OPA140_15 Datasheet, PDF (17/44 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output,11-MHz JFET Op Amp

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Feature Description (continued)
OPA140, OPA2140, OPA4140
SBOS498B – JULY 2010 – REVISED NOVEMBER 2015
10k
E_O
1k
R_S
OPA211
100
OPA140
10
Resistor Noise
1
100
2
2
2
E_O = e_n + (i_n R_S) + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (W)
Figure 33. Noise Performance of the OPA140 and OPA211 in Unity-Gain Buffer Configuration
7.3.5 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 33. The source impedance is usually fixed; consequently, select the
op amp and the feedback resistors to minimize the respective contributions to the total noise.
Figure 34 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. Low impedance
feedback resistors load the output of the amplifier. The equations for total noise are shown for both
configurations.
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