OPA145 Datasheet, PDF (20/40 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output, 5.5-MHz JFET Operational Amplifiers

OPA145, OPA2145, OPA4145
SBOS427 – JUNE 2017
Feature Description (continued)
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10µ
OPA211
1µ
100n
10n
OPA145
1n
0.1n
1
Resistor Noise
10 100
1k
R_S = 3.8 kŸ
10k 100k 1M
10M
Source Resistance, R_S (Ÿ)
C003
R_S = 3.8 kΩ is indicated in Figure 39.
This is the source impedance above which OPA145 will be a lower noise option than the OPA211.
Figure 39. Noise Performance of the OPA145 and OPA211 in Unity-Gain Buffer Configuration
8.3.5 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 39. 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 40 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 OPAx145 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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