Calculator Inputs
Enter CMOS output and input threshold data to evaluate low and high noise margin performance.
Formula Used
NML = VIL(max) - VOL(max)
NMH = VOH(min) - VIH(min)
Adjusted NML = NML - Guard Band
Adjusted NMH = NMH - Guard Band
Worst-Case Margin = min(Adjusted NML, Adjusted NMH)
Target Margin = VDD × Target%
Noise margin estimates how much unwanted voltage disturbance a logic path can tolerate before a valid 0 or 1 is misread. Larger margins usually improve robustness against noise, drift, tolerance spread, and board-level coupling.
How to Use This Calculator
- Enter the guaranteed supply voltage and the device thresholds from the data sheet.
- Use VOH(min) and VOL(max) from the driver side for conservative analysis.
- Use VIH(min) and VIL(max) from the receiving side thresholds.
- Add a guard band to reserve margin for drift, ripple, or uncertainty.
- Set a target percentage of VDD to compare against your design rule.
- Click the calculate button to view margins, status, and the comparison graph.
- Download the CSV for spreadsheet work or the PDF for documentation.
- Review the warning notes if the thresholds overlap or the margins become negative.
Example Data Table
| Scenario | VDD (V) | VOH(min) (V) | VOL(max) (V) | VIH(min) (V) | VIL(max) (V) | Guard Band (V) | Adjusted NML (V) | Adjusted NMH (V) | Status |
|---|---|---|---|---|---|---|---|---|---|
| 5 V CMOS example | 5.00 | 4.90 | 0.10 | 3.50 | 1.50 | 0.10 | 1.30 | 1.30 | Pass |
| 3.3 V logic example | 3.30 | 3.00 | 0.20 | 2.10 | 0.90 | 0.15 | 0.55 | 0.75 | Pass |
| Tight-threshold example | 1.80 | 1.55 | 0.25 | 1.20 | 0.65 | 0.10 | 0.30 | 0.25 | Caution |
Frequently Asked Questions
1. What is CMOS noise margin?
CMOS noise margin is the voltage buffer between guaranteed output levels and guaranteed input thresholds. It shows how much unwanted noise a signal can absorb before the next stage reads the wrong logic state.
2. Why are there two margins, NML and NMH?
Digital logic needs protection for both logic low and logic high states. NML measures low-level tolerance, while NMH measures high-level tolerance. A design is only as strong as the smaller of the two margins.
3. Why should I use VOH(min) and VOL(max)?
Those are worst-case guaranteed output limits. Using them makes the calculation conservative and closer to real design validation. Typical values may look better, but they can hide weak corners and lead to overconfidence.
4. What does the guard band do?
The guard band reserves extra voltage margin for ripple, tolerance spread, aging, interference, and modeling uncertainty. It reduces both raw margins so the final result reflects a more cautious engineering decision.
5. Does temperature change the equations?
The displayed equations are still the same. However, temperature shifts the actual threshold and output values found in device data sheets. That is why this calculator records temperature as operating context for documentation.
6. What if VIH(min) is below VIL(max)?
That means the undefined region has collapsed or overlapped. The calculator warns you because the receiving logic has little or no transition separation, which weakens immunity against input noise and threshold variation.
7. What is a good target margin?
There is no universal value for every product. Many teams use a percentage of VDD or an application-specific voltage reserve. Sensitive, noisy, or long-trace systems usually justify a higher target margin.
8. Can I use this for mixed-voltage interfaces?
Yes, if you enter the driver output guarantees and receiver input thresholds for that exact interface. It is especially useful for checking whether one device family safely drives another under worst-case conditions.