OPA1641 Datasheet, PDF (18/31 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

OPA1641, OPA1642, OPA1644
SBOS484C – DECEMBER 2009 – REVISED DECEMBER 2015
Application Information (continued)
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10k
E_O
1k
R_S
OPA1611
100
OPA1641
10
Resistor Noise
1
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (W)
Figure 32. Noise Performance of the OPA1611 and OPA1641 in Unity-Gain Buffer Configuration
8.1.2 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 operational amplifier 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 32. The source impedance is usually fixed; consequently, select the
operational amplifier and the feedback resistors to minimize the respective contributions to the total noise.
Figure 33 illustrates both noninverting (A) and inverting (B) operational amplifier circuit configurations with gain.
In circuit configurations with gain, the feedback network resistors also contribute noise. In general, the current
noise of the operational amplifier reacts with the feedback resistors to create additional noise components.
However, the extremely low current noise of the OPA164x 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 will load the output of the amplifier. The equations for total noise are shown for
both configurations.
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