PSRR

PSRR stands for power supply rejection ratio. It is a measure of how well a circuit’s output is insulated from variations on its supply rails. In small-signal terms, PSRR quantifies how much the output changes when the input supply changes. A common way to express it is PSRR(dB) = 20 log10(|ΔVdd|/|ΔVout|), or equivalently PSRR(dB) = -20 log10(|ΔVout|/|ΔVdd|). A higher PSRR indicates better rejection of supply disturbances.

PSRR is typically frequency dependent. Measurements are made as a function of the ripple or noise frequency on the supply, because different circuits couple differently to the supply at different frequencies. Low-frequency PSRR is generally better and degrades as frequency increases. The exact values depend on the device and operating conditions.

Applications and relevance: PSRR is a key spec for linear regulators and reference-sensitive circuitry, such as operational amplifiers, analog-to-digital converters, and digital-to-analog converters. For regulators, PSRR describes how well the output remains stable when the input line fluctuates. For op-amps and other analog ICs, PSRR indicates how much rail noise can appear at the output.

Improving PSRR can involve better decoupling and layout, such as placing capacitors close to the device, adding ferrite beads, and using separate analog/digital rails. Choosing devices designed with higher PSRR, or adding an additional regulation stage (for example a low-dropout regulator) can also help. Careful design ensures that supply variations do not unduly affect critical analog performance.