OPA227 Datasheet, PDF (19/46 Pages) Burr-Brown (TI) – High Precision, Low Noise OPERATIONAL AMPLIFIERS

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Feature Description (continued)
OPA227, OPA2227, OPA4227
OPA228, OPA2228, OPA4228
SBOS110B – MAY 1998 – REVISED JUNE 2015
1.00+03
E_O
1.00E+02 ^RS
OPA227
1.00E+01
OPA227
OPA277
OPA277
Resistor Noise
Resistor Noise
1.00E+00
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 OPA227 in Unity-Gain Buffer Configuration
7.3.7 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 shown plotted in Figure 39. Because the source impedance is usually fixed, select the
operational amplifier and the feedback resistors to minimize their contribution to the total noise.
Figure 39 shows 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. Consequently, 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 40 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 in the following images for both configurations.
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