MOTM

Question for Paul, mostly, but maybe others have considered the same thing:
If I'm building a module from scratch, and I want it to play nicely with my
MOTM modules (present and future), and it's drawing all three voltages
through a MOTM-standard 6-pin power connector, and it's a hybrid
analog/digital module, how do I connect together the digital ground and the
analog ground on my PC board? It may be something of a moot point because
AFAICT my Power-One triple supply has the grounds connected together anyway,
but I'm wondering whether there is some little nicety that I should observe,
like maybe using a ferrite bead instead of just plain copper.

Thanks for any opinions or suggestions.

--Adam

If the header has a separate "digital" ground for the 5V source, then
I don't see any reason to connect them on the PCB.  The "star point"
is at the power supply.  Many mixed-signal systems have analog and
digital grounds separated in this way, with the ground pins of the AD
and DA converters connected to the analog system ground.  Putting a
ferrite bead or resistor between the grounds will create an impedance.

On 10/26/06, Adam Schabtach put forth:

>Question for Paul, mostly, but maybe others have considered the same thing:
>If I'm building a module from scratch, and I want it to play nicely with my
>MOTM modules (present and future), and it's drawing all three voltages
>through a MOTM-standard 6-pin power connector, and it's a hybrid
>analog/digital module, how do I connect together the digital ground and the
>analog ground on my PC board? It may be something of a moot point because
>AFAICT my Power-One triple supply has the grounds connected together anyway,
>but I'm wondering whether there is some little nicety that I should observe,
>like maybe using a ferrite bead instead of just plain copper.
>
>Thanks for any opinions or suggestions.

Ah, right you are. For some reason I got it into my head that the grounds
would have to meet at the board, but you're right, the star point should
stay back at the supply. Thanks.

--Adam 

> -----Original Message-----
> From: Mark [mailto:yahoogroups@...] 
> Sent: Thursday, October 26, 2006 3:50 PM
> To: Adam Schabtach; 'MOTM litserv'
> Subject: Re: [motm] MOTM 6-pin power connector ettiquette question
> 
> 
> If the header has a separate "digital" ground for the 5V 
> source, then I don't see any reason to connect them on the 
> PCB.  The "star point"
> is at the power supply.  Many mixed-signal systems have 
> analog and digital grounds separated in this way, with the 
> ground pins of the AD and DA converters connected to the 
> analog system ground.  Putting a ferrite bead or resistor 
> between the grounds will create an impedance.
> 
> On 10/26/06, Adam Schabtach put forth:
> >Question for Paul, mostly, but maybe others have considered 
> the same thing:
> >If I'm building a module from scratch, and I want it to play nicely 
> >with my MOTM modules (present and future), and it's drawing 
> all three 
> >voltages through a MOTM-standard 6-pin power connector, and it's a 
> >hybrid analog/digital module, how do I connect together the digital 
> >ground and the analog ground on my PC board? It may be 
> something of a 
> >moot point because AFAICT my Power-One triple supply has the grounds 
> >connected together anyway, but I'm wondering whether there is some 
> >little nicety that I should observe, like maybe using a 
> ferrite bead instead of just plain copper.
> >
> >Thanks for any opinions or suggestions.
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
> 
> 
>

It depends on whether your circuit keeps analog and digital grounds 
separate.  If it does, then there is no reason to connect these on the 
board; they will be connected at the supply.  If the two grounds do 
connect at the board, it is not cause for concern.  I am running a 
couple of Veeblefetzer's off a MOTM-950, and the analog and digital 
grounds do connect together on the board.  I asked Paul S. if this would 
be a problem and he said no.

Richard Brewster
http://www.pugix.com

Adam Schabtach wrote:

> Ah, right you are. For some reason I got it into my head that the grounds
> would have to meet at the board, but you're right, the star point should
> stay back at the supply. Thanks.
>
> --Adam 
>
>   
>> -----Original Message-----
>> From: Mark [mailto:yahoogroups@...] 
>> Sent: Thursday, October 26, 2006 3:50 PM
>> To: Adam Schabtach; 'MOTM litserv'
>> Subject: Re: [motm] MOTM 6-pin power connector ettiquette question
>>
>>
>> If the header has a separate "digital" ground for the 5V 
>> source, then I don't see any reason to connect them on the 
>> PCB.  The "star point"
>> is at the power supply.  Many mixed-signal systems have 
>> analog and digital grounds separated in this way, with the 
>> ground pins of the AD and DA converters connected to the 
>> analog system ground.  Putting a ferrite bead or resistor 
>> between the grounds will create an impedance.
>>
>> On 10/26/06, Adam Schabtach put forth:
>>     
>>> Question for Paul, mostly, but maybe others have considered 
>>>       
>> the same thing:
>>     
>>> If I'm building a module from scratch, and I want it to play nicely 
>>> with my MOTM modules (present and future), and it's drawing 
>>>       
>> all three 
>>     
>>> voltages through a MOTM-standard 6-pin power connector, and it's a 
>>> hybrid analog/digital module, how do I connect together the digital 
>>> ground and the analog ground on my PC board? It may be 
>>>       
>> something of a 
>>     
>>> moot point because AFAICT my Power-One triple supply has the grounds 
>>> connected together anyway, but I'm wondering whether there is some 
>>> little nicety that I should observe, like maybe using a 
>>>       
>> ferrite bead instead of just plain copper.
>>     
>>> Thanks for any opinions or suggestions.
>>>       
>>  
>> Yahoo! Groups Links
>>
>>
>>
>>
>>
>>
>>     
>
>
>
>  
> Yahoo! Groups Links
>
>
>
>
>
>

On 10/26/06, Richard Brewster put forth:
>It depends on whether your circuit keeps analog and digital grounds
>separate.  If it does, then there is no reason to connect these on the
>board; they will be connected at the supply.  If the two grounds do
>connect at the board, it is not cause for concern.  I am running a
>couple of Veeblefetzer's off a MOTM-950, and the analog and digital
>grounds do connect together on the board.  I asked Paul S. if this would
>be a problem and he said no.

