Figure 7-2 shows the total circuit noise for varying source impedances with the op amp in a unity-gain configuration (no feedback resistor network, and therefore, no additional noise contributions). Two different op amps are shown with the total circuit noise calculated. The OPAx210 have very low voltage noise, making these devices a great choice for low source impedances (less than 2 kΩ). As a comparable, precision FET-input op amp (very low current noise), the OPA827 has somewhat higher voltage noise, but lower current noise. The device provides excellent noise performance at moderate to high source impedance (10 kΩ and up). For source impedance lower than 300 Ω, the OPA211 may provide lower noise.

The equation in Figure 7-2 shows the calculation of the total circuit noise, with these parameters:

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

For more details on calculating noise, see Section 8.1.1.

Figure 7-2 Noise Performance of the OPAx210 and OPA827 in Unity-Gain Buffer Configuration