Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
21
DRDY
AIN
P
−AIN
N
DIN
DOUT
t
18
Settled
Data
RDATA
SYNC/PDWN
Figure 18. Data Retrieval After Synchronization
Settling Time Using the Input Multiplexer
The most efficient way to cycle through the inputs is to
change the multiplexer setting (using a WREG command
to the multiplexer register MUX) immediately after DRDY
goes low. Then, after changing the multiplexer, restart the
conversion process by issuing the SYNC and WAKEUP
commands, and retrieve the data with the RDATA
command. Changing the multiplexer before reading the
data allows the ADS1256 to start measuring the new input
channel sooner. Figure 19 demonstrates efficient input
cycling. There is no need to ignore or discard data while
cycling through the channels of the input multiplexer
because the ADS1256 fully settles before DRDY
goes low,
indicating data is ready.
Step 1: When DRDY
goes low, indicating that data is ready
for retrieval, update the multiplexer register MUX using the
WREG command. For example, setting MUX to 23h gives
AIN
P
= AIN2, AIN
N
= AIN3.
Step 2: Restart the conversion process by issuing a SYNC
command immediately followed by a WAKEUP command.
Make sure to follow timing specification t
11
between
commands.
Step 3: Read the data from the previous conversion using
the RDATA command.
Step 4: When DRDY
goes low again, repeat the cycle by
first updating the multiplexer register, then reading the
previous data.
Table 14
gives the effective overall throughput (1/t
19
) when
cycling the input multiplexer. The values for throughput
(1/t
19
) assume the multiplexer was changed with a 3-byte
WREG command and f
SCLK
= f
CLKIN
/4.
Table 14. Multiplexer Cycling Throughput
DATA RATE
(SPS)
CYCLING THROUGHPUT (1/t
19
)
(Hz)
30,000 4374
15,000 3817
7500 3043
3750 2165
2000 1438
1000 837
500 456
100 98
60 59
50 50
30 30
25 25
15 15
10 10
5 5
2.5 2.5
NOTE: f
CLKIN
= 7.68MHz.
DRDY
DOUT
DIN
MUX
Register
01h
AIN
P
=AIN0,AIN
N
=AIN
1
Data from
MUX = 01h
23h
AIN
P
=AIN2,AIN
N
=AIN3
45h
AIN
P
=AIN4,AIN
N
=AIN5
Data from
MUX = 23h
RDATASYNC WAKEUP SYNC WAKEUP
WREG 23h
to MUX reg
RDATA
WREG 45h
to MUX reg
t
18
t
19
Figure 19. Cycling the ADS1256 Input Multiplexer