I have noticed that there are two main types of DSA boards in the 2001 catalog -- the 4451/4452/4454 "Dynamic Signal Acquisition" devices and the 4551/4552 "Dynamic Signal Analysis" devices. What are the practical differences between these different boards, and when should I choose one over the other?
The quick answer to this question is that the 455x "Dynamic Signal Analysis" boards include a DSP (Digital Signal Processing) chip for performing spectral analysis in hardware. The 445x "Dynamic Signal Acquisition" devices have identical analog hardware, but do not have the DSP chip. That means that any data processing you would like to perform must be done in software on the host computer. This is typically referred to as "host-based processing."
When you program the 445x boards in LabVIEW, LabWINDOWS/CVI, Visual Basic, or any other environment, you make calls to the NI-DAQ hardware driver. This is the same driver used to program National Instruments E Series boards and other general-purpose DAQ products. When you make a call to the DAQ driver to read in data, what you get back is an array of analog voltages in the time-domain. It is up to you to include any additional data processing in your software. Examples of such processing include Fast Fourier Transforms (FFT), Octave Analysis, and smoothing windows.
The 455x boards can do these sorts of the operations in hardware. The DSP processor on these boards does not have the overhead associated with running the Windows operating system -- this chip's only job is to perform DSP operations. This makes it faster and more deterministic than performing the same operations on the host computer. The 455x devices are usually not programmed with NI-DAQ. Instead, they have their own dedicated instrument driver called NI-DSA. If you do elect to program them with NI-DAQ function calls, you will bypass the DSP chip and the boards will function just like the 445x Dynamic Signal Acquisition devices described above.
When should I use one type of the board over the other? This is not always a simple question. In general, 445x boards are sufficient where the data does not be need to analyzed as it is being acquired. If these analysis requirements are fairly modest, 445x may also be acceptable for on-line analysis. More demanding applications requiring deterministic on-line data analysis generally call for a 455x board. The following Developer Zone article contains some very good information on devices: