FinalOutputData=(Input
OFC[2:0])
-
FSC[2:0]
400000h
Modulator
AINP
AINN
Digital
Filter
S
OFC
Register
FinalOutput
OutputData
Clippedto24Bits
+
-
FSCRegister
400000h
SBAS424D
– JUNE 2009 – REVISED AUGUST 2011
Table 11. Offset Calibration Values
OFFSET AND FULL-SCALE CALIBRATION
REGISTERS
OFC REGISTER
7FFFFFh
800001h
The conversion data are scaled by offset and gain
registers before yielding the final output code. As
000001h
FFFFFFh
shown in
Figure 55, the output of the digital filter is
000000h
first subtracted by the offset register (OFC) and then
FFFFFFh
000001h
multiplied by the full-scale register (FSC).
Equation 8800001h
7FFFFFh
shows the scaling:
(1) Ideal output code excluding noise and inherent offset error.
FSC[2:0] Registers
(8)
The full-scale calibration is a 24-bit word, composed
The values of the offset and full-scale registers are
of three 8-bit registers, as shown in
Table 14. The
set by writing to them directly, or they are set by
full-scale calibration value is 24-bit, straight binary,
calibration commands.
normalized to 1.0 at code 400000h.
Table 12summarizes the scaling of the full-scale register. A
OFC[2:0] Registers
register value of 400000h (default value) has no gain
The offset calibration is a 24-bit word, composed of
correction (gain = 1). Note that while the gain
three 8-bit registers, as shown in
Table 13. The offset
calibration register value corrects gain errors above 1
is in twos complement format with a maximum
(gain correction
< 1), the full-scale range of the
positive value of 7FFFFFh and a maximum negative
analog inputs cannot exceed 105% full-scale to avoid
value of 800000h. This value is subtracted from the
input overload.
conversion data. A register value of 00000h has no
offset correction (default value). Note that while the
Table 12. Full-Scale Calibration Register Values
offset calibration register value can correct offsets
FSC REGISTER
GAIN FACTOR
ranging from
800000h
2.0
avoid input overload, the analog inputs cannot
exceed 105% full-scale.
400000h
1.0
200000h
0.5
000000h
0
Figure 55. Calibration Block Diagram
Table 13. Offset Calibration Word
REGISTER
BYTE
BIT ORDER
OFC0
LSB
B7
B6
B5
B4
B3
B2
B1
B0 (LSB)
OFC1
MID
B15
B14
B13
B12
B11
B10
B9
B8
OFC2
MSB
B23 (MSB)
B22
B21
B20
B19
B18
B17
B16
Table 14. Full-Scale Calibration Word
REGISTER
BYTE
BIT ORDER
FSC0
LSB
B7
B6
B5
B4
B3
B2
B1
B0 (LSB)
FSC1
MID
B15
B14
B13
B12
B11
B10
B9
B8
FSC2
MSB
B23 (MSB)
B22
B21
B20
B19
B18
B17
B16
26
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2009–2011, Texas Instruments Incorporated