OPA2140 Datasheet, PDF (14/25 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output 11MHz JFET Op Amp

OPA140
OPA2140, OPA4140
SBOS498A – JULY 2010 – REVISED AUGUST 2010
BASIC NOISE CALCULATIONS
Low-noise circuit design requires careful analysis of
all noise sources. External noise sources can
dominate in many cases; consider the effect of
source resistance on overall op amp noise
performance. Total noise of the circuit is the
root-sum-square combination of all noise
components.
The resistive portion of the source impedance
produces thermal noise proportional to the square
root of the resistance. This function is plotted in
Figure 34. The source impedance is usually fixed;
consequently, select the op amp and the feedback
resistors to minimize the respective contributions to
the total noise.
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Figure 35 illustrates both noninverting (A) and
inverting (B) op amp circuit configurations with gain.
In circuit configurations with gain, the feedback
network resistors also contribute noise. In general,
the current noise of the op amp reacts with the
feedback resistors to create additional noise
components. However, the extremely low current
noise of the OPAx140 means that its current noise
contribution can be neglected.
The feedback resistor values can generally be
chosen to make these noise sources negligible. Note
that low impedance feedback resistors load the
output of the amplifier. The equations for total noise
are shown for both configurations.
space
A) Noise in Noninverting Gain Configuration
R₂
R₁
E_O
R_S
V_S
Noise at the output:
E_O2 =
1+
R₂
R₁
2
e_n2 +
2
2
R₂
R₁
e₁₂ + e₂₂ +
1+
R₂
R₁
e_s2
Where e_S =
e₁ =
e₂ =
4kTR_S = thermal noise of R_S
4kTR₁ = thermal noise of R₁
4kTR₂ = thermal noise of R₂
B) Noise in Inverting Gain Configuration
R₂
R₁
Noise at the output:
2
E_O2 =
1 + ^R2
R₁ + R_S
e2+
n
R₂
R₁ + R_S
2
e²+e²+
1
2
R₂
R₁ + R_S
2
e2
s
R_S
V_S
E_O
Where e_S =
e₁ =
e₂ =
4kTR_S = thermal noise of R_S
4kTR₁ = thermal noise of R₁
4kTR₂ = thermal noise of R₂
For the OPAx140 series of operational amplifiers at 1kHz, e_n = 5.1nV/√Hz.
Figure 35. Noise Calculation in Gain Configurations
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