REV. 0

–16–

AD7927

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 AD7927

When supplies are first applied to the AD7927, the ADC may

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 15.

The three dummy conversion operation outlined in Figure 15

must be performed to place the part into the Auto Shutdown

Mode. The first two conversions of this dummy cycle operation

are performed with the DIN line tied high, and for the third con-

version of the dummy cycle operation, the user should write the

desired Control Register configuration to the AD7927 in order to

place the part into the Auto Shutdown mode. On the third

CS

rising edge after the supplies are applied, the Control Register

will contain the correct information and valid data will result from

the next conversion.

Therefore, to ensure the part is placed into the correct operating

mode, when supplies are first applied to the AD7927, the user

must first issue two serial write operations with the DIN line tied

high, and on the third conversion cycle the user can then write

to the Control Register to place to part into any of the operating

modes. The user should not write to the Shadow Register until

the fourth conversion cycle after the supplies are applied to the

ADC, in order to guarantee the Control Register contains the

correct data.

If the user wishes to place the part into either the Normal or

Full Shutdown Mode, the second dummy cycle with DIN tied

high can be omitted from the three dummy conversion operation

outlined in Figure 15.

POWER VERSUS THROUGHPUT RATE

In Auto Shutdown Mode, the average power consumption of the

ADC may be reduced at any given throughput rate. The power

saving will depend on the SCLK frequency used, i.e., conversion

time. In some cases where the conversion time is quite a propor-

tion of the cycle time, the throughput rate would need to be

reduced in order to take advantage of the power-down modes.

Assuming a 20 MHz SCLK is used, the conversion time is

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. To Place AD7927 into the Required Operating Mode after Supplies Are Applied

800 ns but the cycle time is 5

ms when the sampling rate is at a

maximum of 200 kSPS. If the AD7927 is placed into shutdown

for the remainder of the cycle time, then on average far less

power will be consumed in every cycle compared to leaving the

device in Normal Mode. Furthermore, Figure 16 shows how as

the throughput rate is reduced, the part remains in its shutdown

longer and the average power consumption drops accordingly

over time.

For example, if the AD7927 is operated in a continuous sampling

mode, with a throughput rate of 200 kSPS and an SCLK of

Mode i.e., if PM1 = 0 and PM0 = 1, then the power consumption

is calculated as follows:

The maximum power dissipation during the conversion time is

from Auto Shutdown is 1

ms and the remaining conversion time

is another cycle, i.e., 800 ns, the AD7927 can be said to dissipate

13.5 mW for 1.8

ms during each conversion cycle. For the remain-

der of the conversion cycle, 3.2

ms, the part remains in Shutdown.

The AD7927 can be said to dissipate 2.5

mW for the remaining

3.2

ms of the conversion cycle. If the throughput rate is 200 kSPS,

the cycle time is 5

ms and the average power dissipated during each

cycle is (1.8/5)

¥ (13.5 mW) + (3.2/5) ¥(2.5 mW) = 4.8616 mW.

Figure 16 shows the maximum power versus throughput rate

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

THROUGHPUT – kSPS

10

0

200

0.1

0.01

80

1

100

140

180

20

40

60

120

160

Figure 16. Power vs. Throughput Rate