Before analog-to-digital conversion, how are unwanted (out-of-band) frequencies prevented from corrupting the sampled data? Choose the technique typically applied at the analog front end.

Introduction / Context:
Any analog-to-digital converter (ADC) must obey sampling theory. Frequencies above half the sampling rate (the Nyquist frequency) will fold back into the passband, creating aliasing. The standard way to avoid this is to limit the analog signal's bandwidth before sampling.

Given Data / Assumptions:

• We are preparing an analog signal for digitization.
• ADC has a fixed sampling frequency fs.
• We want to minimize aliasing artifacts.

Concept / Approach:

An anti-aliasing pre-filter (typically a low-pass with cutoff below fs/2) attenuates unwanted high-frequency components so they are not sampled and folded into baseband. The sample-and-hold (S/H) helps the ADC by keeping the voltage constant during conversion but does not remove out-of-band content. Digital signal processing comes after sampling; if aliasing has already occurred, DSP cannot generally recover the original spectrum.

Step-by-Step Solution:

Verification / Alternative check:

Plot the spectrum before and after the analog filter; energy above Nyquist should be greatly reduced, preventing foldover in the sampled spectrum.

Why Other Options Are Wrong:

DSP (post-conversion) cannot undo aliasing. Sample-and-hold reduces droop and aperture errors but does not filter frequency content. “All of the above” is therefore incorrect.

Common Pitfalls:

Choosing too-low filter order; ignoring transition-band requirements; forgetting real-world filter tolerances and ADC input bandwidth.

Final Answer:

Pre-filters