separate grounds

[Jamie Morken]

Hi,

I have two circuits on one PCB, isolated magnetically and optically from
each other, with two grounds, an analog and a digital, right now they
are completely separate, is it ok to leave them separate or should I
connect them with a 1Mohm resistor or even tie the two ground planes
together at one point to keep them both at a similar voltage? I don't
see any reason to tie them together except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.

cheers,
Jamie

 

[Paul Hovnanian P.E.]

Hi,

I have two circuits on one PCB, isolated magnetically and optically from
each other, with two grounds, an analog and a digital, right now they
are completely separate, is it ok to leave them separate or should I
connect them with a 1Mohm resistor or even tie the two ground planes
together at one point to keep them both at a similar voltage? I don't
see any reason to tie them together except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.

Tie them together unless this creates a undesirable ground loop in
combination with the external circuitry. Not knowing what the design
decisions were that led to the initial separation, its difficult for us
to guess what will happen if they are tied together.

The 1 Meg resistor may not do you any good depending on the impedance
behind the voltage differential between each circuit.

What sorts of max ground voltage differences and energies are we talking
about here? There are optoisolaters rated at many KV.

 

[Spehro Pefhany]

Hi,

I have two circuits on one PCB, isolated magnetically and optically from
each other, with two grounds, an analog and a digital, right now they
are completely separate, is it ok to leave them separate or should I
connect them with a 1Mohm resistor or even tie the two ground planes
together at one point to keep them both at a similar voltage? I don't
see any reason to tie them together except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.

cheers,
Jamie

Why are they isolated from each other in the first place? What else is
connected to the boards? What safety concerns are there?

 

[legg]

Hi,

I have two circuits on one PCB, isolated magnetically and optically from
each other, with two grounds, an analog and a digital, right now they
are completely separate, is it ok to leave them separate or should I
connect them with a 1Mohm resistor or even tie the two ground planes
together at one point to keep them both at a similar voltage? I don't
see any reason to tie them together except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.

If the analog is separated from the digital by the isolation specified
then there is no requirement to join them.

However, if the magnetic isolator carries high dv/dt on it's windings,
it may produce a high frequency differential between the two sections
that can try to pump current through capacitances of the optocoupler,
or through external tracking and the common safety earth plane.

Allowing for a capacitive return between the two grounds locally can
provide a return path, if the optocoupler is affecred by this
differential. Laying out a location for a couple of hundred pF of
isolation-rated capacitance is a prudent measure in this case.

RL

 

[IronTom]

If the circuits come together at an AD or DA converter, that is where
you would like to place your single point ground, as part of the trace
underneath the converter. But since you are using optoisolators, it
would probably be ok to leave them completely separate. I like the 1
Mohm resistor idea, to keep them in relatively the same level.

 

[neon]

the idea of optocoupler is to isolate one circuit from another. if you want to tie them together then why isolate them to begin with.

 

[RRogers]

Hi,

I have two circuits on one PCB, isolated magnetically and optically from
each other, with two grounds, an analog and a digital, right now they
are completely separate, is it ok to leave them separate or should I
connect them with a 1Mohm resistor or even tie the two ground planes
together at one point to keep them both at a similar voltage? I don't
see any reason to tie them together except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.

cheers,
Jamie

Like a lot of other design items it depends upon the context.
Is the statement:
except that the two grounds could be
at a voltage larger than that allowed by the optoisolators.
true or hypothetical? What happens when the grounds are tied
together? Enormous currents flow? The Big Bang is recreated?
Leaving chunks of copper floating around is generally a bad idea, but
sometimes necessary. If you don't join the grounds, or even if you
do, how much pickup can you expect; radio stations, transformer
currents, motor currents, etc.... ?

Ray