Here is a follow up on this old thread. I got a temporary solution to work so far and produced some intrinsic noise spectra of 3458. The following is what I did. I switched to the Matlab platform to interface with the 3458 to use a traditional text based programming. In principle, any other programming language should work just as well.
Assuming that the communication is established in the "dmm" object, then the integration time of each sample can be set by the "fprintf(dmm,sprintf('aper %g',tint))", where "tint" is the user defined integration time.
The sampling interval "dt", which should be slightly longer than the integration time, was set by "fprintf(dmm,sprintf('sweep %g %d',dt,num))", where "num" is the number of samples to be measured. I have tried "num"=4855 without a problem.
The data in the 3458 buffer can be retrieved by "[data] = fread(dmm,num,'long')".
In the post analysis, it is the integration time "tint" that should be assigned to the time between each samples regardless of what "dt" is used.
Attached is the intrinsic voltage noises of 3458 at 100 mV, 1 V, and 10 V ranges. With the 17 nV/√Hz best noise figure, it appears that 3458 is a decent DAQ device as well.
Thanks all for the input.