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Linear Stimulus Isolator

Rob Douglas rdouglas at mckellar.com
Fri Jun 9 21:16:25 EST 1995

In article <199506091334.JAA25149 at vp3.med.harvard.edu> Shai Gozani,
gozani at VP3.MED.HARVARD.EDU writes:
>Our lab is interested in obtaining a linear stimulus isolator
>(i.e. isolator reproduces input waveform which, for example,
>can be generated by computer D/A).  I am only aware of the
>A395 from World Precision Instruments (WPI).  Does anybody
>have experience with this particular instrument or similar
>instruments from other vendors.
You don't state what you want to do, so I'll assume electrical
stimulation of nerves, tracts, heaps of cells etc.  The three siolators
that I know about are:

A-M Systems 2200
Axon Instruments: Isolator 10
Neurolog NL800 (?) (in the US available from Medical Systems)

I use the Neurologs in my own lab, as do quite a few others who use my
software.  They require programming in an offset voltage, but are small
and the batteries last for years.  There are also monopolar, so if you
need diphasic pulses you have to use two of them.  Two labs here at UBC
bought the AM 2200 when it first came out but they had problems with
battery life in long recording sessions.  One other lab I dela with has
the Axon isolator and they haven't complained about it to me yet.

While being able to control stimulus parameters from the computer is
great, there are potential problems. Two that have caused me grief are:

(1) Making lesions is too easy. On some boards I have used (eg the
NB-MIO-16) the DAC's default to -10V when the computer is turned on.  I
sometimes run a little program at startup that zeros my DAC's, but that
is still way too late.  Even with other boards just about any software or
hardware glitch leaves a steady voltage on the output, and hence DC
current going through your prep.  I have toggle switches in my lab to
allow the outputs to be disabled quickly, and that helps some (but can
cause other problems).

(2) Noise.  I bought a linear isolator from Bak 15 years ago and it
worked great except for the noise it injected into my recording system
and I could not record any units when it was on.  If the circuit is
active all the time the S/N ratio of the isolator AND the DAC's are
critical.  I like the Neurologs because they are OFF between pulses and
only turn on when the voltage exceeds ~.6V.  I do not know how Axon or AM
Systems deal with this issue, but I presume they do otherwise I would
have two fewer friends.

I don't want to scare you off though.  There are pitfalls of doing
qualitative work without such isolators too. Good luck.

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