8.1.3 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 56. The source impedance is typically fixed; consequently, select the op amp and the feedback resistors to minimize the respective contributions to the total noise.

Figure 57 shows noninverting (A) and inverting (B) op amp circuit configurations with gain. In circuit configurations with gain, the feedback network resistors contribute noise. In general, the current noise of the op amp reacts with the feedback resistors to create additional noise components.

The selected feedback resistor values 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.

1. e_N is the voltage noise of the amplifier. For the OPAx169x series of operational amplifiers, e_N = 4.2 nV/√Hz at 1 kHz.

2. i_N is the current noise of the amplifier. For the OPA169x series of operational amplifiers, i_N = 370 fA/√Hz at 1 kHz.

3. For additional resources on noise calculations, see TI's Precision Labs Series.

Figure 57. Noise Calculation in Gain Configurations