AD7908/AD7918/AD7928

REV. A

CS

SCLK

DOUT

DIN

114

16

1

14

16

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

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

Figure 13. Full Shutdown Mode Operation

1

CS

SCLK

DOUT

DIN

16

116

1

16

DUMMY CONVERSION

CHANNEL IDENTIFIER BITS + CONVERSION RESULT

INVALID DATA

CHANNEL IDENTIFIER BITS + CONVERSION RESULT

DATA INTO CONTROL/SHADOW REGISTER

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

DATA INTO CONTROL/SHADOW REGISTER

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

12

12

12

Figure 14. Auto Shutdown Mode Operation

1

CS

SCLK

DOUT

DIN

16

116

1

16

DUMMY CONVERSION

INVALID DATA

DATA IN TO CONTROL REGISTER

CORRECT VALUE IN CONTROL

REGISTER, VALID DATA FROM

NEXT CONVERSION, USER CAN

WRITE TO SHADOW REGISTER

IN NEXT CONVERSION

KEEP DIN LINE TIED HIGH FOR FIRST TWO DUMMY CONVERSIONS

CONTROL REGISTER IS LOADED ON THE FIRST

12 CLOCK EDGES

12

12

12

DUMMY CONVERSION

INVALID DATA

INVALID DATA

Figure 15. Placing AD7928 into the Required Operating Mode after Supplies are Applied

POWER VS. THROUGHPUT RATE

By operating in Auto Shutdown mode on the AD7908/AD7918/

AD7928, the average power consumption of the ADC decreases

at lower throughput rates. Figure 16 shows how as the through-

put rate is reduced, the part remains in its shutdown state longer

and the average power consumption over time drops accordingly.

For example if the AD7928 is operated in a continuous sam-

pling mode, with a throughput rate of 100 kSPS and an SCLK

Shutdown mode, i.e., if PM1 = 0 and PM0 = 1, then the power

consumption is calculated as follows:

The maximum power dissipation during normal operation is

is one dummy cycle, i.e., 1 µs, and the remaining conversion

time is another cycle, i.e., 1 µs, then the AD7928 can be said

to dissipate 13.5 mW for 2 µs during each conversion cycle.

For the remainder of the conversion cycle, 8 µs, the part

remains in Auto Shutdown mode. The AD7928 can be said

to dissipate 2.5 µW for the remaining 8 µs of the conversion

cycle. If the throughput rate is 100 kSPS, the cycle time is 10 µs

and the average power dissipated during each cycle is

(2/10)

(13.5 mW) + (8/10)

(2.5 µW) = 2.702 mW.

Figure 16 shows the maximum power versus throughput rate

when using the Auto Shutdown mode with 3 V and 5 V supplies.

–20–