ADC Resolution Calculator
The number of bits in an analog input card sets the smallest change it can detect. See the step size in signal units and in your own engineering units.
- Counts over span = 2^16 − 1 = 65,535
- Step = 16 ÷ 65,535 = 0.0002441 (for a 16 mA span this is in mA)
- In engineering units = 100 ÷ 65,535 = 0.001526
Resolution by bit depth for your span
| Bits | Counts | Step (signal units) | Step (EU) | Resolution |
|---|---|---|---|---|
| 8 bit | 256 | 0.06275 | 0.3922 | 0.392 % |
| 10 bit | 1,024 | 0.01564 | 0.09775 | 0.0978 % |
| 12 bit | 4,096 | 0.003907 | 0.02442 | 0.0244 % |
| 14 bit | 16,384 | 0.0009766 | 0.006104 | 0.0061 % |
| 15 bit | 32,768 | 0.0004883 | 0.003052 | 0.00305 % |
| 16 bit | 65,536 | 0.0002441 | 0.001526 | 0.00153 % |
| 24 bit | 16,777,216 | 9.537 × 10^−7 | 5.960 × 10^−6 | 5.96 × 10^−6 % |
How ADC Resolution Works
An n bit converter divides its input range into 2n steps. The size of one step, the least significant bit (LSB), is the smallest change the card can see.
Resolution % = 100 ÷ (2n − 1)
Many PLC cards do not use all counts for the nominal span. A Siemens card spans 4 to 20 mA with 27648 counts even though the converter has 15 bits plus sign, so enter 27648 in the counts box for those cards.
Worked example
A 12 bit card spans 16 mA (4 to 20 mA) with 4095 steps, so one count is 16 ÷ 4095 = 3.9 µA. On a 0 to 100 °C range that is about 0.024 °C per count.
Frequently Asked Questions
How many counts does a 16 bit ADC have?
65536 counts (0 to 65535). Signed 16 bit cards use 0 to 32767 for positive values.
Is a higher bit ADC always better?
Only up to the noise level of the signal. Beyond about 14 to 16 bits, electrical noise in a typical 4 to 20 mA loop is larger than one count.
What resolution do I need?
Aim for a step at least ten times smaller than the accuracy you need. For ±0.1 % accuracy, 0.01 % resolution (about 14 bits) is enough.
Learn more on the blog
Analog input stability and cable length
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