Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
25
Self-Calibration
Self-calibration corrects internal offset and gain errors.
During self-calibration, the appropriate calibration signals
are applied internally to the analog inputs.
SELFOCAL performs a self offset calibration. The analog
inputs AIN
P
and AIN
N
are disconnected from the signal
source and connected to AVDD/2. See Table 19 for the
time required for self offset calibration for the different data
rate settings. As with most of the ADS1255/6 timings, the
calibration time scales directly with f
CLKIN
. Self offset
calibration updates the OFC register.
Table 19. Self Offset and System Offset
Calibration Timing
DATA RATE
(SPS)
SELF OFFSET CALIBRATION AND
SYSTEM OFFSET CALIBRATION TIME
30,000 387μs
15,000 453μs
7500 587μs
3750 853μs
2000 1.3ms
1000 2.3ms
500 4.3ms
100 20.3ms
60 33.7ms
50 40.3ms
30 67.0ms
25 80.3ms
15 133.7ms
10 200.3ms
5 400.3ms
2.5 800.3ms
NOTE: For f
CLKIN
= 7.68MHz.
SELFGCAL performs a self gain calibration. The analog
inputs AIN
P
and AIN
N
are disconnected from the signal
source and AIN
P
is connected internally to VREFP while
AIN
N
is connected to VREFN. Self gain calibration can be
used with any PGA setting, and the ADS1255/6 has
excellent gain calibration even for the higher PGA settings,
as shown in the Typical Characteristics section. Using the
buffer will limit the common-mode range of the reference
inputs during self gain calibration since they will be
connected to the buffer inputs and must be within the
specified analog input range. When the voltage on VREFP
or VREFN exceeds the buffer analog input range
(AVDD – 2.0V), the buffer must be turned off during self
gain calibration. Otherwise, use system gain calibration or
write the gain coefficients directly to the FSC register.
Table 20 shows the time required for self gain calibration
for the different data rate and PGA settings. Self gain
calibration updates the FSC register.
Table 20. Self Gain Calibration Timing
DATA RATE
PGA SETTING
DATA RATE
(SPS)
1 2 4 8 16, 32, 64
30,000 417μs 417μs 451μs 517μs 651μs
15,000 484μs 484μs 484μs 551μs 551μs
7500 617μs 617μs 617μs 617μs 751μs
3750 884
2000 1.4ms
1000 2.4ms
500 4.5ms
100 21.0ms
60 34.1ms
50 41.7ms
30 67.8ms
25 83.0ms
15 135.3ms
10 207.0ms
5 413.7ms
2.5 827.0ms
NOTE: For f
CLKIN
= 7.68MHz.
SELFCAL performs first a self offset and then a self gain
calibration. The analog inputs are disconnected from the
from the signal source during self-calibration. When using
the input buffer with self-calibration, make sure to observe
the common-mode range of the reference inputs as
described above. Table 21 shows the time required for
self-calibration for the different data rate settings.
Self-calibration updates both the OFC and FSC registers.
Table 21. Self-Calibration Timing
DATA RATE
PGA SETTING
DATA RATE
(SPS)
1 2 4 8 16, 32, 64
30,000 596μs 596μs 692μs 696μs 892μs
15,000 696μs 696μs 696μs 762μs 896μs
7500 896μs 896μs 896μs 896μs 1029μs
3750 1.3ms
2000 2.0ms
1000 3.6ms
500 6.6ms
100 31.2ms
60 50.9ms
50 61.8ms
30 101.3ms
25 123.2ms
15 202.1ms
10 307.2ms
5 613.8ms
2.5 1227.2ms
NOTE: For f
CLKIN
= 7.68MHz.