OPA1641_15 Datasheet, PDF (11/29 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

www.ti.com
Figure 32 illustrates both noninverting (A) and
inverting (B) op amp circuit configurations with gain.
In circuit configurations with gain, the feedback
network resistors also contribute noise. In general,
the current noise of the op amp reacts with the
feedback resistors to create additional noise
components. However, the extremely low current
noise of the OPA164x means that its current noise
contribution can be neglected.
OPA1641
OPA1642
OPA1644
SBOS484B – DECEMBER 2009 – REVISED AUGUST 2010
The feedback resistor values can generally be
chosen to make these noise sources negligible. Note
that low impedance feedback resistors will load the
output of the amplifier. The equations for total noise
are shown for both configurations.
space
space
A) Noise in Noninverting Gain Configuration
R₂
R₁
E_O
R_S
V_S
Noise at the output:
E_O2 =
1+
R₂
R₁
2
e_n2 +
2
2
R₂
R₁
e₁₂ + e₂₂ +
1+
R₂
R₁
e_s2
Where e_S =
e₁ =
e₂ =
4kTR_S = thermal noise of R_S
4kTR₁ = thermal noise of R₁
4kTR₂ = thermal noise of R₂
B) 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 +
R₂
R₁ + R_S
2
e₁₂ + e₂₂ +
R₂
R₁ + R_S
2
e_s2
Where e_S =
e₁ =
e₂ =
4kTR_S = thermal noise of R_S
4kTR₁ = thermal noise of R₁
4kTR₂ = thermal noise of R₂
For the OPA164x series op amps at 1kHz, e_n = 5.1nV/ÖHz
Figure 32. Noise Calculation in Gain Configurations
Copyright © 2009–2010, Texas Instruments Incorporated
11
Product Folder Link(s): OPA1641 OPA1642 OPA1644