Hi DanCee,
Yes this is correct.
If sampling on a single channel of the 6259 you may acquire up to 1.25 MHz. For multi-channel acquisitions, the specified aggregate rate is only 1 MHz. The reason for the difference is that the switching speed (or settling) of the multiplexer is the limiting factor when you have more than one channel. When you are acquiring from a single channel, the MUX doesn't have to switch so the limiting factor becomes the conversion rate (or settling) of the ADC.
It is true that for N channels the per-channel maximum sample rate is 1 MHz / N. This is true for single-ended or differential channels (i.e. a differential channel still counts as one channel--a separate MUX is used for the + and - terminal of the measurement so no additional switching time is needed for a differential input).
It might be useful to point out that you can probably sample a bit faster than these specified rates if you are feeling adventurous; however, settling time specs wouldn't apply and if you go too fast you would eventually receive a hardware error (e.g. ADC overrun).
NI does sell simultaneously sampled DAQ devices which are specified with a per channel rate (e.g. the 6358 can go up to 1.25 MHz/ch). The multiplexing considerations do not aplpy here of course (e.g. 1.25 MHz/ch is true regardless of the number of channels in your task).
Best Regards,
John Passiak
