8.1.2 Noise Performance

Figure 31 shows the total circuit noise for varying source impedances with the operational amplifier in a unity-gain configuration (with no feedback resistor network and therefore no additional noise contributions). The op amp itself contributes a voltage noise component and a current noise component. The voltage noise is commonly modeled as a time-varying component of the offset voltage. The current noise is modeled as the time-varying component of the input bias current and reacts with the source resistance to create a voltage component of noise. For a CMOS-input device, the noise resulting from the input current is negligible; therefore, the total noise is dominated by the voltage noise of the OPA1671 at low source resistance, and the resistor noise > 1 kΩ.

Figure 31 shows the calculation of the total circuit noise, with these parameters:

• e_n = voltage noise
• R_S = source impedance
• k = Boltzmann's constant = 1.38 × 10^–23 J/K
• T = temperature in kelvins (K)

For more details on calculating noise, see Basic Noise Calculations.

Figure 32. Noise Performance of the OPA1671 in a Unity-Gain Buffer Configuration