Application is time correlated single photon counting to measure nanosecond fluorescent lifetimes. A laser pulse generates a start pulse. The first photon emitted from the sample generates a stop pulse. The time difference is converted to a voltage pulse (0 to +10V), 2 usec duration (I could extend that with an external sample/hold) by a time-to-amplitude converter. The overall lifetime of the sample is constructed by repeating this at least tens of thousands of times. For each laser shot, the voltage pulse is digitized and its binary code summed into a histogram bin. A plot is made of number of occurances vs adc binary code value. At least 12 bits of resolution is desired. The pulses to be digitized occur randomly in time at least 10 usec apart (maybe 100 usec).
For accurate data, the width of each histogram bin should be equal. I test this by making a histogram from a triangle wave. The number of occurances vs raw adc counts should be flat. Instead, it exhibits variations of 20% (equivalent to 0.2 lsb of dnl). So what I'd like is an adc with bin widths equal to say 1% that can digitize a voltage within 10 usec. Its type should not really matter as long as it can accomplish this. A 20% deviation in adc bin width seems like a high value for something electronic in nature ike this. What is it that makes this not typically so, since most adc's has 0.2 lsb DNL specs ? On Mon, 5 Apr 2004 21:43:39 -0500 (CDT), LocalDSP <[EMAIL PROTECTED]> wrote: >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.
