Short Note: Anti-aliasing Filter

An anti-aliasing filter is a low-pass filter placed before the ADC to remove signal components above half the sampling frequency ($f_s/2$), preventing aliasing.

• Cut-off frequency is set at or just below $f_s/2$.
• RC filter (first order): $f_c = 1/(2\pi RC)$, 20 dB/decade roll-off.
• Higher-order Butterworth or Bessel active filters give sharper roll-off. Oversampling with a simple filter is an alternative.
• Aliasing cannot be corrected after sampling, so the filter is essential.

Exam tip

Draw the block diagram and give the RC cut-off formula.