AN-6605 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – Noise of Sources

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AN-6605
Noise of Sources
INTRODUCTION
The elimination or minimization of noise is one of the
most perplexing problems facing engineers today. Many
preamplifiers and components come with outstanding
noise specifications, only to disappoint the user. The
problem is the difference between specification and
application, as the amplifiers are specified under ideal
conditions not the real conditions, (i.e., a transducer
connected to the input). Many times the transducer noise
is as large, or even greater than, the amplifier noise
degrading the signal to noise ratio. Before amplifier or
component noise can be considered, familiarity with the
source noise is essential.
REVIEW OF NOISE BASICS
There are 3 types of transducers: resistive, capacitive and
inductive. The noise of a passive network is thermal
noise, generated by the real part of the complex impe-
dance, as given by Nyquist's relation:
V
= 4kTRe(Z) f
(1)
Where:
V
= Mean square noise voltage (V²)
k
= Boltzmann’s constant (1.38x10^-23 VAS/K)
T
= Absolute temperature (K)
Re(Z) = Real part of complex impedance ()
f
= Noise bandwidth (Hz)
The noise may be represented as a spectral density of
(V²/Hz) or more commonly in V/ or nV/ and is
given by:
e
=
V
f
(2)
© 2014 Fairchild Semiconductor Corporation
Rev. 1.0 • 6/25/15
Figure 1. Thermal Noise Voltage vs. Resistance
The total noise voltage in a frequency band can be readily
calculated if it is white noise (i.e., Re(Z) is frequency
independent). This is not the case for capacitive or
inductive sources or most real world noise
problems.
Rapidly changing network impedance and amplifier gain
equalization combine to complicate the issue. The total
source noise in a non-ideal case can be calculated by
breaking the noise spectrum into several small bands
where the noise (Re(Z)) is nearly white and calculating
the noise of each band. The total source noise is the RMS
sum of the noise in each of the bands N1-Nn.
V_NOISE = (V^2N1 ₊ V +    + V ) ^1/2 (3)
2
n
The expression does not take amplifier gain equalization
(like RIAA) into account, which will change the character
of the noise at the amplifier output. By reflecting the gain
equalization to the amplifier input and normalizing the
gain to 0 dB at 1 kHz, the equalized source noise may
then be calculated.
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