2 Current Noise Versus Frequency

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.

Figure 2-1 Current and Voltage Noise Density for a Typical Bipolar Device OPA277

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, i_n = e_n/X_c, where X_c decreases with frequency).

Figure 2-2 Current and Voltage Noise Density for a Typical Bipolar CMOS OPA350