Noise in Noninverting Gain Configuration
R2
Noise at the output:
2
2
R2
R2
R1
R1
2
2
2
2
2
2
2
EO = 1+
en + e1 + e2 + i R
+ eS + i R
1+
(
)
(
)
n
2
n
S
R1
EO
R2
Where eS = √4kTRS •
= thermal noise of RS
1+
R1
RS
R2
R1
e1 = √4kTR1 •
e2 = √4kTR2
= thermal noise of R1
= thermal noise of R2
VS
Noise in Inverting Gain Configuration
R2
Noise at the output:
2
R1
R2
2
2
2
2
2
2
EO = 1+
en + e1 + e2 + i R
+ eS
(
)
n
2
R1 + RS
EO
RS
R2
Where eS = √4kTRS •
= thermal noise of RS
R1 + RS
VS
R2
e1 = √4kTR1 •
e2 = √4kTR2
= thermal noise of R1
= thermal noise of R2
R1 + RS
For the OPA227 and OPA228 series op amps at 1kHz, en = 3nV/√Hz and in = 0.4pA/√Hz.
FIGURE 5. Noise Calculation in Gain Configurations.
OPA227, 2227, 4227
OPA228, 2228, 4228
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