I don't know what a Veeblefetzer does, or what sort of noise
considerations there are in "Veeblefetzering".  However, if noise is
a concern, as it would be in an audio or precision voltage module,
then, imho, it is best to keep the digital and analog grounds
separate.  If it were something like a logic module, then it wouldm't
matter much.

There are two questions here.  One is whether analog and digital grounds 
can be connected together on a PC board connected to a MOTM-950.  The 
answer to that is, Yes, because the two are connected on the MOTM-950 
itself anyway.

It's a different question whether a module can propagate noise through 
the power supply connections into other modules.  I am sure that is 
possible.  If noise could get through simply due to the analog and 
digital grounds connected, then the MOTM-950 would be subject to this 
problem for *any* set of modules supplied by it.  Analog and digital 
grounds have to connect somewhere.  At one point is ideal.  The 950 
provides that point.

In the end whether you have a noise problem comes down to whether you 
*detect* a noise problem, i.e. you have audible noise or noise visible 
on a 'scope.  If you do, then it becomes a matter of tracing the source, 
and, if you suspect coupling through power supplies, it is a simple 
matter to disconnect the suspected modules to see if the noise goes 
away.  I haven't noticed noise from my Veeblefetzers (which have a bunch 
of LED's), but if I detected a noise problem, I would disconnect them to 
see if it fixed it.

I have my digital modules, including Veeblefetzers, the MOTM-650, CGS 
digital circuits, Blacet Miniwaves, etc., on separate power supplies 
from the VCO's, VCF's and VCA's in my synth.   Right now I have a 
MOTM-950 powering nothing but two Veeblefetzers, the MOTM-650, and two 
MOTM-800 envelope generators.  The other digital modules are on a MOTM-900.

-Richard Brewster
http://www.pugix.com

Mark wrote:

> On 10/26/06, Richard Brewster put forth:
>   
>> It depends on whether your circuit keeps analog and digital grounds
>> separate.  If it does, then there is no reason to connect these on the
>> board; they will be connected at the supply.  If the two grounds do
>> connect at the board, it is not cause for concern.  I am running a
>> couple of Veeblefetzer's off a MOTM-950, and the analog and digital
>> grounds do connect together on the board.  I asked Paul S. if this would
>> be a problem and he said no.
>>     
>
> I don't know what a Veeblefetzer does, or what sort of noise
> considerations there are in "Veeblefetzering".  However, if noise is
> a concern, as it would be in an audio or precision voltage module,
> then, imho, it is best to keep the digital and analog grounds
> separate.  If it were something like a logic module, then it wouldm't
> matter much.
>
>
>
>

I'm not sure what your point is, but the question was, "how do I
connect together the digital ground and the analog ground on my PC
board?" and the answer is that there is no need to connect them on
the PC board.  Saying that something is unnecessary is not the same
thing as saying that it will cause a problem.

The issue is not whether a module can propagate noise through the
power supply connections to other modules.  The issue is whether the
digital circuitry on a module can propagate noise to the analogue
circuitry on the same module.  One way to try to prevent that is to
separate the analogue and digital grounds on the board, and connect
them near the supply where the ground impedance is lowest.

You are saying that it is ideal to have the grounds connect at only
one point, and I agree, but you are also saying that you can have
them connected both at the board and at the 950, which would be
connecting them at two points.  Whether or not that causes a problem
depends on the module.

On 10/30/06, Richard Brewster put forth:

>There are two questions here.  One is whether analog and digital grounds
>can be connected together on a PC board connected to a MOTM-950.  The
>answer to that is, Yes, because the two are connected on the MOTM-950
>itself anyway.
>
>It's a different question whether a module can propagate noise through
>the power supply connections into other modules.  I am sure that is
>possible.  If noise could get through simply due to the analog and
>digital grounds connected, then the MOTM-950 would be subject to this
>problem for *any* set of modules supplied by it.  Analog and digital
>grounds have to connect somewhere.  At one point is ideal.  The 950
>provides that point.

You connect the analog and digital grounds through a 600ohm, 3A rated ferrite 
bead.

Paul S.

On 10/31/06, Paul Schreiber put forth:
>You connect the analog and digital grounds through a 600ohm,
>3A rated ferrite bead.

A better way to put that would be "I connect the analog and digital
grounds..."  While I'm curious why you do that, and I'm sure your
reasons are excellent, the point is that is what works for what you
are doing.  I have no idea what the original poster was trying to do.
Back when I testing sequencer ideas, I was getting "blips" on the
output until I separated the grounds.  Then again, the discrete CMOS
I was using is antique compared to the fast, high-impedance logic I
presume MOTM uses.  I've been reading quite a bit about mixed signal
design over the last few months.  I find it to be an interesting
topic, and there seems to be many different opinions about what is
"best".  Regardless, my concern here is that it is the etiquette of
this list that seems to have suffered a bit lately.

>>You connect the analog and digital grounds through a 600ohm,
>>3A rated ferrite bead.
>
> A better way to put that would be "I connect the analog and digital
> grounds..."  While I'm curious why you do that.

Ferrite beads are a short at DC, but as the frequency increases, they become a 
high(er) impedance. A bead that is spec'd at '600ohms' means the complex 
impedance at 10Mhz is 600ohms.

Paul S.