Another Log on the Fire
Howard Shapiro (HPANDA@HARVARDA.HARVARD.EDU)
Wed, 15 Dec 93 18:12:36 EST
Following up on the log amp discussion a few weeks ago, I now report
that we have successfully tested new Cytomutt electronics incorporating
peak detector modules from OEI (Tucson, AZ; Model AH504), which provide
accurate detection of peaks to below 3 mv, 3 mv being the noise level in
our electronics. When signals are digitized using an Analogic HSDAS-16
200KHz 16-bit data acquisition system, and converted to a 4-decade, 256-
channel log scale, there are no gaps in the data down to channel 34,
which is about in the middle of the noise. Spherotech particles with
250 MESF come out about channel 50. The log transformation would take
about 6 microseconds on a 33 MHz 486; it's faster to use a log table,
with the lookup requiring about 300 nanoseconds. Since the noise level
is around 3 mv and the biggest signal the board takes is 10 volts, the
actual range of signals is 3 1/2 decades, which means a 4-decade scale
is useful primarily for cosmetic purposes. From measurements made on
some commercial systems, I'd guess it wasn't easy to do significantly
better than this in the noise department. It's absolutely wonderful
not to have to deal with real-world log amps; once you go digital,
you'll never go back. -- Happy Holidays -- Howard Shapiro
CD ROM Vol 2 was produced by staff at the Purdue University Cytometry Laboratories
and distributed free of charge as an educational service to the cytometry community.
If you have any comments please direct them to Dr. J. Paul Robinson, Professor & Director, PUCL,
Purdue University, West Lafayette, IN 47907. Phone:(317) 494-0757; FAX (317) 494-0517; Web http://www.cyto.purdue.edu
EMAIL robinson@flowcyt.cyto.purdue.edu
