Difference between -3dB and alias-free bandwidth

What is the difference between the -3dB bandwidth and the alias-free bandwidth?

Why is such a relatively small portion of the 200kS/s bandwidth usable (44 kHz vs. the Nyquist limit of 100kS/s)? Am I missing something?

Hi,

-3 dB bandwidth is the frequency where the signal is attenuated by factor of sqrt(2). Alias free bandwidth is the frequency where the sine waves with frequencies above Nyquist is attenuated to the noise floor, so you can be sure that up to alias-free frequency limit all the signals are real.

There are three regions that ADC has: up to 50 kHz, up to 100 kHz and up to full speed. The sharpness of the filter is different in these three regions because at higher speed ADC can use less coefficients for calculations, so at 200 kHz the alias-free frequency is lower in ratio to the sampling rate than for example at 50 kHz.

The choice of ADC is always a compromise between different properties. The group delay is one important criteria for signal conditioning applications. Low group delay brings these tradeoff, especially at higher sample rates.

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