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:
E =
O
2
Wheree = Ö
S
4kTR
S
´ =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
4kTR
1
´ =thermalnoiseofR
1
2
1+
R
2
R
1
e = Ö
2 2
4kTR
2
=thermalnoiseofR
Noiseattheoutput:
E =
O
2
Wheree = Ö
S
4kTR
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 = Ö
1
4kTR
1
´ =thermalnoiseofR
1
e = Ö
2 2
4kTR =thermalnoiseofR
2
OPA209
OPA2209
OPA4209
www.ti.com SBOS426C –NOVEMBER 2008–REVISED OCTOBER 2013
Figure 34 illustrates both noninverting (Figure 34a) The feedback resistor values can generally be
and inverting (Figure 34b) op amp circuit chosen to make these noise sources negligible. Note
configurations with gain. In circuit configurations with that low-impedance feedback resistors load the
gain, the feedback network resistors also contribute output of the amplifier. The equations for total noise
noise. The current noise of the op amp reacts with are shown for both configurations.
the feedback resistors to create additional noise
components.
NOTE: For the OPA209 series op amps at 1kHz, e
n
= 2.2nV/√Hz and I
n
= 530fA/√Hz.
Figure 34. Noise Calculation in Gain Configurations
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