Datasheet
ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
22
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. When using one−shot
mode, an additional delay is required for the modulator to
power up and settle. This delay may be up to 64 modulator
clocks (64 x 4 x τ
CLKIN
) or 33.3μs for a 7.68MHz master
clock. Following the settling time (t
18
+ 256 x τ
CLKIN
), 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. When using one−shot mode, an
additional delay is required for the modulator to power up and
settle. This delay may be up to 64 modulator clocks (64 x 4
x
τ
CLKIN
or 33.3μs for a 7.68MHz master clock. 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.
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.
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
the previous and current input signal and should therefore
be discarded. Figure 21 shows an example of readback in
this situation.
Table 15. Data Settling Delay vs Data Rate
DATA RATE
(SPS)
SETTLING TIME
(DRDY Periods)
30,000 5
15,000 3
7500 2
3750 1
2000 1
1000 1
500 1
100 1
60 1
50 1
30 1
25 1
15 1
10 1
5 1
2.5 1
DOUT
Settled
Data
STANDBY
WAKEUP
DRDY
DIN
ADS1255/6
Status
RDATA
Performing One−Shot Conversion
Standby
Mode
Standby
Mode
STANDBY
t
18
+ 256 x τ
CLKIN
Figure 20. One-Shot Conversions Using the STANDBY Command
DRDY
V
IN
=AIN
P
−
AIN
N
DIN
DOUT
Old V
IN
Data
Settled
Data
RDATA
Fully Settled
New V
IN
Data
Mix of
Old and New
V
IN
Data
Old V
IN
New V
IN
Figure 21. Step Change on V
IN
while Continuously Converting for Data Rates ≤ 3750SPS