WG: A shockingly basic question

Haible Juergen Juergen.Haible at nbgm.siemens.de
Fri Jun 27 13:03:57 CEST 1997

> ----------
> Von: 	Haible_Juergen#Tel2743
> Gesendet: 	Freitag, 27. Juni 1997 13:03
> An: 	Martin Czech
> Betreff: 	AW: A shockingly basic question
> >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. 
> Right. Four resistors. You still can use 2 pairs of different values
> to get
> gain larger than one.
> >The problem with this simple circuit is that common mode
> >supression is not very good, you need very high precision resistors.
> No problem to select a few with a multimeter.
> Real problem is that this simple circuit has different impedance for
> both inputs, and common mode suppression will decrease at
> high frequencies. But It should still suppress 50 or 60Hz hum
> remarkably, if the driving impedance is is low.
> >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.
> Really ? I think it still works with an unsymmetrical sender, as long
> as
> you don't have a direct connection of the two GND's, other than in
> the PSU star ground. You replace the second (ground-loop-forming)
> GND connection between sender and receiver with this differential amp.
> Sure, for higher frequencies you'd also want balanced signals with the
> screen of the cable at gnd - but I think even this simple circuit
> connected
> to an assymetrical sender will give improovement in the low frequency
> range.
> JH.

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