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54
DM35425HR User’s Manual
7.2
ADC Calibration
Two procedures are used to calibrate the ADC for all input voltage ranges. The first procedure calibrates the converter for the bipolar ranges
(±5, ±10 V), and the second procedure calibrates the unipolar range (0 to +10 V). Table 25 shows the ideal input voltage for each bit weight for
the bipolar ranges, and Table 27 shows the ideal voltage for each bit weight for the unipolar ranges.
7.2.1
B
IPOLAR
C
ALIBRATION
Bipolar Range Adjustment: -5 to +5 V
Two adjustments are made to calibrate the ADC for the bipolar range of -5 to +5 V. One is the offset adjustment, and the other is the full scale,
or gain, adjustment. Trimpot
TR4
is used to make the offset adjustment, and trimpot
TR5
is used for gain adjustment. Before making these
adjustments, make sure that the board is programmed for a range of ±5 V.
Use AIN0 and set it for a gain of 1 while calibrating the board. Connect you precision voltage source to AIN0. Set the voltage source to -
1.22070 mV, start a conversion and read the resulting data. Adjust trimpot
TR4
until the reading flickers between the values listed in the table
below. Next, set the voltage to -4.99878 V, and repeat the procedure, this time adjusting
TR5
until the data flickers between the values in the
table below.
Table 23: Data Values for Calibrating Bipolar 10 V Range
Offset (TR4)
Input Voltage = -1.22mV
Converter Gain (TR5)
Input Voltage = -4.99878V
ADC Converted Data
0000 0000 0000
1000 0000 0000
1111 1111 1111
1000 0000 0001
Bipolar Range Adjustment: -10 to +10 V
To adjust the bipolar 20 V range (-10 to +10 V), program the board for ± 10 V input range. Then, set the input voltage to +5.0000 V and adjust
TR2
until the output matches the data in the table below.
Table 24: Data Values for Calibrating Bipolar 20 V Range
Offset (TR4)
Input Voltage = +5.0000 V
ADC Converted Data
0100 0000 0000
Bipolar Range Ideal Bit Weight
Below is a table listing the ideal input voltage for each bit weight for the bipolar ranges.
Table 25: ADC Bit Weights, Bipolar
Sign
ADC Bit Weight
Ideal Output Voltage (mV)
-5 to +5 V -10 to +10 V
1
1111 1111 1111
-2.44
-4.88
1
1000 0000 0000
-5000.00
-10000.00
0
0100 0000 0000
+2500.00
+5000.00
0
0010 0000 0000
+1250.00
+2500.00
0
0001 0000 0000
+625.00
+1250.00
0
0000 1000 0000
+312.50
+625.00
0
0000 0100 0000
+156.25
+312.50
0
0000 0010 0000
+78.13
+156.25
0
0000 0001 0000
+39.06
+78.13
0
0000 0000 1000
+19.53
+39.06
0
0000 0000 0100
+9.77
+19.53
0
0000 0000 0010
+4.88
+9.77
0
0000 0000 0001
+2.44
+4.88
0
0000 0000 0000
0.00
0.00
7.2.2
U
NIPOLAR
C
ALIBRATION
One adjustment is made to calibrate the ADC for the unipolar range of 0 to +10 V. Trimpot
TR6
is used to make the offset adjustment. This
calibration procedure is performed with the module programmed for a 0 to +10 V input range. Before making these adjustments, make sure that
the module is programmed properly and has been calibrated for bipolar ranges.
