Instrumentation
Analog Input Resolution
Resolution in engineering units per count for an analog input, from converter bit depth and calibrated range, with the smallest meaningful change.
Results
- Resolution
- 0.0003815ft per count
- Usable counts across 4-20 mA
- 65,536
- Resolution in current
- 0.244microamps per count
- Resolution as percent of span
- 0.00153%
- Total converter counts
- 65,536
- Smallest meaningful display step
- 0.0007629ft
16 bit
Displaying more precision than about two counts shows converter noise rather than process change.
The arithmetic
- Total counts = 2 ^ 16 = 65,536
- Counts across 4-20 mA = 65,536 x 1.00 = 65,536
- Resolution = 25.000 ft / 65,536 counts = 0.0003815 ft per count
What this does not account for
- Resolution is not accuracy. A 16-bit card can still be several counts off due to gain error, offset, linearity, and drift. Check the module accuracy specification.
- Displaying more decimal places than the resolution supports invents precision that is not there, and it makes operators chase noise.
- Filtering below the resolution accomplishes nothing. If a value is stepping by one count, the process is not moving.
How this is calculated
Analog resolution is the calibrated engineering span divided by the number of counts across that span. A 16-bit converter has 65,536 total counts, but a 4-20 mA input uses only the portion of the range representing 4 to 20 mA, so the usable resolution is coarser than the bit depth alone suggests.
Formulas
Resolution = engineering span / usable counts
Assumptions built into this calculator
- Ideal converter with no gain, offset, or linearity error.
- The usable fraction depends on how the module maps its input range onto its count range.
Frequently asked questions
- Is a 16-bit card four times better than a 14-bit card?
- In resolution, yes. In accuracy, often not, because accuracy is dominated by gain, offset, linearity, and temperature drift rather than by the converter step size. Read the accuracy specification, not just the bit depth.
Direct contact
Have a controls question?
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