On 13 Sep 2000, at 20:49, Sean H. Breheny wrote:

> A side question: No one has ever been able to explain to me why the third
> terminal (ground) is necessary. I even asked a bioengineering prof here and
> he at first made an argument about the inputs being ground referenced, but
> that doesn't hold up in my understanding of it because the inst. amp
> measures the difference between the two main inputs, regardless of where
> ground is. When I said this he reverted to "well, it might not be strictly
> necessary, but it is required by law, supposedly for safety although it
> actually makes things slightly more dangerous".
>
My understanding was that  the reference terminal is to help keep
the source within 'coo-ee' (read 'ballpark' for the U.S.) of the
reference used by the inst-amp psu.  i.e.. with a high input
impedence and a nice insulated patient, things might staticly (or by
other mechanisms) float a great many volts apart. Sure input
protection will save the amp but measurement might be difficult.



_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

_____________________________

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