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.