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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
the previous and current input signal and should therefore
be discarded. Figure 21 shows an example of readback in
this situation.
Settling Time Using One-Shot Mode
A dramatic reduction in power consumption can be achieved
in the ADS1255/6 by performing one-shot conversions using
the STANDBY command; the sequence for this is shown in
Figure 20. Issue the WAKEUP command from Standby
mode to begin a one-shot conversion. Following the settling
time (t18), DRDY will go low, indicating that the conversion is
complete and data can be read using the RDATA command.
The ADs1255/6 settles in a single cycle—there is no need to
ignore or discard data. Following the data read cycle, issue
another STANDBY command to reduce power consumption.
When ready for the next measurement, repeat the cycle
starting with another WAKEUP command.
Table 15. Data Settling Delay vs Data Rate
DATA RATE
(SPS)
SETTLING TIME
(DRDY Periods)
30,000
15,000
7500
3750
2000
1000
500
100
60
5
3
2
1
1
1
1
1
1
1
1
1
1
1
1
1
Settling Time while Continuously Converting
After a synchronization, input multiplexer change, or
wakeup from Standby mode, the ADS1255/6 will
continuously convert the analog input. The conversions
coincide with the falling edge of DRDY. While continuously
converting, it is often more convenient to consider settling
times in terms of DRDY periods, as shown in Table 15.
The DRDY period equals the inverse of the data rate.
50
30
25
15
If there is a step change on the input signal while
continuously converting, performing a synchronization
operation to start a new conversion is recommended.
Otherwise, the next data will represent a combination of
10
5
2.5
Standby
Mode
Standby
Mode
ADS1255/6
Status
Performing One−Shot Conversion
t18
DRDY
DIN
STANDBY
RDATA
WAKEUP
STANDBY
DOUT
Settled
Data
Figure 20. One-Shot Conversions Using the STANDBY Command
New VIN
−
V
IN = AINP AINN
Old VIN
Mix of
Old and New
VIN Data
Fully Settled
New VIN Data
Old VIN Data
DRDY
RDATA
DIN
Settled
Data
DOUT
Figure 21. Step Change on V while Continuously Converting for Data Rates ≤ 3750SPS
IN
22
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