Noise from the power supply, linear regulators, and voltage references is a major contributor to the limitations of system performance, especially in instrumentation and communications products. In analog-to-digital converter (ADC) applications, the noise from regulators and references results in clock jitter, which can significantly degrade ADC characteristics such as signal-to-noise ratio (SNR), signal-to-noise and distortion (SINAD) ratio, and bit error rate (BER). Low-noise amplifiers (LNAs) also suffer from phase noise and modulation effects related to power supply noise.
Oscilloscopes often are used to measure power supply, linear regulator, and reference noise. Since an oscilloscope has a sensitivity in the range of 2 mV per division, a substantial voltage gain must be added to see the ripple and noise, which is often in microvolts. This gain is usually accomplished using a “low-noise” operational amplifier or several cascaded low-noise op amps. An active filter follows the op amps, providing high pass and low pass elements, to meet the desired measurement bandwidth with the entire circuit constructed in a Faraday shield. (A paint can serves this purpose.) Several IC manufacturers have application notes describing the measurement.