This application note looks at how current noise impacts the total system noise, and when it is a concern. The emphasis is on low current circuits where current noise is less than 10fA/√Hz. This application note uses simulations and basic calculations to explain the role current noise, source impedance, and parasitic capacitance have on current noise. A simplified explanation describing the device level origin of the noise is also covered. Finally, methods used to measure this noise as well as practical board level precautions for current noise are covered.
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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 ( ). The source impedance itself will generate a thermal noise voltage proportionate to the source impedance. This thermal noise is given by Johnson’s equation . Current noise is a concern when it translates into a larger voltage noise than the thermal noise of the source impedance ( ).
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 then current noise will dominate.
Current Noise | Source Resistance | Total Noise Density |
---|---|---|
0.1fA/√Hz | 1.65 TΩ | 233uV/√Hz |
1fA/√Hz | 16.5 GΩ | 23.3uV/√Hz |
10fA/√Hz | 165 MΩ | 2.33uV/√Hz |
100fA/√Hz | 1.65 MΩ | 232nV/√Hz |
1pA/√Hz | 16.5 kΩ | 23.2nV/√Hz |
10pA/√Hz |
160Ω |
2.33nV/√Hz |
Sometimes amplifier data sheets provide a plot for current noise density versus frequency, and in other cases current noise is specified in the specification table at a particular frequency. Generally, the plot of current noise versus frequency is given for devices where the current noise is larger in magnitude and has a flicker region. Figure 2-1 illustrates a typical bipolar device where the current noise is in hundreds of femtoamps and has a flicker region.
Devices with low current noise generally give the noise in the specification table, but in some cases provide a graph of current noise versus frequency. Figure 2-2 illustrates a CMOS device where current noise density is provided versus frequency. Note that at low frequency the current noise is only 5fA/√Hz, but at about 2kHz the noise starts to increase. This increase in current noise is sometimes called blowback or f-squared noise. F-squared noise gets its name because the slope of the power spectrum is equal to f-squared in this region. This increase in noise is related to internal amplifier noises sources that interact with the amplifier input capacitance. To understand the increase, consider that capacitive reactance of the input capacitors will decrease with frequency translating to higher current noise (for example, in = en/Xc, where Xc decreases with frequency).