High slew rate, unity-gain stable, voltage-feedback op amps usually achieve a high slew rate at the expense of a higher input noise voltage. However, the 9.2nV/√Hz input voltage noise for the OPA2830 is much less 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 8-16 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 8-16 Noise Analysis Model

The total output spot noise voltage is computed as the square root of the sum of all squared output noise voltage contributors. Equation 6 shows the general form for the output noise voltage using the terms shown in Figure 8-16:

Equation 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 shown in Equation 7:

Equation 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.