5.2: Sources of Noise in Instrumental Analyses

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Johnson Noise

Also called thermal noise, this source of noise results in random voltage fluctuations produced by the thermal agitation of electrons as they pass through resistive elements in the electronics of an instrument. The relationship between Johnson Noise and experimental parameters is as follows:

\[\mathrm{V_{RMS} =\sqrt{4kTRΔf}}\]

where

V_RMS: Root-mean-square noise voltage with a frequency bandwidth of Δf (in Hertz).
k: Boltzmann’s constant (1.38 x 10^-23 J/K)
T: Temperature (K)
R: Resistance of resistive element (Ω)

Reduction of Johnson Noise is accomplished most easily by:

Cooling the detector (reducing T)
Decreasing the frequency bandwidth of the signal (reducing Δf)
Actual measurements of Johnson Noise may be found by clicking here

Shot Noise

This source of noise results in current fluctuations produced by electrons crossing a junction in a random fashion, which highlights the quantized nature of electron flow. The relationship between Shot Noise and experimental parameters is as follows:

\[\mathrm{i_{RMS} = \sqrt{2IeΔf}}\]

with

i_RMS: Root-mean-square current fluctuation (in Amperes)
I: Average direct current (A)
e: electronic charge (1.60 x 10^-19 C)
Δf: frequency bandwidth (Hz)

Reduction of Shot Noise is accomplished most easily by decreasing the frequency bandwidth of the signal (reducing Δf). A good discussion of Shot Noise may be found by clicking here

Flicker Noise

Flicker noise is also called 1/f noise because the magnitude of flicker noise is inversely proportional to frequency. The source of flicker noise is uncertain and it seems to be significant only at low frequencies (<100 Hz). A good summary of flicker noise (and Johnson noise) may be found by clicking here

Environmental Noise

These are sources of noise that interfere with analytical measurements. Examples of such sources include:

electrical power lines (e.g. 50 or 60 Hz line noise)
electrical equipment (e.g. motors, fluorescent lights, etc.)
RF sources (e.g. cell phones)
environmental factors (drift in temperature, aging of electronic components)