To help converging toward your actual requirements, could you be a little more specific on what exactly you need?
Remember that the actual DNL accuracy specified in "% of lsb" depends on the actual resolution (lsb) of your converter, so 40% for an 18 bit converter is as accurate as 10% for a 16 bit converter. In other words, "percent of full-scale" would be a more useful specification. Also, Sigma-Delta converters tend to have significantly better DNL specifications than "traditional" sample rate type converters, but they may not track ramps or triangular wave as well. Would that be a problem for you? Do you need a true integrating ADC? Have you looked at the NI-5911? http://www.ni.com/pdf/products/us/4mi444-447.pdf This PCI board is a multi-bit SD-ADC that will give you more than 19 bit INL at 100 kHz rate and much better DNL. ... but it's a sigma-delta type.
