power supply rejection ratio (PSRR)
Generally, the supply voltage of active amplifier circuits is superimposed by interference voltages. These unwanted signals can be noise or hum signals from the mains voltage, as well as interference generated in the switching power supplies.
These interferences are coupled into the active circuits via the supply voltage and capacitively. They have a direct influence on the signals to be amplified and affect, among other things, the signal-to-noise ratio. For this reason, the interference voltage component should be as low as possible when generating the supply voltage.
The ratio of the supply voltage change to the resulting change in the useful voltage is the power supply rejection ratio (PSRR). In other words, it is the voltage change in the supply voltage that causes the same amplitude changeas a change in the useful signal at the input of an active circuit. For example, if a supply voltage change of 1 V corresponds to a wanted signal change of 1 mV, then the PSRR, expressed in decibels, is `10^3` or 60 dB.
At low frequencies such as ripple voltage, the PSRR is higher and can reach values of 120 dB and more; at higher frequencies of a few megahertz( MHz), this value can drop to 40 dB and below.