A shockingly basic question

Martin Czech martin.czech at itt-sc.de
Fri Jun 27 11:31:35 CEST 1997


SNIP
> > 
> Ok, here's the question:  Assuming you're just doing something simple
> (like buffering the signal), could you hook it up as follows:
> 
>           +--R3--+
>           | |\   |
> TIP: --R1-|-|-\  |
>             |  \_|___ Out
>             |  /
> RING:--R2---|+/
>             |/
> R1=R3
> R2=(R1R2)/(R1+R2)
> 
> This would (I think) accurately buffer the signal (inverted, of course), but
> without a ground connection between the two devices.
> Am I right?  Is this a pointless observation?  Thanks for any responses.
> 

Ok, there are no stupid questions, just stupid answers, though I hope mine
will be not so dumb.

You have just re-invented symmetrical inputs. From my head, the above circuit won't work
'cause you ommited r4 betwenn + und gnd. 
I may be wrong, but if r1=r2=r3=r4 you'll have a simple differential amp:
Vout=(Vring-Vtip)*1.0. The problem with this simple circuit is that common mode
supression is not very good, you need very high precision resistors.
There are more clever circuits for this purpose (SSM2017 e.g.).

However, ground loops will only be prevented, if the signal source is also
symmetrical, that is inverted and non inverted, so both lines (tip and ring)
have nothing to do with gnd.

Yesterday, some old magazine article came across my way, with a hifi symm->unsymm
and unsymm->symm converter, pure silicon solution, no expensive transformers.
It is a bit expensive.

If somebody is interested, I could dig it up again.

m.c.





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