High slew rate, unity-gain stable, voltage-feedback op amps usually achieve a slew rate at the expense of a higher input noise voltage. The 9.2nV/√Hz input voltage noise for the OPA830, however, is much lower than comparable amplifiers. The input-referred voltage noise and the two input-referred current noise terms (2.8pA/√Hz) combine to give low output noise under a wide variety of operating conditions. Figure 8-6 shows the op amp noise analysis model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/√Hz or pA/√Hz.

Figure 8-6 Noise Analysis Model

The total output spot noise voltage can be computed as the square root of the sum of all squared output noise voltage contributors. Figure 8-4 shows the general form for the output noise voltage using the terms shown in Figure 8-6:

Dividing this expression by the noise gain (NG = (1 + R_F / R_G)) gives the equivalent input-referred spot noise voltage at the noninverting input, as shown in Figure 8-7:

Evaluating these two equations for the circuit and component values shown in Figure 8-1 gives a total output spot noise voltage of 19.3nV/√Hz and a total equivalent input spot noise voltage of 9.65nV/√Hz. This result includes the noise added by the resistors. This total input-referred spot noise voltage is not much greater than the 9.2nV/√Hz specification for the op amp voltage noise alone.