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Old 27th February 2004, 10:57 AM   #21
00940 is offline 00940  Belgium
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I've done my homework.

Here's a single sided pcb, analog and digital grounds connected. Power supply traces are 40mil, signal is 16 mil. Clearance is 8mil. Grid is 1 inch. I hadn't a correct layout for the inductor, so I used a cap layout. The computer ground is not directly connected to the groundplane but to the chip only.

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Old 27th February 2004, 02:29 PM   #22
Gyula is offline Gyula  Hungary
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Hi Rookie!

In this way, everything is coupled capacitively to your ground plane, and it's Full of ground loops!

Good luck,
Gyula
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Old 27th February 2004, 02:31 PM   #23
Gyula is offline Gyula  Hungary
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Oops!

I have to apologize from Rookie, I wrote the reply above to 00940.
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Old 27th February 2004, 03:22 PM   #24
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Gyula--I would have to say that with this single sided board the capacitive coupling from a trace to the ground plane is next to nil. Capacitance between two plates (trace and ground plane for example) is proportional to the area of the plates in parallel, and traces side by side with a ground plane on the same layer are coupled only by the thickness of the 1 ounce copper, NOT the width of the traces.

We do this all the time on low noise photoamps here at work. it works fine, no ground loops. And it's not really a problem with 2 or 4 sided boards either. Calculate the capacitance with 0.062 FR-4 separating two plates, for example (2 sided standard board). It's pretty small. I have in production 4 layer boards (0.015 inch insulating layers between 1 ounce copper traces, 2 signal layers with a ground plane and power plane) with 10 megohm feedback resistors in current-to-voltage converters where 1 picofarad is a big deal. These boards work fine, and the typical parasitic capacitance is on the order of 1/4 picofarad. Not enough to create much of a ground loop....

cheers

Adam
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Old 27th February 2004, 05:19 PM   #25
Rookie is offline Rookie  Serbia
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Here is my PCB. According to Gyula, it is fool of ground loops too.
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Old 27th February 2004, 05:42 PM   #26
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That looks pretty nice, Rookie!

One other comment I forgot to add to the previous post: Ground loops are created when various parts of a circuit have different return paths to ground with significant resistances to those paths. In an extreme case, where a circuit drawing significant current causes a measurable voltage drop across a return trace and shares that ground return with, say, a low level analog signal, real havoc can result--oscillation, noise, etc. As far as the analog circuit is concerned, its ground reference is bouncing up and down as the load varies.

The main reason a fill, or ground plane, is used is so that all the ground return paths are through a large area and thus low impedance conductor, thus insuring that all return paths have a low impedance to ground. And still, if you have a significant current draw (hundreds of milliamps or more), you should have a separate ground plane for the high current circuit only, tied (as I described in the previous post) to the other ground planes where power comes in to the board.

There is a more elegant way to do this--a star ground, where **every** ground path has its own trace back to a single grounding point. I have a picture of a preamp designed by Dan Schmalle (Doc Bottlehead) where every signal or power path has a separate wire, and all the return wires lead to one single point on the chassis. It's definitely a 3-D rats nest and nearly impossible to do properly on a PC board...but technically speaking that is the BEST way to design a circuit. But then again that is why PC boards have ground planes instead...much easier to implement and nearly as effective.

cheers
Adam
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Old 27th February 2004, 08:53 PM   #27
00940 is offline 00940  Belgium
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If the first wasn't good, would this be better ?
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Old 27th February 2004, 10:31 PM   #28
Gyula is offline Gyula  Hungary
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Hi Adam!

Had wrote that observation because I think the tracks and the ground plane are too close to each other.
The capacitance depends on the Area and the Distance of the conductors at a given permittivity.

Gyula
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Old 27th February 2004, 10:41 PM   #29
Gyula is offline Gyula  Hungary
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Hi Adam!

The ground loop is coming from the parasitic impedance and capacitive coupling. These are the two terms of oscillation. This is not coming from the resistive voltage drop as you wrote, the current division is an another phenomeon. Every path has her own LC, and begin to oscillate with each other.

Gyula
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Old 1st March 2004, 10:34 AM   #30
00940 is offline 00940  Belgium
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no comment on the new layout ?
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