# Sampling frequency of ADC and maximum frequency of input signal

As pair the datasheet of LTC2207, maximum input frequency can be 700MHz. whereas the sampling frequency is 105MSPS. Two times 700MHz is way less than 105MSPS. So how is this not contradicting with Nyquist theorem? ,

• A signal extending from 500 MHz to 501 MHz has a bandwidth of 1 MHz and a Nyquist rate of 2 MHz. However, it still requires the ADC input frequency to extend to 501 MHz even if sampled at 2 MHz. Apr 17 at 12:19

it simply doesn't, because "input frequency up to" has no bearing on bandwidth.

If you know you have signal only between, say, 700 to 740 MHz, you can perfectly well sample it with a 80 MS/s ADC and fully reconstruct it from the sampled signal; no information is lost.

The keyword you might be looking for is subsampling.

This is an especially useful property in superheterodyne receivers, where a signal might not be in baseband, but in a passband, but everything else (up to 700 MHz) is sufficiently suppressed by filtering.

This is called "undersampling" or "super-Nyquist sampling".

If you worry about the reconstruction of the signal, the aliasing (which is a result of the undersampling and basically the production of the frequency difference between the sampling and the signal-to-be-sampled) can still be used to reconstruct the signal.

As Marcus pointed out in his answer, Nyquist theorem states that the sampling frequency should be at least twice the maximum input frequency, but there's also a modified version, Nyquist-Shannon, stating that the sampling frequency should be at least twice the bandwidth.

If you research the terms above you can find a lot of details.

An ADC is built from an analog component, the sampling-gate, followed by a analog-to-digital converter (1). The maximum frequency of the input signal is limited by the time duration for which the sampling-gate is turned on, the maximum bandwidth of the signal (for Shannon reconstruction) is limited by how long a conversion takes in the analog-to-digital converter.

(1) For a traditional ADC design the datasheet may call the sampling-gate a track-and-hold or a sample-and-hold. A modern design may replace the sampling-gate with a bank of narrow-band filters, RF mixers, and converters, followed by a digital reconstruction filter.