OPA211_15 Datasheet, PDF (22/45 Pages) Texas Instruments – Low Power, Precision Operational Amplifier

OPA211, OPA2211
SBOS377H – OCTOBER 2006 – REVISED NOVEMBER 2015
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Application Information (continued)
9.1.3 Noise Performance
Figure 45 shows total circuit noise for varying source impedances with the operational amplifier in a unity-gain
configuration (no feedback resistor network, and therefore no additional noise contributions). Two different
operational amplifiers are shown with total circuit noise calculated. The OPA211 has very low voltage noise,
making it ideal for low source impedances (less than 2 kΩ). A similar precision operational amplifier, the
OPA227, has somewhat higher voltage noise but lower current noise. It provides excellent noise performance at
moderate source impedance (10 to 100 kΩ). Above 100 kΩ, a FET-input operational amplifier such as the
OPA132 (very low current noise) may provide improved performance. The equation in Figure 45 is shown for the
calculation of the total circuit noise.
NOTE
e_n = voltage noise, I_n = current noise, R_S = source impedance, k = Boltzmann’s constant =
1.38 × 10^–23 J/K, and T is temperature in K.
10k
1k
R_S
100
E_O
OPA227
OPA211
Resistor Noise
10
1
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (Ω)
Figure 45. Noise Performance of the OPA211 and OPA227 in Unity-Gain Buffer Configuration
9.1.4 Basic Noise Calculations
Design of low-noise operational amplifier circuits requires careful consideration of a variety of possible noise
contributors: noise from the signal source, noise generated in the operational amplifier, 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. This function is plotted in Figure 45. The source impedance is usually fixed; consequently, select the
operational amplifier and the feedback resistors to minimize the respective contributions to the total noise.
Figure 45 depicts total noise for varying source impedances with the operational amplifier in a unity-gain
configuration (no feedback resistor network, and therefore no additional noise contributions). The operational
amplifier 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 operational amplifier for a given application depends on the source
impedance. For low source impedance, current noise is negligible and voltage noise generally dominates. For
high source impedance, current noise may dominate.
Figure 42 shows both inverting and noninverting operational amplifier circuit configurations with gain. In circuit
configurations with gain, the feedback network resistors also contribute noise. The current noise of the
operational amplifier 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.
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