If you are learning PLC or working in automation, you already know that analog signals are the heart of real-world control systems.
Temperature, pressure, level, flow, speed, all these values reach the PLC through analog inputs like 4–20 mA or 0–10 V.
But here is a very important question that many beginners ignore:
How accurately does the PLC actually read this analog signal?
What is the smallest change in current or voltage that the PLC can detect?
This is where analog signal resolution in PLC becomes extremely important.
In this article, I will explain how to calculate analog signal resolution in PLC step by step, using very simple formulas and real examples with 12-bit and 16-bit PLCs.
I will also show you how this directly affects your 4–20 mA measurement accuracy in real plants.
Let’s start from the basics.
What Is Analog Signal Resolution in PLC?
In simple words:
Analog resolution is the smallest change in an analog signal that a PLC can detect and convert into a digital value.
Your field instrument (pressure transmitter, temperature transmitter, flow transmitter, etc.) sends a continuous analog signal like:
- 4–20 mA
- 0–10 V
But the PLC cannot understand continuous signals directly. Internally, the PLC works only with digital numbers (0s and 1s). So it uses a device called an ADC (Analog-to-Digital Converter) to convert the continuous signal into digital values.
The resolution depends mainly on the number of bits used by the ADC.
What Does 12-Bit, 14-Bit, or 16-Bit Mean in PLC?
When a PLC analog input card is described as:
- 12-bit
- 14-bit
- 16-bit
It simply means how many digital steps the analog signal is divided into.
The general formula is:
Number of Steps = 2ⁿ
Where n = number of bits
So:
- 12-bit → 2¹² = 4096 steps
- 14-bit → 2¹⁴ = 16384 steps
- 16-bit → 2¹⁶ = 65536 steps
This means a 16-bit PLC divides the same analog signal into many more smaller steps than a 12-bit PLC. Therefore, the 16-bit PLC can detect much smaller signal changes.
This is the foundation of analog resolution.
Step 1: Understand Your Analog Signal Range
Before calculating resolution, you must clearly know your input signal range.
The most common ranges are:
- 4–20 mA
- 0–20 mA
- 0–10 V
- ±10 V
Let us take the most widely used industrial signal:
4–20 mA Range
Signal span = 20 mA – 4 mA = 16 mA
This 16 mA is your total measurable analog span.
Step 2: Know Your PLC Bit Resolution
Now check your PLC analog input module manual. It will clearly mention:
- 12-bit
- 14-bit
- or 16-bit resolution
Let us take two very common cases:
- 12-bit PLC analog input
- 16-bit PLC analog input
Step 3: Calculate Total Digital Counts
Use the formula:
Digital Counts = 2ⁿ
For 12-bit PLC:
2¹² = 4096 counts
For 16-bit PLC:
2¹⁶ = 65536 counts
These counts represent how many digital divisions the PLC makes across your 4–20 mA signal.
Step 4: Calculate Analog Resolution (Important Formula)
Now comes the most important formula of this article:
Analog Signal Resolution = Signal Span ÷ Number of Digital Counts
Let us calculate it practically.
Example 1: 12-Bit PLC Resolution for 4–20 mA
Signal Span = 16 mA
Digital Counts = 4096
So,
Resolution = 16 mA ÷ 4096 = 0.0039 mA per count
This means:
The PLC can detect a change of only 0.0039 mA in the input signal.
Example 2: 16-Bit PLC Resolution for 4–20 mA
Signal Span = 16 mA
Digital Counts = 65536
So,
Resolution = 16 mA ÷ 65536 = 0.000244 mA per count
This means:
The PLC can detect a change of just 0.000244 mA, which is extremely fine.
Now you can clearly see: 16-bit resolution is far more accurate than 12-bit resolution.
Step 5: Convert Resolution into Engineering Units (Real Plant Meaning)
Now let’s convert this into something meaningful for a plant engineer.
Assume you have a pressure transmitter with this range:
- 4–20 mA = 0–10 bar
So: - 16 mA = 10 bar
- 1 mA = 0.625 bar
For 12-Bit PLC
We already calculated:
Resolution = 0.0039 mA
So pressure resolution becomes:
0.0039 × 0.625 = 0.00244 bar per count
Meaning:
The smallest pressure change your PLC can detect is 0.00244 bar.
For 16-Bit PLC
Resolution = 0.000244 mA
So pressure resolution becomes:
0.000244 × 0.625 = 0.0001525 bar per count
Meaning:
The PLC can now detect pressure changes as small as 0.00015 bar.
This is why high-accuracy applications always use 16-bit or higher resolution cards.
Why Analog Resolution Is So Important in Real Plants
Let me talk to you practically.
Many students think:
“As long as I am getting a value on the HMI, it is fine.”
But in real industrial control, small errors can create big problems.
Low analog resolution can cause:
- Poor temperature control
- Unstable pressure loops
- Hunting in control valves
- Inaccurate flow measurement
- Wrong level readings in tanks
In PID control systems, resolution directly affects control stability. If your PLC cannot detect small changes in the analog signal, your controller reacts late or incorrectly.
Difference Between Accuracy, Resolution, and Precision (Very Important)
Many beginners confuse these three terms.
Let me simplify:
- Resolution → Smallest detectable change
- Accuracy → How close the measured value is to the true value
- Precision → Repeatability of measurement
A PLC can have high resolution but poor accuracy if the analog card is not calibrated properly. So always remember: resolution alone does not guarantee accuracy, but it is a very important contributor.
How PLC Displays Raw Analog Values Internally
Internally, the PLC does not directly work in mA or volts. It works with raw digital counts.
For example:
In many PLCs:
=> 4 mA ≈ 0
=> 20 mA ≈ 27648 or 32767 (depends on manufacturer)
These raw values are then scaled in the PLC program to convert into:
- bar
- °C
- %
- m³/h
Resolution defines how smoothly this scaling works.
Quick Comparison: 12-Bit vs 16-Bit PLC Resolution
| Feature | 12-Bit | 16-Bit |
|---|---|---|
| Digital Counts | 4096 | 65536 |
| 4–20 mA Resolution | 0.0039 mA | 0.000244 mA |
| Pressure Sensitivity (0–10 bar) | ~0.00244 bar | ~0.00015 bar |
| Suitable for | Basic automation | High-accuracy control |
| Cost | Lower | Higher |
What we learn today?
If you are just starting your career in PLC and automation, this topic of analog signal resolution may look small, but trust me, it plays a huge role in real industrial accuracy and control quality.
Now you clearly know how to calculate analog signal resolution in PLC, and why it plays such a critical role in real industrial automation. Whether you are working with a small PLC or a high-end automation system, understanding PLC analog resolution calculation will help you achieve better accuracy, smoother control, and more reliable measurement.
Always remember:
- Higher bit resolution = better signal sensitivity
- Proper analog resolution in PLC = better process control
- Wrong PLC resolution selection = unstable and inaccurate systems
If you truly master how to calculate analog signal resolution in PLC, you will be far ahead of many beginners in automation engineering.
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