OPA2140AIDR datasheet(17/48 Pages) TI

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7.3.5 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 7-1. The source impedance is usually fixed; consequently, select the

op amp and the feedback resistors to minimize the respective contributions to the total noise.

Noise Calculation in Gain Configurations 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 OPAx140 means that its 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.

A)NoiseinNoninvertingGainConfiguration

B)NoiseinInvertingGainConfiguration

Noiseattheoutput:

Wheree =

4kTR

=thermalnoiseofR

e =

Noiseattheoutput:

R +R

Wheree =

4kTR

=thermalnoiseofR

e =

R +R

e +e +

e +

e +e +

e +

For the OPAx140 series of operational amplifiers at 1 kHz, en = 5.1 nV/√ Hz.

Figure 7-2. Noise Calculation in Gain Configurations

7.3.6 Phase-Reversal Protection

The OPA140, OPA2140, and OPA4140 family has internal phase-reversal protection. Many FET- and bipolar-

input op amps exhibit a phase reversal when the input is driven beyond its linear common-mode range. This

condition is most often encountered in noninverting circuits when the input is driven beyond the specified

common-mode voltage range, causing the output to reverse into the opposite rail. The input circuitry of the

OPA140, OPA2140, and OPA4140 prevents phase reversal with excessive common-mode voltage; instead, the

output limits into the appropriate rail (see No Phase Reversal).

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OPA140, OPA2140, OPA4140

SBOS498E – JULY 2010 – REVISED JULY 2021

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