A small noise is superimposed on the output voltage of an LDO. What can I do to reduce the noise?

For noise-sensitive loads such as analog circuits and IC power supplies, use an LDO with low output noise voltage (VNO) and a high ripple rejection ratio (R.R.). The sources of noise that appear at the output of an LDO can be broadly divided into two categories. One is the inherent noise generated within an LDO, which includes:

  • 1/f noise:
    Also called flicker noise or pink noise, 1/f noise is considered to be caused by defects on the semiconductor surface, etc. It is called 1/f noise because the noise level is inversely proportional to frequency.
  • Popcorn noise:
    Also called burst noise, popcorn noise is considered to be caused by lattice defects in semiconductor, etc. It is called popcorn noise because it produces an acoustic noise that sounds like popping popcorn when it enters an audio speaker. Popcorn noise has considerable adverse effects on applications that are very sensitive to noise such as CMOS sensors.
  • White noise:
    Also called thermal noise, white noise is caused by the thermal motion of electrons. It is not dependent on frequency. The white noise voltage is expressed as:
White noise:
  • Shot noise:
    Shot noise occurs whenever carriers cross the potential barrier of a p-n junction. It is generated because the resulting current flow is discontinuous. As is the case with white noise, shot noise is not dependent on frequency, and is expressed as:
Shot noise
Figure 1 Noise figure vs. frequency
Figure 1 Noise figure vs. frequency

The other type of noise source is the ripple noise superimposed on the input voltage (supply voltage) of an LDO. Although an LDO internally suppresses ripple noise, some ripple still appears at the output. The ability of an LDO to suppress ripple in the input voltage to its output is called the power supply rejection ratio (PSRR) or the ripple rejection ratio (R.R.). Regarding ripple rejection ratio, also see the section “How can a low-noise power supply be created using an LDO regulator?” in the Frequently Asked Questions (FAQs) page on our website.