Data Sheet

AD7927

Rev. D | Page 21 of 28

1

SCLK

DOUT

DIN

16

1

1

16

DUMMY CONVERSION

INVALID DATA

KEEP DIN LINE TIED HIGH FOR FIRST TWO DUMMY CONVERSIONS

12

12

12

DUMMY CONVERSION

INVALID DATA

INVALID DATA

16

CORRECT VALUE IN CONTROL REGISTER,

VALID DATA FROM NEXT CONVERSION,

USER CAN WRITE TO SHADOW REGISTER

IN NEXT CONVERSION

CONTROL REGISTER IS LOADED ON THE FIRST

12 CLOCK EDGES

DATA INTO CONTROL REGISTER

CS

Figure 24. Three-Dummy-Conversions to Place AD7927 into the Required Operating Mode After Power Supplies Are Applied

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 24.

The three-dummy-conversion operation outlined in Figure 24

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

conversion of the dummy cycle operation, the user should

write the desired control register configuration to the AD7927

to place the part into the auto shutdown mode. On the third CS

rising edge after the supplies are applied, the control register

contains the correct information and valid data results 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 oper-

ating modes. The user should not write to the shadow register

until the fourth conversion cycle after the supplies are applied

to the ADC, 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 24.

POWER VS. THROUGHPUT RATE

In auto shutdown mode, the average power consumption of

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

power saving depends on the SCLK frequency used, that is,

conversion time. In some cases where the conversion time is

quite a proportion of the cycle time, the throughput rate needs

to be reduced to take advantage of the power-down modes.

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

800 ns, but the cycle time is 5 μs 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

is consumed in every cycle compared to leaving the device in

normal mode. Furthermore, Figure 25 shows how as the through-

put 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 20 MHz

that is, if PM1 = 0 and PM0 = 1, then the power consumption is

calculated as follows.

The maximum power dissipation during the conversion time is

up time from auto shutdown is 1 μs and the remaining conversion

time is another cycle, that is, 800 ns, the AD7927 can be said to

dissipate 13.5 mW for 1.8 μs during each conversion cycle. For

the remainder of the conversion cycle, 3.2 μs, the part remains

in shutdown. The AD7927 can be said to dissipate 2.5 μW for

the remaining 3.2 μs of the conversion cycle. If the throughput

rate is 200 kSPS, the cycle time is 5 μs and the average power

dissipated during each cycle is (1.8/5) × (13.5 mW) + (3.2/5) ×

(2.5 μW) = 4.8616 mW.

Figure 25 shows the maximum power vs. 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 25. Power vs. Throughput Rate