Introduction
Noise Equivalent Power (NEP) is a key figure of merit for infrared detectors. It quantifies the minimum detectable signal power. This article explains what NEP means for PIR sensors and how to use it in design.
Definition of NEP
NEP is the amount of incident infrared power required to produce a signal-to-noise ratio of 1 (i.e., signal equals noise). It is typically specified in watts per square root hertz (W/√Hz) or simply in watts for a given bandwidth.
Lower NEP = Better sensitivity.
NEP in PIR Sensor Datasheets
For the Murata IRA-S200ST01, NEP is specified as 1.5×10⁻⁹ W/√Hz. This means that in a 1 Hz bandwidth, the sensor can detect 1.5 nW of infrared power.
Calculating Minimum Detectable Signal
To find the minimum detectable signal power in your application:
P_min = NEP × √(Δf)
Where Δf is the noise bandwidth of your system. For a PIR system with a 10 Hz bandwidth:
P_min = 1.5e-9 × √10 = 1.5e-9 × 3.16 = 4.74e-9 W (4.74 nW).
Relating NEP to Detection Distance
The infrared power received from a human at distance d depends on:
- Human’s emitted power (≈100 W total, but only a fraction in the sensor’s band).
- Atmospheric attenuation.
- Lens collection area.
A rough estimate: at 10m, received power might be on the order of 1-10 nW, which is near the NEP limit. This explains why range is limited.
NEP vs. Detectivity (D*)
Detectivity (D*) is another common figure, defined as:
D* = √(A_d × Δf) / NEP
Where A_d is the detector area. D* normalizes for area, allowing comparison between different sensors. Higher D* is better.
Factors Affecting NEP
- Sensor material: Lithium tantalate has lower NEP than PZT.
- Temperature: Cooling reduces NEP (but not practical for most PIR).
- Element size: Larger elements have higher signal but also higher noise.
- Electrical bandwidth: Limiting bandwidth reduces total noise.
Using NEP in System Design
NEP helps you:
- Compare different sensors objectively.
- Estimate maximum detection range.
- Design amplifier gain (signal must be above noise floor).
- Choose optimal bandwidth (wider bandwidth gives faster response but higher noise).
Example: HC-SR501 vs. Murata IRA-S200ST01
HC-SR501 uses a cheap pyroelectric element (likely PZT-based) with higher NEP (less sensitive) than the Murata. This explains why HC-SR501 has shorter range and more false triggers.
Conclusion
NEP is a fundamental specification that quantifies a PIR sensor’s sensitivity. Understanding it helps in selecting the right sensor and designing optimal signal conditioning.