AN-940 데이터 시트보기 (PDF) - Analog Devices
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AN-940 Datasheet PDF : 12 Pages
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Application Note
Resistors
For the purposes of this application note, the resistor noise is
limited to thermal (Johnson) noise. To keep a low level of this
type of noise, resistance values should be as low as possible
because RMS voltage of thermal (Johnson) noise is proportional
to the square root of the resistor value. For example, a 1 kΩ
resistor has a thermal noise of ~4 nV/√Hz at room temperature.
For an in-depth analysis and low noise designs, other types of
resistor noise should be accounted for, such as contact noise
and shot noise. A few practical notes follow and they should
be considered when selecting a resistor.
Choose the largest practical wattage resistors, as the contact
noise is decreased with a larger volume of material.
Choose low noise resistive element material
Resistive elements composed of pure metals and/or
metal alloys in bulk exhibits low noise characteristics.
Such as Vishay Bulk Metal® foil technology resistors
(such as, S102C, Z201)
Wirewound technology resistors composed of metal
alloys have similar noise characteristics as Bulk Metal
foil technology, but are much more inductive.
Metal film technology resistors as thin film are noisier
than Bulk Metal foil or wirewound technology resistors
because of significant noise contributions from occlusions,
surface imperfections, and nonuniform depositions.
Thick film and carbon composition resistors are the
nosiest resistors.
Reactances
Reactances, such as capacitors and inductors, do not generate
noise, but the noise current through reactances develops noise
voltage as well as the associated parasitic.
Practical Tips
Output noise from a circuit can be reduced by lowering the
total component resistance or by limiting the circuit bandwidth.
Temperature reduction is generally not very helpful unless a
resistor can be made very cold, because noise power is propor-
tional to the absolute temperature,
T(x) in Kelvin = x°C + 273.15°
(3)
All resistors in a circuit generate noise. The effect of generated
noise must always be considered. In practice, only resistors in
the input and feedback paths (typically in high gain configu-
rations) are likely to have an appreciable effect on total circuit
noise. The noise can be considered as coming from either
current sources or voltage sources (whichever is more conve-
nient in a given circuit).
AN-940
INTERNAL NOISE SOURCES
Noise appearing at the amplifier’s output is usually measured as
a voltage. However, it is generated by both voltage and current
sources. All internal sources are generally referred to the input,
that is, treated as uncorrelated or independent random noise
generators in series or in parallel with the inputs of an ideal
noise-free amplifier (see Figure 1). Because these noise sources
are considered random and/or exhibit Gaussian distribution
behavior, it is important to take proper care when summing the
noise sources as discussed in the Summing the Noise Sources
section.
If the same noise appears at two or more points in a circuit (that
is, input bias current cancellation circuitry), the two noise sources
are correlated noise sources and a correlation coefficient factor
should be included in the noise analysis. Further analysis of
correlated noise is limited in this application note as typical
correlation noise sources are less than 10% to 15% and they
can usually be disregarded.
Internal amplifier noise falls into four categories:
Input-referred voltage noise
Input-referred current noise
Flicker noise
Popcorn noise
Input-referred voltage noise and input-referred current noise
are the most common specifications used for amplifier noise
analysis. They are often specified as an input-referred spectral
density function or the rms noise contained in Δf bandwidth
and usually given in terms of nV/√Hz (for voltage noise) or
pA/√Hz (for current noise). The /√Hz is needed because the
noise power adds with (is cumulative over) bandwidth (Hz) or
the voltage and current noise density adds with square root of
the bandwidth (√Hz) (see Equation 1 and Equation 2).
en
–+ +
RS
in
–
in
R2
R1
Figure 1. Op Amp Noise Model
Rev. D | Page 3 of 12
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