OPA141_1 Datasheet, PDF (14/25 Pages) Texas Instruments – Single-Supply, 10MHz, Rail-to-Rail Output,Low-Noise, JFET Amplifier

OPA141
OPA2141
OPA4141
SBOS510B – MARCH 2010 – REVISED MAY 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 31. 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 32 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 OPAx141 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
e_n2 +
R₂
R₁ + R_S
2
e₁₂ + e₂₂ +
R₂
R₁ + R_S
2
e_s2
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 OPAx141 series of operational amplifiers at 1kHz, e_n = 6.5nV/√Hz.
Figure 32. Noise Calculation in Gain Configurations
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