplies are applied. In

this dummy cycle, the user simply sets the power management

bits, PM1, PM0 = 1, 0, and upon the rising edge of CS at the

end of that serial transfer, the part enters full shutdown. If the

desired mode of operation is auto shutdown after supplies are

applied, two dummy cycles are required, the first with DIN tied

high and the second dummy cycle to set the power manage-

ment bits PM1 and PM0 = 0,1. On the second CS rising edge

after the supplies are applied, the control register contains the

correct information and the part enters auto shutdown mode as

programmed. If power consumption is of critical concern, then

in the first dummy cycle the user may set PM1, PM0 = 1, 0, that

is, full shutdown, and then place the part into auto shutdown in

the second dummy cycle. For illustration purposes, Figure 25 is

shown with DIN tied high on the first dummy cycle in this case.

Figure 23, Figure 24, and Figure 25 each show the required

dummy cycle(s) after supplies are applied in the case of normal

mode, full shutdown mode, and auto shutdown mode, respect-

ively, being the desired mode of operation.

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 INTO CONTROL REGISTER

t12

Figure 21. Full Shutdown Mode Operation

SCLK

DOUT

DIN

DUMMY CONVERSION

CHANNEL IDENTIFIER BITS + CONVERSION RESULT

INVALID DATA

CHANNEL IDENTIFIER BITS + CONVERSION RESULT

CONTROL REGISTER IS LOADED ON THE

FIRST 12 CLOCKS, PM1 = 0, PM0 = 1

CONTROL REGISTER CONTENTS SHOULD

NOT CHANGE, WRITE BIT = 0

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 BEGINS

TO POWER

UP ON CS

FALLING EDGE

PART ENTERS

SHUTDOWN ON CS

RISING EDGE AS

PM1 = 0, PM0 =1

PART ENTERS

SHUTDOWN ON CS

RISING EDGE AS

PM1 = 0, PM0 =1

DATA INTO CONTROL REGISTER

Figure 22. Auto Shutdown Mode Operation

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