The OPA695 offers an excellent balance between voltage and current noise terms to achieve low output noise. The inverting current noise (22 pA/√Hz) is lower than most other current-feedback operational amplifiers, while the input voltage noise (1.8 nV/√Hz) is lower than any unity-gain stable, wideband, voltage-feedback operational amplifier. This low-input voltage noise was achieved at the price of a higher noninverting input current noise (18 pA/√Hz). As long as the ac source impedance looking out of the noninverting node is less than 50 Ω, this current noise does not contribute significantly to the total output noise. The operational amplifier input voltage noise and the two input current noise terms combine to give low output noise under a wide variety of operating conditions. Figure 7-4 shows the operational amplifier 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 7-4 Operational Amplifier Noise Figure 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 1 shows the general form for the output noise voltage using the terms shown in Figure 7-8.

Equation 1.

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 Equation 2:

Equation 2.

Evaluating these two equations for the OPA695 circuit and component values shown in Figure 6-1 gives a total output spot-noise voltage of 18.7 nV/√Hz and a total equivalent input spot-noise voltage of 2.3 nV/√Hz. This total input referred spot-noise voltage is higher than the 1.8-nV/√Hz specification for the operational amplifier voltage noise alone. This reflects the noise added to the output by the inverting current noise times the feedback resistor. If the feedback resistor is reduced in high-gain configurations (as suggested previously), the total input referred voltage noise given by Equation 2 just approaches the 1.8 nV/√Hz of the operational amplifier. For example, going to a gain of +20 (using R_F = 200 Ω) gives a total input referred noise of 2.0 nV/√Hz.

For a more complete discussion of operational amplifier noise calculation, see the Noise Analysis for High Speed Op Amps application note, available through www.ti.com.