PSRRdB

PSRRdB, short for Power Supply Rejection Ratio in decibels, is a parameter used to quantify how well a circuit suppresses variations in its supply voltage. It describes the relationship between changes in the supply (ΔVin) and the resulting changes at the output (ΔVout), with higher values indicating better rejection of supply ripple.

The standard definition for PSRRdB is a voltage ratio expressed in decibels, typically written as PSRRdB = 20 log10 (ΔVin / ΔVout). Because both signals are voltages, the 20 log10 form is used. In practice, PSRRdB is frequency dependent; measurements are often taken across a range of ripple frequencies to show how rejection changes with frequency.

PSRRdB values vary by device and technology. Linear regulators and low-dropout regulators (LDOs) commonly exhibit strong PSRR at low frequencies, often in the 60–100 dB range, with some reaching higher values. At higher frequencies, PSRR generally degrades due to finite loop gain, internal filtering, and parasitics. Operational amplifiers, comparators, and data converters also specify PSRRdB, typically reflecting how well their analog stages reject supply-induced disturbances under specified conditions.

Measurement typically involves injecting a known, small-amplitude ripple into the supply while keeping load and temperature controlled, then measuring the induced output ripple to compute PSRRdB across frequencies. Practitioners use PSRRdB to compare regulators and circuits for applications sensitive to supply noise, such as precision analog, audio, and high-resolution data conversion. Design techniques to improve PSRRdB include better decoupling, regulator selection with higher intrinsic rejection, and inputs filtering.