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| OPA1622 Datasheet, PDF (20/36 Pages) Texas Instruments – OPA1622 SoundPlus™ High-Fidelity, Bipolar-Input, Audio Operational Amplifier | |||
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OPA1622
SBOS727A â NOVEMBER 2015 â REVISED NOVEMBER 2015
www.ti.com
Application Information (continued)
8.1.1.3 Basic Noise Calculations
Designing low-noise op amp circuits requires careful consideration of a variety of possible noise contributors,
such as noise from the signal source, noise generated in the op amp, and noise from the feedback network
resistors. The total noise of the circuit is the root sum squared combination of all noise components.
The resistive portion of the source impedance produces thermal noise proportional to the square root of the
resistance. Figure 49 plots this function. 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 50 shows both inverting and noninverting op amp circuit configurations with gain. In circuit configurations
with gain, the feedback network resistors also contribute noise.
The current noise of the op amp reacts with the feedback resistors to create additional noise components.
Choose feedback resistor values that make these noise sources negligible. The equations for total noise are
shown for both configurations.
Noise in Noninverting Gain Configuration
R2
R1
Noise at the output:
EO2
=
êé1+
ë
R2
R1
ù2
ú
û
en2
+ e12
+
e22
+ (in R2 )2
+ eS2
+
(in RS )2
êé1+
ë
R2
R1
ù2
ú
û
RS
VS
EO
where
·
eS =
4kTRS
´
éê1+
ë
R2
R1
ù
ú
û
=
therma l
noise
of
RS
·
e1 =
4kTR1
´
é
ê
ë
R2
R1
ù
ú
û
=
therma l
noise
of
R1
· e2 = 4kTR2 = thermal noise of R2
Noise in Inverting Gain Configuration
R2
R1
Noise at the output:
EO2
=
éê1+
ë
R2
R1 + RS
ù2
ú
û
en2
+
e12
+
e22
+ (in R2 )2
+
eS2
RS
VS
EO
where
·
eS =
4kTRS
´
é
ê
ë
R2
R1 + RS
ù
ú
û
=
therma l
noise
of
RS
·
e1 =
4kTR1
´
é
ê
ë
R2
R1 + RS
ù
ú
û
=
therma l
noise
of
R1
· e2 = 4kTR2 = thermal noise of R2
For the OPA1622 at 1 kHz, en = 2.8 nV/âHz and in = 800 fA/âHz.
Figure 50. Noise Calculation in Gain Configurations
20
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