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Since the transfer function of a good log amp fluctuates by at least
4 times the resolution of a 10 bit ADC, is there any reason to measure
log fluorescence at 1024 channel resolution?
My definition of the transfer function for log amps is taken from
the poster abstract and handout from the ISAC meeting in Sept. 1988
at Breckenridge Colorado by Dave Parks, Marty Bigos and Wayne Moore.
The abstract describes the transfer function as the "actual input-output
relationship" of a log amp; i.e., voltage pulse from detector/preamp is
input and subsequent output voltage pulse from the log amp.
The transfer function is evaluated by measuring the ratio of two beads with
different fluorescence intensities across the "usable" range of the log amp
by
adjusting the photomultiplier voltage. According to their data and
calibrations I've made, the ratio (mean or median) channel number can
fluctuate by AT LEAST 2 channels (plus or minus) from the average on
an 8 bit ADC (256 channel). This corresponds to plus or minus 62 mV
for an 8 volt full scale system or 78 mV on a 10 volt full scale system.
My data was collected on a FACS IV and looks identical to the data in the
handout from D. Parks et. al. which was collected on FACStar.
For a 10 bit ADC (1024 channels) the increment for each channel is
7.8 or 9.8 mV for an 8 volt or 10 volt system ,respectively. If the output
voltage pulse from a log amp can vary by AT LEAST 60 mV across the
"usable range" why use 1024 channel resolution?
Am I missing something here? Is my understanding and reasoning completely
out of whack? What kills me is data files collected with 1024 resolution are
twice as large as 256, then we "smooth" histograms to make them look
better. What are we smoothing? The fluctuation in the log amp's transfer
function?
Any comments would be appreciated, esp. from Dave Parks or Marty Bigos.
thanks tom d.
-- ============================================================================== Tom Delohery | Internet: tdeloher@bigmac.mskcc.org Manager, Flow Cytometry Core Facility | Phone: (212) 639-8729 Memorial Sloan-Kettering Cancer Center | Fax: (212) 794-4019 ==============================================================================
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