Datasheet

R
1
R
2
E
O
R
1
R
2
E
O
R
S
V
S
R
S
V
S
NoiseinNoninvertingGainConfiguration
NoiseinInvertingGainConfiguration
Noiseattheoutput:
E =
O
2
Wheree = Ö
S S
4kTR ´ =thermalnoiseofR
S
2
1+
R
2
R
1
e +e
n
1 2
n
2 S S
+e +(i R ) +e +(i
n
R )
2 2 2 2 2 2
1+
R
2
R
1
R
2
R
1
e = Ö
1 1
4kTR ´ =thermalnoiseofR
1
2
1+
R
2
R
1
e = Ö
2 2 2
4kTR =thermalnoiseofR
Noiseattheoutput:
E =
O
2
Wheree = 4kTRÖ
S S
´ =thermalnoiseofR
S
2
1+
R
2
R +R
1 S
e +e
n 1 2 n 2 S
+e +(i R ) +e
2 2 2 2 2
R
2
R +R
1 S
R
2
R +R
1 S
e = 4kTRÖ
1 1
´ =thermalnoiseofR
1
e = 4kTRÖ
2 2 2
=thermalnoiseofR
FortheOPA211seriesopampsat1kHz,e =1.1nV/ Hzandi =1.7pA/ Hz.Ö Ö
n n
OPA211
OPA2211
SBOS377G – OCTOBER 2006 – REVISED MAY 2009 ...................................................................................................................................................... www.ti.com
Figure 46 depicts total noise for varying source Therefore, the lowest noise op amp for a given
impedances with the op amp in a unity-gain application depends on the source impedance. For
configuration (no feedback resistor network, and low source impedance, current noise is negligible and
therefore no additional noise contributions). The voltage noise generally dominates. For high source
operational amplifier itself contributes both a voltage impedance, current noise may dominate.
noise component and a current noise component.
Figure 47 illustrates both inverting and noninverting
The voltage noise is commonly modeled as a
op amp circuit configurations with gain. In circuit
time-varying component of the offset voltage. The
configurations with gain, the feedback network
current noise is modeled as the time-varying
resistors also contribute noise. The current noise of
component of the input bias current and reacts with
the op amp reacts with the feedback resistors to
the source resistance to create a voltage component
create additional noise components. The feedback
of noise.
resistor values can generally be chosen to make
these noise sources negligible. The equations for
total noise are shown for both configurations.
Figure 47. Noise Calculation in Gain Configurations
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