OPA1612-Q1 Datasheet, PDF (19/31 Pages) Texas Instruments – SoundPlus High-Performance, Bipolar-Input Audio Operational Amplifier

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OPA1612-Q1
SLOS931A – NOVEMBER 2015 – REVISED NOVEMBER 2015
Typical Application (continued)
9.2.1 Design Requirements
Use Equation 1 to calculate the total circuit noise.
E_O2 = e_n2 + (i_nR_S)² + 4kTR_S
where
• e_n = voltage noise
• I_n = current noise
• R_S = source impedance
• k = Boltzmann’s constant = 1.38 × 10^–23 J/K
• T = temperature in degrees Kelvin (K)
(1)
9.2.2 Detailed Design Procedure
9.2.2.1 Noise Performance
Figure 40 shows the total circuit 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 OPA1612-Q1 device (GBW = 40 MHz, G = +1) is shown with total circuit noise calculated. The op amp
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. The low voltage noise of
the OPA1612-Q1 device makes it a good choice for use in applications where the source impedance is less than
1 kΩ.
9.2.2.1.1 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 40 plots this function. 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 38 shows 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.
Copyright © 2015, Texas Instruments Incorporated
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