OPA211AIDRGR Datasheet, PDF (16/37 Pages) Texas Instruments – 1.1nV/√Hz Noise, Low Power, Precision Operational Amplifier in Small DFN-8 Package

OPA211
OPA2211
SBOS377G – OCTOBER 2006 – REVISED MAY 2009...................................................................................................................................................... www.ti.com
Figure 46 depicts total noise for varying source
impedances with the op amp in a unity-gain
configuration (no feedback resistor network, and
therefore no additional noise contributions). The
operational amplifier itself contributes both a voltage
noise component and a current noise component.
The voltage noise is commonly modeled as a
time-varying component of the offset voltage. The
current noise is modeled as the time-varying
component of the input bias current and reacts with
the source resistance to create a voltage component
of noise.
Therefore, the lowest noise op amp for a given
application depends on the source impedance. For
low source impedance, current noise is negligible and
voltage noise generally dominates. For high source
impedance, current noise may dominate.
Figure 47 illustrates both inverting and noninverting
op amp circuit configurations with gain. In circuit
configurations with gain, the feedback network
resistors also contribute noise. The current noise of
the op amp reacts with the feedback resistors to
create additional noise components. The feedback
resistor values can generally be chosen to make
these noise sources negligible. The equations for
total noise are shown for both configurations.
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 47. Noise Calculation in Gain Configurations
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