DI-194 (was Re: Data Acquisition newbie question)

[Phil Hunter]

The way I read the DI-194 specs, max samples per
second is 240 divided by number of channels enabled,
so if you are using 3 of the 4 channels, each will
be chugging along at only 80S/s.

Nyquist's theorem says you'll need at least 2 samples
for each cycle of the waveform you are trying to
measure, so that means w/ only 1 channel enabled to
get the max of 240S/s, the max frequency you'll be
able to measure is 120Hz, which is 7200 RPM. Enable a
second channel and your RPM tops out at 3600. Three
workarounds I can see, a) Brian's suggestion of the
LM2907/2917 Freq-Volt converters rather than trying to
measure frequency directly, b) divide the frequency
down such as using a cam vs. crank sensor or some kind
of electronics, c) you can "sum" 2 (or more) signals
together into 1 input channel using something like a
resistor summing node or an op-amp arrangement. Max
input voltage is 10V, so you'll probably need a
resistor divider anyway.

I ran the introduction demo, was very impressed w/ the
s/w, unfortunately it only runs on Win95, Win98 & NT.
There is a calibration you run, which I assume would
have to be stored in the PC rather than the unit, so
this would seem to limit you to only 1 unit per PC, I
don't think you can get 8 or more channels using more
serial ports and multiple units, unless you get tricky
w/ something like VMware or Plex86 s/w to run multiple
instances of Windows simultaneously.

All in all, even playing w/ it just a little, I'd say
for the price this is something for every DIYer's
tool kit. http://www.dataq.com

regards,
phil

"That's my theorem, an' I'm stickin' to it..."


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