Good catch guys. Yes I can confirm that 1.25 Gsa/s is actually possible on channel 4
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And I've been caught by this trigger channel thing more than once already! It's actually caused not only by the trigger setting, but also if any of the other channels are used in measurements, counter, and, I think, math as well. In other words, other channels, even if they are turned off, must not be referenced by anything to allow for full sampling rate to be active. This can easily be considered a bug, or at least a serious UX issue: when a channel is off and its respective button is not lit on the front panel, then it should not matter whether anything else references it. If it's off, it can't be used by anything anyway, and it should be automatically disabled for everything that may be concerned.