A battery powered application can see a change in power supply voltage of several hundreds of milivolts over the life of the product. Moreover, op-amp is also often supplied from noisy lines and must therefore reject the noise adequately. For these reasons, it is important to know how a change or noise on the power supply manifests itself at the output of an op-amp.
It quantifies the amplifier’s sensitivity to power supply changes. Ideally, the power supply rejection ratio should be infinite, which means power supply changes do not affect the output . Typical specifications for a power supply rejection ratio of an amplifier range from 60dB to 100dB.
The picture below shows the MCP601 power supply rejection specification.