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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
CALIBRATION
PGA · VIN
2VREF
OFC
Output + ǒ
ǓFSC
*
·
b
a
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.
(3)
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
VREFP VREFN
AINP
AINN
Analog
Modulator
Digital
Filter
PGA
Output
Σ
X
OFC
Register
FSC
Register
strongly recommended to issue
a
self-calibration
Figure 23. Calibration Block Diagram
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.
The output of the ADS1255/6 after calibration is shown in
Equation 3.
Table 18. Calibration Values for Different Data Rate Settings
DATA RATE
(SPS)
α
β
IDEAL OFC
000000
IDEAL FSC
44AC08
30,000
15,000
7500
3750
2000
1000
500
100
60
400000
1.8639
1.8639
1.8639
1.8639
1.7474
1.7474
1.7474
2.1843
1.8202
2.1843
1.8202
2.1843
1.8202
2.7304
2.7304
2.7304
H
H
H
400000
000000
44AC08
H
44AC08
H
44AC08
H
494008
H
494008
H
494008
H
3A99A0
H
4651F3
H
3A99A0
H
4651F3
H
3A99A0
H
4651F3
H
2EE14C
H
2EE14C
H
2EE14C
H
H
H
400000
000000
H
H
400000
000000
H
H
3C0000
000000
H
H
3C0000
000000
H
H
3C0000
000000
H
H
4B0000
000000
H
H
3E8000
000000
H
H
50
4B0000
000000
H
H
30
3E8000
000000
H
H
25
4B0000
000000
H
H
15
3E8000
000000
H
H
10
5DC000
000000
H
H
5
5DC000
000000
H
H
2.5
5DC000
000000
H
H
24
BB [ BURR-BROWN CORPORATION ]
