OPA211_15 Datasheet, PDF (19/45 Pages) Texas Instruments – Low Power, Precision Operational Amplifier

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Feature Description (continued)
OPA211, OPA2211
SBOS377H – OCTOBER 2006 – REVISED NOVEMBER 2015
NOTE
The input signal and load applied to the operational amplifier are the same as with
conventional feedback without R₃. The value of R₃ should be kept small to minimize its
effect on the distortion measurements.
Validity of this technique can be verified by duplicating measurements at high gain and/or high frequency where
the distortion is within the measurement capability of the test equipment. Measurements for this data sheet were
made with an Audio Precision System Two distortion/noise analyzer, which greatly simplifies such repetitive
measurements. The measurement technique can, however, be performed with manual distortion measurement
instruments.
Noise in Noninverting Gain Configuration
R₂
R₁
E_O
R_S
V_S
Noise at the output:
2
2
E_O2 =
1+
R₂
R₁
e_n2 + e₁₂ + e₂₂ + (i_nR₂)² + e_S2 + (i_nR_S)²
1+
R₂
R₁
Where e_S = Ö4kTR_S ´
1+
R₂
R₁
= thermal noise of R_S
e₁ = Ö4kTR₁ ´
R₂
R₁
= thermal noise of R₁
e₂ = Ö4kTR₂ = thermal noise of R₂
Noise in Inverting Gain Configuration
R₂
R₁
R_S
E_O
V_S
Noise at the output:
2
E_O2 =
1 + ^R2
R₁ + R_S
e_n2 + e₁₂ + e₂₂ + (i_nR₂)² + e_S2
Where e_S = Ö4kTR_S ´
R₂
R₁ + R_S
= thermal noise of R_S
e₁ = Ö4kTR₁ ´
R₂
R₁ + R_S
= thermal noise of R₁
e₂ = Ö4kTR₂ = thermal noise of R₂
For the OPA211 series op amps at 1kHz, e_n = 1.1nV/ÖHz and i_n = 1.7pA/ÖHz.
Figure 42. Noise Calculation in Gain Configurations
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