Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
24
CALIBRATION
Offset and gain errors can be minimized using the
ADS1255/6 onboard calibration circuitry. Figure 23 shows
the calibration block diagram. Offset errors are corrected
with the Offset Calibration (OFC) register and, likewise,
full-scale errors are corrected with the Full-Scale
Calibration (FSC) register. Each of these registers is
24-bits and can be read from or written to.
Analog
Modulator
PGA
Digital
Filter
Output
X
OFC
Register
Σ
VREFNVREFP
AIN
P
AIN
N
FSC
Register
Figure 23. Calibration Block Diagram
The output of the ADS1255/6 after calibration is shown in
Equation 3.
Output +
ǒ
PGA · V
IN
2V
REF
*
OFC
a
Ǔ
FSC · b
where α and β vary with data rate settings shown in
Table 18 along with the ideal values (assumes perfect
analog performance) for OFC and FSC. OFC is a Binary
Two’s Complement number that can range from
−8,388,608 to 8,388,607, while FSC is unipolar ranging
from 0 to 16,777,215.
The ADS1255/6 supports both self-calibration and system
calibration for any PGA setting using a set of five
commands: SELFOCAL, SELFGCAL, SELFCAL,
SYSOCAL, and SYSGCAL. Calibration can be done at
any time, though in many applications the ADS1255/6 drift
performance is low enough that a single calibration is all
that is needed. DRDY
goes high when calibration begins
and remains so until settled data is ready afterwards.
There is no need to discard data after a calibration. It is
strongly recommended to issue a self-calibration
command after power-up when the reference has
stabilized. After a reset, the ADS1255/6 performs
self-calibration. Calibration must be performed whenever
the data rate changes and should be performed when the
buffer configuration or PGA changes.
Table 18. Calibration Values for Different Data Rate Settings
DATA RATE
(SPS)
α β IDEAL OFC IDEAL FSC
30,000 400000
H
1.8639 000000
H
44AC08
H
15,000 400000
H
1.8639 000000
H
44AC08
H
7500 400000
H
1.8639 000000
H
44AC08
H
3750 400000
H
1.8639 000000
H
44AC08
H
2000 3C0000
H
1.7474 000000
H
494008
H
1000 3C0000
H
1.7474 000000
H
494008
H
500 3C0000
H
1.7474 000000
H
494008
H
100 4B0000
H
2.1843 000000
H
3A99A0
H
60 3E8000
H
1.8202 000000
H
4651F3
H
50 4B0000
H
2.1843 000000
H
3A99A0
H
30 3E8000
H
1.8202 000000
H
4651F3
H
25 4B0000
H
2.1843 000000
H
3A99A0
H
15 3E8000
H
1.8202 000000
H
4651F3
H
10 5DC000
H
2.7304 000000
H
2EE14C
H
5 5DC000
H
2.7304 000000
H
2EE14C
H
2.5 5DC000
H
2.7304 000000
H
2EE14C
H
(3)