ZHCSIQ4B September 2018 – December 2018 OPA828
Figure 48 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 OPA828 and OPA211 are shown with total circuit noise calculated. The op amp itself contributes both 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. Therefore, the lowest noise op amp for a given application depends on the source impedance. For low source impedance, current noise is negligible, and voltage noise generally dominates. The OPA828 device has both low voltage noise and extremely low current noise because of the FET input of the op amp. As a result, the current noise contribution of the OPA828 is negligible for any practical source impedance, which makes it the better choice for applications with high source impedance.
The equation in shows the calculation of the total circuit noise, with these parameters: