Datasheet

R
1
R
2
E
O
R
1
R
2
E
O
R
S
V
S
R
S
V
S
A)NoiseinNoninvertingGainConfiguration
B)NoiseinInvertingGainConfiguration
Noiseattheoutput:
Wheree =
S
4kTR
S
4kTR
1
4kTR
2
=thermalnoiseofR
S
=thermalnoiseofR
1
=thermalnoiseofR
2
e =
1
e =
2
Noiseattheoutput:
E =
O
2
1+
R
2
R +R
1 S
R
2
R +R
1 S
2 22
Wheree =
S
4kTR
S
4kTR
1
4kTR
2
=thermalnoiseofR
S
=thermalnoiseofR
1
=thermalnoiseofR
2
e =
1
e =
2
R
2
R +R
1 S
2
1+
R
2
R
1
1+
R
2
R
1
2
R
2
R
1
2
e +e +
1 2
2 2
E =
O
2
e +
n
2
e
s
2
e +e +
1 2
2 2
e
s
2
e +
n
2
OPA140
OPA2140, OPA4140
SBOS498A –JULY 2010–REVISED AUGUST 2010 www.ti.com
BASIC NOISE CALCULATIONS Figure 35 illustrates both noninverting (A) and
inverting (B) op amp circuit configurations with gain.
Low-noise circuit design requires careful analysis of
In circuit configurations with gain, the feedback
all noise sources. External noise sources can
network resistors also contribute noise. In general,
dominate in many cases; consider the effect of
the current noise of the op amp reacts with the
source resistance on overall op amp noise
feedback resistors to create additional noise
performance. Total noise of the circuit is the
components. However, the extremely low current
root-sum-square combination of all noise
noise of the OPAx140 means that its current noise
components.
contribution can be neglected.
The resistive portion of the source impedance
The feedback resistor values can generally be
produces thermal noise proportional to the square
chosen to make these noise sources negligible. Note
root of the resistance. This function is plotted in
that low impedance feedback resistors load the
Figure 34. The source impedance is usually fixed;
output of the amplifier. The equations for total noise
consequently, select the op amp and the feedback
are shown for both configurations.
resistors to minimize the respective contributions to
the total noise. space
For the OPAx140 series of operational amplifiers at 1kHz, e
n
= 5.1nV/√Hz.
Figure 35. Noise Calculation in Gain Configurations
14 Copyright © 2010, Texas Instruments Incorporated
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