Re: multiplexers

Alison Teasdale (ateasdal@interchange.ubc.ca) Wed, 29 Nov 2000 21:45:45 -0800 

From: "Alison Teasdale" <ateasdal@interchange.ubc.ca>
Subject: Re: multiplexers
Date: Wed, 29 Nov 2000 21:45:45 -0800




Multiplexers will only read one channel (or signal) at a time, but will
switch through all channels of the multiplexer very quickly so that it
'appears' that all channels are read instantaneously.  The number of
channels that a multiplexer has depends on the application you are wanting
to use it for and the price you want to pay.  Companies like Keithley and
National Instruments make multiplexers with anywhere from 4 to 100+
channels and some of their products are expandable to hook in more
channels.  Companies like Boeing require 1000s of channels to record strain
gage inputs for their stress testing of their structures.  

The problem with multiplexers is the switching time between channels
reduces the overall sample rate, so you don't want too many excess channels
that aren't being used if you need a fast sample rate.  This means that you
can't just divide the rated maximum sampling speed (usually, although not
explicitly stated, the rated sampling speed is for one channel only) by the
number of channels because the time to switch between channels is usually
appreciable for sampling rates of kilohertz or greater.  

I know that with National Instruments equipment the multiplexer is separate
from the A/D converter.  The multiplexer switches between the analog
signals and sends the analog signal to the A/D converter.  


----------
> From: Keri Orbeck <electricbluejaye@hotmail.com>
> To: eece485@fs3.ece.ubc.ca
> Subject: multiplexers
> Date: Wednesday, November 29, 2000 3:59 PM
> 
> Is there a limit to how many signals a multiplexer can monitor at one
time?  
> Does the multiplexer also convert multiple digital signals to analog or
is 
> there a similar device that does this?
>
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