OPA2211-EP_16 Datasheet, PDF (15/32 Pages) Texas Instruments – Low-Power, Precision Operational Amplifier

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Feature Description (continued)
-
Input
R_I
+
R_F
OPA2211-EP
Output
OPA2211-EP
SBOS761 – NOVEMBER 2015
Figure 38. Pulsed Operation
7.3.2 Noise Performance
Figure 39 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 OPA2211-EP has very low voltage noise,
making it ideal for low source impedances (<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 kΩ 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 39 is shown for the
calculation of the total circuit noise. Note that 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 39. Noise Performance of the OPA2211-EP and OPA227 in Unity-Gain Buffer Configuration
7.3.3 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 39. The source impedance is usually fixed; consequently, select the
operational amplifier and the feedback resistors to minimize the respective contributions to the total noise.
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