In electronic systems, power supply rejection ratio, also supply-voltage rejection ratio, is a term widely used to describe the capability of an electronic circuit to suppress any power supply variations to its output signal. In the specifications of operational amplifiers, the PSRR is defined as the ratio of the change in supply voltage to the equivalent output voltage it produces, often expressed in decibels. An idealop-amp would have infinite PSRR. The output voltage will depend on the feedback circuit, as is the case of regular input offset voltages. But testing is not confined to DC ; often an operational amplifier will also have its PSRR given at various frequencies. Unwanted oscillation, including motorboating, can occur when an amplifying stage is too sensitive to signals fed via the power supply from a later power amplifier stage. Some manufacturers specify PSRR in terms of the offset voltage it causes at the amplifiers inputs; others specify it in terms of the output; there is no industry standard for this issue. The following formula assumes it is specified in terms of output: where is the voltage gain. For example: an amplifier with a PSRR of 100 dB in a circuit to give 40 dB closed-loopgain would allow about 1 millivolt of power supply ripple to be superimposed on the output for every 1 volt of ripple in the supply. This is because And since that's 60 dB of rejection, the sign is negative so: Note:
The PSRR doesn't necessarily have the same poles as A, the open-loop gain of the op-amp, but generally tends to also worsen with increasing frequency.
For amplifiers with both positive and negativepower supplies, the PSRR for each supply voltage may be separately specified, but normally the PSRR is tested with opposite polarity signals applied to both supply rails at the same time.
For voltage regulators the PSRR is occasionally quoted, but often the concept is transferred to other terms relating changes in output voltage to input: Ripple rejection for low frequencies, line transient response for high frequencies, and line regulation for DC.
