Effect of analog and digital signal conditioning filters on signal to noise ratio

Digital processing of analog signals involves sampling and analog-to-digital (A/D) conversion. A prefilter may be inserted in analog implementation prior to sampling, or in digital form after A/D conversion. The effects on signal-to-noise (S/N) ratio of these alternative forms of prefiltering are studied. Significant differences in S/N ratio may result, depending upon the relative spectral characteristics of the input signals and noise. The impulse-invariant technique and other methods for the design of digital filters seem to imply the equivalence of the two signal conditioning subsystems analyzed. However, the subsystem with analog prefilter, which is usually recommended on the basis of preventing aliasing of the signal x(t), is shown to be preferable on the basis of output signal-to-noise ratio