OPA1662-Q1_16 Datasheet, PDF (16/33 Pages) Texas Instruments – Operational Amplifier

OPA1662-Q1
SLOS805C – JULY 2012 – REVISED AUGUST 2016
www.ti.com
Feature Description (continued)
8.3.4 Basic Noise Calculations
Design of low-noise op amp circuits requires careful consideration of a variety of possible noise contributors:
noise from the signal source, noise generated in the op amp, and noise from the feedback network resistors. The
total noise of the circuit is the root-sum-square combination of all noise components.
The resistive portion of the source impedance produces thermal noise proportional to the square root of the
resistance. Figure 45 plots this equation. The source impedance is usually fixed; consequently, select the op
amp and the feedback resistors to minimize the respective contributions to the total noise.
Figure 46 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.
A) Noise in Noninverting Gain Configuration
R₂
R₁
E_O
Noise at the output:
E_O2 =
1+
R₂
R₁
2
e_n2 +
2
2
R₂
R₁
e₁₂ + e₂₂ +
1+
R₂
R₁
e_s2
R_S
V_S
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
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
R_S
E_O
V_S
Where e_S = 4kTR_S = thermal noise of R_S
e₁ = 4kTR₁ = thermal noise of R₁
e₂ = 4kTR₂ = thermal noise of R₂
For the OPA1662-Q1 op amp at 1 kHz, e_n = 3.3 nV/√Hz.
Figure 46. Noise Calculation in Gain Configurations
8.3.5 Total Harmonic Distortion Measurements
The OPA1662-Q1 op amp has excellent distortion characteristics. THD + noise is below 0.0006% (G = 1,
V_O = 3 V_RMS, BW = 80 kHz) throughout the audio frequency range, 20 Hz to 20 kHz, with a 2-kΩ load (see
Figure 7 for characteristic performance).
The distortion produced by the OPA1662-Q1 op amp is below the measurement limit of many commercially
available distortion analyzers. However, a special test circuit (such as Figure 47 shows) can be used to extend
the measurement capabilities.
16
Submit Documentation Feedback
Copyright © 2012–2016, Texas Instruments Incorporated
Product Folder Links: OPA1662-Q1