Noise Lower Limit Detectivity

  1. What is NEP and how do I use it in my design?
  2. What is the lowest optical power a photodiode can detect?
  3. How does dark current (shunt resistance) relate to noise?
  4. What is noise bandwidth?
  5. Are the photodiodes susceptible to EMI?


1. What is NEP and how do I use it in my design?
The Noise Equivalent Power or NEP, is the incident optical power, which generates a photocurrent equal to the noise of the photodiode. In another words, it is the optical power for which the Signal-to-Noise Ratio is equal to one. This value is used as the minimum detectable incident power.

2. What is the lowest optical power a photodiode can detect?
Theoretically there is no lower limit for the optical power that a photodiode can detect. However, the dark current limits the minimum detectable optical power by reducing the signal to noise ratio to less than one.

3. How does dark current (shunt resistance) relate to noise?
Pn the photovoltaic mode measurements, the limiting source of noise is the Johnson (or thermal) noise in the source or shunt resistance. I another words shunt or source resistance is a noise-generating resistance. These are the charged particles produced as a result of the thermal energy in the resistance. This relation is given by:

where kb is the Boltzman constant, T the temperature in Kelvin, Df the noise measurement bandwidth, and RSH the shunt resistance of the photodiode.

In the Photoconductive mode, however, the dominant source of noise is the Shot noise and is related to dark current by:

where IP is the photocurrent, ID the dark current, q the elementary charge and Df the noise measurement bandwidth.

4. What is noise bandwidth?
Noise is the random electrical signal that interferes with the measurement of the signal of interest. Since both shot and Johnson noises are distributed over a wide frequency range, reducing the noise bandwidth effectively reduces the noise in the measurement. The high frequency noise cutoff point is approximately equal to the smallest of either:

- p/2 times the upper 3 dB frequency limit of the analog DC measuring circuitry or

- 0.55/tr where tr is the photodiode's 10%-90% rise time.

5. Are the photodiodes susceptible to EMI?
If not properly shielded, electromagnetic energy can be picked up at the pins of a device. Also, to a lesser extent, the same energy can come through unshielded windows..

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