SERIES IP320 INDUSTRIAL I/O PACK 12-BIT HIGH DENSITY ANALOG INPUT BOARD
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- 10 -
m = Gain * (VoltCALHI - VoltCALLO ) / (CountCALHI -
CountCALLO) (2)
Gain
=
The Programmable Gain Amplifier
Setting Used (See Previous Table)
VoltCALHI
=
High Calibration Voltage
(See Previous Table)
VoltCALLO
=
Low Calibration Voltage
(See Previous Table)
CountCALHI
=
Actual ADC Data Read With High
Calibration Voltage Applied
CountCALLO
=
Actual ADC Data Read With Low
Calibration Voltage Applied
Ideal_Volt_Span
=
Ideal ADC Voltage Span
(See Following Table)
Count_Actual
=
Actual Uncorrected ADC Data For
Input Being Measured
Ideal_Zero
=
Ideal ADC Input For Zero Count
(See Following Table)
Table 3.5: Ideal Voltage Span and Zero For Input Ranges
Input Range
(Volts)
PGA
Gain
ADC
Range
(Volts)
"Ideal_Volt
_Span"
(Volts)
"Ideal_
Zero"
(Volts)
-5 to +5
1
-5 to +5
10.0000
-5.0000
-2.5 to +2.5
2
"
"
"
-1.25 to +1.25
4
"
"
"
-0.625 to +0.625
8
"
"
"
-10 to +10
1
-10 to +10
20.0000
-10.0000
-5 to +5
2
"
"
"
-2.5 to +2.5
4
"
"
"
-1.25 to +1.25
8
"
"
"
0 to +10
1
0 to +10
10.0000
0.0000
0 to +5
2
"
"
"
0 to +2.5
4
"
"
"
0 to +1.25
8
"
"
"
The calibration parameters (CountCALHI and CountCALLO) for
each active input range should be determined at startup and updated
periodically (e.g. once an hour, or more often if ambient
temperatures change) to obtain the best accuracy. Note that several
readings (e.g. 16) of the calibration parameters should be taken via
the ADC and averaged to reduce the measurement uncertainty.
Calibration Programming Example 1
Assume that the input range is -10 to +10 volts. Channel 0 is
connected differentially, and corrected input channel data is desired.
From Tables 3.4 & 3.5, several calibration parameters can be
determined:
Gain = 1 (From Table 3.4)
VoltCALHI = 4.9000 volts (CAL0; From Table 3.4)
VoltCALLO = 0.0000 volts (Auto Zero; From Table 3.4)
Ideal_Volt_Span = 20.0000 volts (From Table 3.5)
Ideal_Zero = -10.0000 volts (From Table 3.5)
The calibration parameters (CountCALHI and CountCALLO)
remain to be determined before uncorrected input channel data can
be taken and corrected.
1. To prepare to measure CountCALLO, write to the Control
Register (@Base + 00H) to setup the auto zero acquisition
mode and PGA gain = 1 by writing 0300H. Note that "not used"
and "don't care" bits are set to zero.
2. Delay to allow for input settling.
3. Execute ADC Convert Command (@Base + 10H).
4. Execute Read ADC Data Command (@Base + 20H). Note that
the 12-bit data is left-justified within the 16-bit word.
5. Repeat steps 3 and 4 several times (e.g. 16) and take the
average of the ADC results. Save this number as
CountCALLO.
6. To prepare to measure CountCALHI, write to the Control
Register (@Base + 00H) to setup the CAL0 acquisition mode
and PGA gain = 1 by writing 0014H. Note that "not used" bits
are set to zero.
7. Delay to allow for input settling.
8. Execute ADC Convert Command (@Base + 10H).
9. Execute Read ADC Data Command (@Base + 20H). Note that
the 12-bit data is left-justified within the 16-bit word.
10. Repeat steps 8 and 9 several times (e.g. 16) and take the
average of the ADC results. Save this number as CountCALHI.
11. Calculate m = actual_slope from equation 2, since all parameters
are known.
It is now possible to correct input channel data from any input
channel using the same input range (i.e. -10 to +10 volts with a
PGA gain = 1). Repeat steps 1-11. periodically to re-measure
the calibration parameters (CountCALHI and CountCALLO) as
required.
12. To prepare to measure channel 0 differentially, write to the
Control Register (@Base + 00H) to setup the differential input
channel 0 acquisition mode and PGA gain = 1 by writing 0000H.
Note that "not used" bits are set to zero.
13. Delay to allow for input settling.
14. Execute ADC Convert Command (@Base + 10H).
15. Execute Read ADC Data Command (@Base + 20H). Note that
the 12-bit data is left-justified within the 16-bit word. This data
represents the uncorrected "Count_Actual" term in equation 1.
Since all parameters on the right hand side of equation 1 are
known. Calculate the calibrated value "Corrected_Count". This
is the desired, corrected value for input channel 0.
16. Repeat steps 12-15 to re-measure channel zero's data as
desired.
Calibration Programming Example 2
Assume that the input range is 0 to +1.25 volts. Channel 39 is
connected single-ended, and corrected input channel data is
desired. From Tables 3.4 and 3.5, several calibration parameters
can be determined:
Gain = 8 (From Table 3.4)
VoltCALHI = 1.2250 volts (CAL2; From Table 3.4)
VoltCALLO = 0.6125 volts (CAL3; From Table 3.4)
Ideal_Volt_Span = 10.0000 volts (From Table 3.5)
Ideal_Zero = 0.0000 volts (From Table 3.5)
The calibration parameters (CountCALHI and CountCALLO)
remain to be determined before uncorrected input channel data can
be taken and corrected.
