Re: [CAD_CAM_EDM_DRO] Re: Proper size of capacitors for transient suppression?

Andy Wander wrote:

>Very interesting, and I will have to digest it.
>
>I still don't see how a noise that gets induced on a ground wire will "stay"
>on that wire at the input to the next electronics stage.
>

It won't, you are quite right. But, if capacitive coupling is the
culprit, then
splitting the coupled field into many wires reduces the magnitude of the
interference on any one wire. It is a poor substitute for coax or
shielded cable,
but many wires is better than one ground.

> It seems to me that
>whatever noise is induced on the ground should practically disappear(as far
>as a voltage) as far as the input is concerned. The impedance(to ground) of
>the grounded wire will almost certainly be different(probably much lower)
>from that of the "signal" wire, and so the noise voltage that is seen by the
>input will be different on the ground wire and on the signal wire.
>Therefore, a "differential" input(which you correctly point out covers an
>unbalanced as well as a balanced input) will "see" some of the noise
>voltage. I do see how this _might_ reduce the noise voltage that gets
>through the chain, depending on what the impedances are on both wires.
>
>

If a signal recieving chip is comparing the signal line against a
reference level
sense wire (a ground at the encoder or source end, but not grounded at the
receive end) it will be essentially the same as a differential input. any
interference coupled into the cable will equally affect both wires, anc be
cancelled out by subtraction at the receive end.

If the coupling is magnetic, the impedance at the end doesn't matter.
The same
current will be developed in each wire. If you use a differential
reciever of
some sort, you don't want to ground the reference input wire. So, you are
right, the impedance at the receive end needs to be the same for reference
and signal. The reference wire can be connected to the encoder's ground
voltage at the source end.

Jon