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Prometheus CPU User Manual V1.44 Page 50
14.7 Convert the numerical data to a meaningful value
Once you have the A/D value, you need to convert it to a meaningful value. The first step is to
convert it back to the actual measured voltage. Afterwards you may need to convert the voltage to
some other engineering units (for example, the voltage may come from a temperature sensor,
and then you would need to convert the voltage to the corresponding temperature according to
the temperature sensor’s characteristics).
Since there are a large number of possible input devices, this secondary step is not included
here; only conversion to input voltage is described. However you can combine both
transformations into a single formula if desired.
To convert the A/D value to the corresponding input voltage, use the following formulas:
Conversion Formula for Bipolar Input Ranges
Input voltage = A/D value / 32768 * Full-scale input range
Example: Input range is ±5V and A/D value is 17761:
Input voltage = 17761 / 32768 * 5V = 2.710V
For a bipolar input range, 1 LSB = 1/32768 * Full-scale voltage.
Here is an illustration of the relationship between A/D code and input voltage for a bipolar input
range (V
FS
= Full scale input voltage):
A/D Code Input voltage symbolic formula Input voltage for ±5V range
-32768 -V
FS
-5.0000V
-32767 -V
FS
+ 1 LSB -4.9998V
… … …
-1 -1 LSB -0.00015V
0 0 0.0000V
1 +1 LSB 0.00015V
… … …
32767 V
FS
- 1 LSB 4.9998V
Conversion Formula for Unipolar Input Ranges
Input voltage = (A/D value + 32768) / 65536 * Full-scale input range
Example: Input range is 0-5V and A/D value is 17761:
Input voltage = (17761 + 32768) / 65536 * 5V = 3.855V
For a unipolar input range, 1 LSB = 1/65536 * Full-scale voltage.
Here is an illustration of the relationship between A/D code and input voltage for a unipolar input
range (V
FS
= Full scale input voltage):
A/D Code Input voltage symbolic formula Input voltage for 0-5V range
-32768 0V 0.0000V
-32767 1 LSB (V
FS
/ 65536) 0.000076V
… … …
-1 V
FS
/ 2 - 1 LSB 2.4999V
0 V
FS
/ 2 2.5000V
1 V
FS
/ 2 + 1 LSB 2.5001V
… … …
32767 V
FS
- 1 LSB 4.9999V