High slew rate, unity-gain stable, voltage-feedback op amps usually achieve the slew rate at the expense of a higher input noise voltage. The 9.2nV/√ Hz input voltage noise for the OPA4830 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 8-18 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 8-18 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. Equation 10 shows the general form for the output noise voltage using the terms shown in Figure 8-18:

Equation 10.

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; this result is shown in Equation 11:

Equation 11.

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 value is including the noise added by the resistors. This total input-referred spot noise voltage is not much higher than the 9.2nV/√ Hz specification for the op amp voltage noise alone.