High slew rate, unity-gain stable, voltage feedback op amps usually achieve slew rate at the expense of a higher input noise voltage. However, the 4nV/√Hz input voltage noise for the OPA698 is much less than comparable amplifiers. The input-referred voltage noise, and the two input-referred current noise terms, combine to give low output noise under a wide variety of operating conditions. Figure 8-5 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.

The total output spot noise voltage can be computed as the square root of the sum of all squared output noise voltage contributors. Equation 5 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:

Evaluating these two equations for the OPA698 circuit and component values (see Figure 8-8) gives a total output spot noise voltage of 9.5nV/√Hz and a total equivalent input spot noise voltage of 4.8nV/√Hz. This total input-referred spot noise voltage is only slightly greater than the 4nV/√Hz specification for the op amp voltage noise alone. The total noise is dominated by the input-referred spot noise of the OPA698 as long as the impedance appearing at each op amp input is limited to a maximum value of 300Ω. Keep both (R_F || R_G) and the noninverting input source impedance less than 300Ω to satisfy both noise and frequency response flatness considerations. The resistor-induced noise is relatively negligible; therefore, additional capacitive decoupling across the bias current cancellation resistor (R_T) for the inverting op amp configuration of Figure 8-10 is not required, but is still desirable.