AN-940 View Datasheet(PDF) - Analog Devices
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AN-940 Datasheet PDF : 12 Pages
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Application Note
FLICKER NOISE
The noise of op amps is Gaussian with constant spectral density
(white noise), over a wide range of frequencies. As frequency
decreases, the spectral density starts to rise because of the fabri-
cation process, the IC device layout, and the device type at a
rate of about 3 dB per octave for CMOS amplifiers, 3.5 dB to
4.5 dB per octave for bipolar amplifiers, or up to 5 dB per
octave for JFET amplifiers.
This low frequency noise characteristic is known as flicker
noise or 1/f noise because the noise power spectral density
goes inversely with frequency (1/f). It has a −1 slope on a log
plot. The frequency at which an extrapolated −3 dB per octave
(for a CMOS-type amplifier) spectral density line intersects the
broadband constant spectral density value is known as the 1/f
corner frequency and is a figure of merit for the amplifier (see
Figure 2). Bipolar and JFET amplifiers typically have lower 1/f
corner frequency than CMOS amplifiers.
100
EXTRAPOLATED 1/f
SPECTRAL NOISE DENSITY
10
1
EXTRAPOLATED
CONSTANT SPECTRAL
NOISE DENSITY
1/f CORNER FREQUENCY
0.1
0.1
1
10
100
1k
10k
FREQUENCY (Hz)
Figure 2. Spectral Noise Density
AN-940
POPCORN NOISE
Popcorn noise (not specified or advertised) is an abrupt shift in
offset voltage or current lasting for several milliseconds with
amplitude from several microvolts to hundreds of microvolts.
This burst or pop is random. Low temperatures and high source
resistances usually produce the most favorable conditions for
popcorn noise. Although the root cause of popcorn noise is
not absolute, both metallic contamination and internal or
surface defects in the silicon lattice can cause popcorn noise
in ICs. Although considerable work has been done to reduce
the sources of popcorn noise in modern wafer fabrication, it
cannot be eliminated. Further analysis of popcorn noise is
beyond the scope of this application note.
SUMMING THE NOISE SOURCES
If the noise sources are uncorrelated (that is, one noise signal
cannot be transformed into the other), the resulting noise is
not their arithmetic sum, but the square root of the sum of
their squares.
Vni, TOTAL (en )2 (RS in )2 Vn (REX )2
(5)
where:
Vni, TOTAL is the total noise referred-to-input (RTI).
en is input-referred voltage noise.
in is input-referred current noise.
RS is an equivalent source or input resistance to the amplifier.
Vn (REX) is voltage noise from external circuitry.
Note the following:
Any resistance in the noninverting input has Johnson noise
and converts current noise to a voltage noise.
Johnson noise in feedback resistors can be significant in
high resistance circuits.
Figure 3 visually shows the Equation 5 as the summation of
vectors by using the Pythagorean Theorem.
Vni, TOTAL
Vn (REX)
RS × in
en
Figure 3. Vector Summation of Noise Sources
Rev. D | Page 5 of 12
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