Re: AGND+DGND+EARTH=NOISE....true or false?

[robin szemeti <robin-at-rszemeti.demon.co.uk>]

On Tuesday 10 September 2002 05:40, Babar wrote:
> Hi All,
> In our Hurco Milling  Servo drives are such that
> digital and analog grounds are internally shorted i.e
> DAC output of STG,tacho of motor ,limit switches and
> encoder have common grounds which are then tied up to
> Machine's body which is connected to earth, We are
> facing problems of Noise I was wondering if this is
> the reason ??

umm .. more of an electrical design than CNC software question I guess .. but 
here goes.

more than likely ... they will all be connected together somewhere .. the 
usual method is to connect all your computer bits to the machine via 
opto-isolators .. the 'ground' on the optos is never connectoed to the rest 
of the machine ... the computer bits 'float' with respect to the machinery.

the analogue and digital grounds within the machine should usually be 'star' 
connected and seperate.  You would normally have seperate analogue and 
digital grounds *which may or may not* be connected together in some place 
... but so long as you take all your digital ground connections form *a 
single point* (eg the output tab of the power supply) then it really doesnt 
matter.  In effect there can't be any noise on it because it *is* the 
reference point .. noise compared to what?  same applies to the analogue 
grounds run the high current ones back to a single point on the PSU output .. 
and keep high current motor wiring clear of signal wiring and you should be OK

-- 
Robin Szemeti