In the majority EEG amplifiers, ALL channels must be sampled at the same moment in time. Otherwise the results make no sense. Phase relationships in EEG are very important.
One clarification. The Pocket-Neurobics Q-Wiz amp is what you would consider a counter example. It is a 4 channel amp, but has a DB-25 connector for attachment of an Electro-cap with 19 or so channels. Pete Van Deusen's assessment procedure records 4 channels at a time, sequentially, until the entire set of 10-20 sites is recorded. The Q-Wiz assists in this by stepping through the DB-25 pins when prompted by the recorder.
This approach is a compromise, because Pete's assessment is only interested in relative band amplitudes at the 10-20 sites. The phase information is lost, and even the amplitude information is not as accurate as what would be attained with simultaneous sampling. But it does cut costs and allows a 4 channel amp to gather sequentially a full head 10-20 map. Brainmaster actually pioneered this approach with their Atlantis 4 channel amplifier and an external switch box called the MiniQ-II. But the cost of the Atlantis plus MiniQ is way up there.
All amplifiers used in BCI, LORETA QEEG, etc., require simultaneous sampling on all channels. This is the norm in the EEG community. Q-Wiz and Mini-Q are exceptions that sacrifice accuracy.
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