1 Introduction

The input of all op amps have current noise sources as shown in Figure 1-1. The current noise will translate into voltage noise when it flows through the source impedance ( ${e_n=i}_n{R}_s$ ). The source impedance itself will generate a thermal noise voltage proportionate to the source impedance. This thermal noise is given by Johnson’s equation $e_n=\sqrt{4kT{R}_s}$ . Current noise is a concern when it translates into a larger voltage noise than the thermal noise of the source impedance ( ${ i}_n{R}_s\ge \sqrt{4kT{R}_s}$ ).

Figure 1-1 Current Noise Source and Voltage Noise Source

Both the current noise and voltage noise relationships are dependent on source resistance, so it is possible to plot current noise, thermal noise, and total noise versus source resistance. Note that thermal noise increases proportionate to the square root of resistance, whereas when current noise translates to a voltage it is directly proportionate to source resistance. Thus, current noise will increase at a faster rate than voltage noise with increasing resistance. Figure 1-2 shows current noise, voltage noise, and total noise versus source impedance for a 10fA/√Hz current noise source. Note that for low resistance values thermal noise dominates and at higher resistance values the current noise dominates. For any level of current noise there will always be a value of resistance above which the current noise will dominate because the slope of current noise is greater than the slope of voltage noise.

Table 1-1 shows the resistance where thermal and current noise are equal for different current noise sources. In this table, if the source resistance is less than the specified resistance, the impact of the current noise on overall noise is negligible. In general, if $R_s>4kT/{\left(i_n\right)}^2$ then current noise will dominate.

Figure 1-2 Comparing Current and Thermal Noise Components in Total Noise