OPA189 Datasheet, PDF (18/34 Pages) Texas Instruments – Precision, 36-V, 14-MHz, MUX-Friendly Low-Noise, Rail-to-Rail Output, Zero-Drift Operational Amplifiers

OPA189, OPA2189, OPA4189
SBOS830 – JUNE 2017
www.ti.com
8.4 Noise Performance
Figure 8 shows the total circuit noise for varying source impedances with the operational amplifier in a unity-gain
configuration (with no feedback resistor network and therefore no additional noise contributions). The OPAx189
and OPA211 are 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 OPA189, OPA2189, and OPA4189 family has both low
voltage noise and low current noise because of the CMOS input of the op amp. As a result, the current noise
contribution of the OPAx189 series is negligible for any practical source impedance, which makes this device the
better choice for applications with high source impedance.
The equation in Figure 8 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)
For more details on calculating noise, see Basic Noise Calculations.
10µ
OPA211
1µ
100n
10n
OPAx189
1n
0.1n
1
Resistor Noise
R_S = 3.6 kŸ
10 100
1k 10k 100k 1M 10M
Source Resistance, R_S (Ÿ)
Copyright © 2017, Texas Instruments Incorpora_Cte₀₀d₃
R_S = 3.6 kΩ is indicated in Figure 8.
This is the source impedance above which OPAx189 is a lower noise option than the OPA211.
Figure 8. Noise Performance of the OPAx189 and OPA211 in Unity-Gain Buffer Configuration
8.5 Basic Noise Calculations
Low-noise circuit design requires careful analysis of all noise sources. External noise sources can dominate in
many cases; consider the effect of source resistance on overall op amp noise performance. 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. This function is plotted in Figure 8. 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 9 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 OPAx189 means that the current noise contribution can be neglected.
The feedback resistor values can generally be chosen to make these noise sources negligible. Low impedance
feedback resistors load the output of the amplifier. The equations for total noise are shown for both
configurations.
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