OPA1611_15 Datasheet, PDF (15/31 Pages) Texas Instruments – SoundPlus High-Performance, Bipolar-Input Audio Operational Amplifiers

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OPA1611, OPA1612
SBOS450C – JULY 2009 – REVISED AUGUST 2014
8 Application and Implementation
8.1 Application Information
The OPA1611 and OPA1612 are unity-gain stable, precision op amps with very low noise; these devices are also
free from output phase reversal. Applications with noisy or high-impedance power supplies require decoupling
capacitors close to the device power-supply pins. In most cases, 0.1-μF capacitors are adequate.
8.2 Noise Performance
Figure 32 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 OPA1611 (GBW = 40 MHz, G = +1) is shown with total circuit noise calculated. The op amp 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. The low voltage noise of
the OPA161x series op amps makes them a good choice for use in applications where the source impedance is
less than 1 kΩ.
8.2.1 Detailed Design Procedure
The equation in Figure 32 shows the calculation of the total circuit noise, with these parameters:
• 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)
8.2.2 Application Curve
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
1k
R_S
E_O
Total Output
Voltage Noise
100
Resistor
10
Noise
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1
100
1k
10k
100k
1M
Source Resistance, R_S (W)
Figure 32. Noise Performance of the OPA1611 In Unity-Gain Buffer Configuration
8.2.3 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 32 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.
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