Datasheet

ADC
S
OFC
Register
Final
Output
OutputData
Clippedto24Bits
´
+
-
FSCRegister
400000h
FinalOutputData=(Input OFC[2:0])- ´
FSC[2:0]
400000h
- 1.251V > |Offset Scaling|
2V
Gain Scaling
ADS1246
ADS1247
ADS1248
www.ti.com SBAS426G –AUGUST 2008–REVISED OCTOBER 2011
Offset Calibration Register: OFC[2:0]
CALIBRATION
The offset calibration is a 24-bit word, composed of
The conversion data are scaled by offset and gain
three 8-bit registers. The offset is in twos complement
registers before yielding the final output code. As
format with a maximum positive value of 7FFFFFh
shown in Figure 66, the output of the digital filter is
and a maximum negative value of 800000h. This
first subtracted by the offset register (OSC) and then
value is subtracted from the conversion data. A
multiplied by the full-scale register (FSC). A digital
register value of 000000h provides no offset
clipping circuit ensures that the output code does not
correction. Note that while the offset calibration
exceed 24 bits. Equation 7 shows the scaling.
register value can correct offsets ranging from –FS to
+FS (as shown in Table 12), make sure to avoid
overloading the analog inputs.
Table 12. Final Output Code versus Offset
Calibration Register Setting
FINAL OUTPUT CODE WITH
OFFSET REGISTER V
IN
= 0
7FFFFFh 8000000h
Figure 66. Calibration Block Diagram
000001h FFFFFFh
000000h 000000h
FFFFFFh 000001h
(7)
8000000h 7FFFFFh
The values of the offset and full-scale registers are
1. Excludes effects of noise and inherent offset
set by writing to them directly, or they are set
errors.
automatically by calibration commands.
The gain and offset calibration features are intended
Full-Scale Calibration Register: FSC[2:0]
for correction of minor system level offset and gain
The full-scale or gain calibration is a 24-bit word
errors. When entering manual values into the
composed of three 8-bit registers. The full-scale
calibration registers, care must be taken to avoid
calibration value is 24-bit, straight binary, normalized
scaling down the gain register to values far below a
to 1.0 at code 400000h. Table 13 summarizes the
scaling factor of 1.0. Under extreme situations it
scaling of the full-scale register. Note that while the
becomes possible to over-range the ADC. To avoid
full-scale calibration register can correct gain errors
this, make sure to avoid encountering situations
> 1 (with gain scaling < 1), make sure to avoid
where analog inputs are connected to voltages
overloading the analog inputs. The default or reset
greater than the reference/PGA.
value of FSC depends on the PGA setting. A different
Care must also be taken when increasing digital gain.
factory-trimmed FSC Reset value is stored for each
When implementing custom digital gains less than
PGA setting which provides outstanding gain
20% higher than nominal and offsets less than 40%
accuracy over all the ADS1246/7/8 input ranges.
of full scale, no special care is required. When
Note: The factory-trimmed FSC reset value loads
operating at digital gains greater than 20% higher
automatically loaded whenever the PGA setting
than nominal and offsets greater than 40% of full
changes.
scale, make sure that the offset and gain registers
follow the conditions of Equation 8.
Table 13. Gain Correction Factor versus
Full-Scale Calibration Register Setting
FULL-SCALE REGISTER GAIN SCALING
(8)
800000h 2.0
400000h 1.0
200000h 0.5
000000h 0
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