This is the major problem (the cables) when use a single connector for
power, analog and digital signals.
The power cables and digital wires are aggressors for the analog signals
carried by wires, even if using shielded cables.

So in such system (but not only) the major concern is to reject the noise
induced in the analog paths by the digital and power signals, the last
being the most aggressive.
As long a bunch of cables will go from the same connector to your external
devices, is likely you'll have plenty noise induced by solenoids or motors.

Once you can separate the cable groups, the noise may be (not *is* for
sure) smaller. Every group of cable should have it's own local ground, but
finally you'll have just one ground. The trick is to avoid the passing of
the noise from one ground to the other as well from one power supply to the
other.

Vasile

On Thu, Aug 27, 2015 at 4:15 AM, <stephen.forrest@agilent.com> wrote:

> Hi Neil -
>
> I'd be more concerned with the bundling of the cable - do the different
> wires bundle together in a single cable? Do they go to one destination? H=
ow
> long is it? Any shielding? Is this a ribbon/IDC connector or
> individual/crimped? Is there another ground path (chassis)?
>
> Also, how much resolution do you expect from your analog signals (I assum=
e
> you will digitize). What is the source impedance? Amplitude? How many?
>
> I personally would associate any spare ground with the analog signals.
>
> Good luck!
> Stephen
>
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Neil
> Sent: Wednesday, 26 August 2015 4:55 AM
> To: Microcontroller discussion list - Public.
> Subject: [EE] Best practices for reducing noise
>
> I'm trying to figure out how best to allocate connector pins for a
> mixed-signal board.  The system has high-current, high-voltage,
> repeated-switching signals (motors and solenoids), serial data signal
> pairs, and a number of analog sensor signals coming in.  The catch is tha=
t
> this board has only one 30-pin connector (3 rows of 10 pins each).
>
> Currently, I have, in order starting from one end towards the other:
> - Motor + and - (12V, 6A max)
> - Solenoids (up to 4A, but with reverse spikes will see 30V+)
> - Power & ground (12V)
> - Serial signals (3 pairs)
> - Digital I/O
> - Analog sensor signals
>
> The PCB has the motor and solenoid drivers on one side of the board, powe=
r
> supply in the middle (but off towards the back), and 2 PICs that handle a=
ll
> the logic near the other side.  The analog circuitry is kept really short
> and off in one corner of the board nearest the end of the connector with
> those signals.
>
> So what I'm wondering is:
> - Is there a better way to organize the signals on the connector?
> - I have one spare pin -- should I run a separate logic ground?
> - If so, where would I connect these on the board (or wouldn't this cause
> a ground loop)?
> - If I do or don't, should I split the ground plane so the logic side is
> separate from the motor/solenoid side?
> - Anything else I should do?
>
> Cheers,
> -Neil.
>
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