Power supply rejection ratio

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is... 

Fig. 6.1.10 shows a variation of the current sense approach where the amplifier is connected differentially to the sense capacitors. Compared to the single-ended version in Fig. 6.1.8 b, the differential circuit has improved power supply rejection ratio (PSRR) and ability to reject errors such as substrate noise or coupling from other signals. This is because any error that couples into both inputs of the amplifier equally will be rejected by the amplifier s common-mode rejection ratio (CMRR). 

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