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AD7927 View Datasheet(PDF) - Unspecified

Part NameDescriptionManufacturer
AD7927 8-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 20-Lead TSSOP ETC
Unspecified 
AD7927 Datasheet PDF : 20 Pages
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AD7927
CS
DUMMY CONVERSION
DUMMY CONVERSION
1
SCLK
12
16
1
12
16
1
CORRECT VALUE IN CONTROL
REGISTER, VALID DATA FROM
NEXT CONVERSION, USER CAN
WRITE TO SHADOW REGISTER
IN NEXT CONVERSION
12
16
DOUT
INVALID DATA
INVALID DATA
INVALID DATA
DIN
DATA IN TO CONTROL REGISTER
KEEP DIN LINE TIED HIGH FOR FIRST TWO DUMMY CONVERSIONS
CONTROL REGISTER IS LOADED ON THE FIRST
12 CLOCK EDGES
Figure 15. To Place AD7927 into the Required Operating Mode after Supplies Are Applied
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
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
20 MHz (AVDD = 5 V), and the device is placed in Auto Shutdown
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
13.5 mW (IDD = 2.7 mA max, AVDD = 5 V). If the power-up time
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.
10
AVDD = 5V
1
AVDD = 3V
0.1
0.01
0
20 40 60 80 100 120 140 160 180 200
THROUGHPUT – kSPS
Figure 16. Power vs. Throughput Rate
–16–
REV. 0
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