AW: To earth, or not to earth...

[Magnus Danielson]

>>>>> "TC" == Tim Cockram <tim at redragon.demon.co.uk> writes:

 >> The earth "dustbin approach is also quite usefull (use an op-amp
 >> unity gain buffer with the non-inverting input connected to ground,
 >> the output is a goood solid ground (limited by the outputs
 >> compliance).  

 >> (I've never heard it called a "dustbin" before!)

 >> It also suffers from other opamp problems; noise, rising output
 >> impedance with frequency, offset voltage, etc.  'Sounds like it makes
 >> any grounding problem several times worse!

 TC>    Good points Don.  It does work in practice (in several high broadcast mixers (high end) and I belive on the chroma voice 
 TC> boards.  If you use the op-amps output for a reference ground (that scheme is used in some Harrison desks) you could run in 
 TC> to the problems  you have described but if you just connect stuff like leds or even logic to the output you transfer the 
 TC> garbage (hence dustbin) to the supply rails not the real ground.

This would then move the problem away somewhat from earthing problems
to power-rail problems. If a cursuit has a bad PSRR (some has a really
bad PSRR, few boxes really deal with it) then it would stick in there
instead. PSRR related problems should however be less problematic than
a moving ground reference.

To avoid having too much noise on the power-rails should the op-amp
have a HF decoupling to the mains, this would however certainly change
the output impedance of the op-amp in the high frequency range.

In my experience is too strong or too weak coupling between signal earth
(really power-rail earth) and protection/chassi earth both problematic.
Too strong coupling help the ground-loops.
Too weak create the basis for a lot of electro-mechanical spurious
effects (like when we had a FIR filter selfoscillate!).

Most main transformers will create a AC difference between primary and
secondary ground. This comes from the capacitive coupling in the
transformer and there should be a double sheild between the primary
and secondary side to decouple them. The two sides should be connected
earth-wise with a resistor at say 100k for a normal signal processor box.

Cheers,
Magnus