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Old 19th April 2005, 10:40 AM   #1
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Default Help with grounding

Hi,

I need some help and would be greatful for any advice offered.

I am in the process of designing a PCB that comprises:
- FRED Rectifiers and filter caps
- Positive and Negative Super Regulators (ALWSR)
- Active crossover for IB Sub to Linkwitz Orion
- A couple of other bits and pieces

The power supply will power both the sub crossover and the linkwitz Orion active crossover that will be co-located.

I am keeping the +ve and -ve supply grounds separate through the regulators and then bringing them together at a single star earth point.

I am using a 4 layer PCB and have a number of grounding options.

1) Do not use ground plane; star earth all Opamp decoupling caps and signal earths.

2) Use a ground plane connected to the star earth at a single point; star earth all Opamp decoupling caps and signal earths (do not connect any circuitry to ground plane).

3) Use a ground plane connected to the star earth at a single point, connect all Opamp decoupling caps and signal earths to the ground plane.

4) Any other combination of the above.

Even though I am a qualified elec. eng., I have gone into comms. networks. integration and don't have any practical understanding of best earthing schemes for audio frequencies.

Thanks.

Mark
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Old 19th April 2005, 12:35 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
star earth all your decoupling caps.
star earth all your signal returns.
Take care to keep the feedback routes (power and ground) of your regs completely clean.
Ensure short low inductance grounds for the decouping. This forces you to adopt a compact layout.
Now help me; what advantage is there with a ground plane? I have even seen ground planes with areas removed to lower capacitive coupling.
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Old 19th April 2005, 01:19 PM   #3
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Thanks for your response Andrew... much appreciated.

I may struggle to star all earths while keeping paths short... there will be 9 opamps in the circuit, requiring 18 decoupling earths, as well as 30 odd signal earths...

Perhaps I could treat the circuit as functional units and star earth each functional unit and then star the earth within that functional unit (an earth tree if you will)?

As for advantages of ground planes, my understanding is that at RF frequencies, minimising path length to a low impedence ground node is of critical importance... at audio frequencies, I'm uncertain as to the benefit (other than allowing for lazy routing... its pretty bloody easy to drop a via to a ground plane... not so easy to star to 48 points on a PCB!)

I'm sure that I'm telling you nothing you don't already know.

Regards

Mark
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Old 19th April 2005, 02:11 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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A 4 layer PCB? Isn't that overkill? What are you going to do with the other layers? One ground plane, one signal layer and one each for + and - rails?

Quote:
Originally posted by AndrewT
...Now help me; what advantage is there with a ground plane? I have even seen ground planes with areas removed to lower capacitive coupling.
Low inductance. You can pretty much connect a supply bypass capacitor to any point and be sure that it will have a nice low impedance return. Also capacitive coupling to various parts of the circuit (e.g. supply rails, which should be routed over the ground plane) helps keep RF out.
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Old 19th April 2005, 02:29 PM   #5
dhaen is offline dhaen  Europe
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Default Ground planes

Opamps used at video frequencies often have the ground plane removed around the inverting input. Fractions of a pF here will cause HF peaking. Presumably as audio opamps improve, this could become an stability issue. An opamp "hooting"out-of-band could cause some serious audio impairment.

John
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Old 20th April 2005, 07:21 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
thanks for all that ground plane info.
Mr Evil's comment seems to solve the lo inductance return for the decoupling caps.
If you go for local stars grounds remember to have a dirty return and a clean return from each to the master star ground.
How about 2 ground planes? one each to power and signal? This would solve all the return problems. Any disadvantages?
Just remembered, Borbely shows his grounds slightly different from the norm. He puts a resistor between signal ground and power ground and connects power ground to safety earth.
Whereas most others who go to the bother of separating the grounds connect signal and power grounds together and then resistor connection to safety earth.
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Old 20th April 2005, 10:46 AM   #7
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Thanks everyone for the info...


Quote:
A 4 layer PCB? Isn't that overkill?
Kind of... I'm sure with time and patience I could do it on a 2 layer board, and if I were designing for minimum cost large production runs, then I would go to the effort, but I am doing 4 boards only and the incremental cost is not so much that it does not compensate for the effort saved. Also, I get the potential advantages of power and earth planes.

Quote:
How about 2 ground planes? one each to power and signal? This would solve all the return problems. Any disadvantages?
I have been seriously considering this particular approach... a power ground plane and a signal star earth on separate layers.

However, I still struggle to understand why, at audio frequencies, a star earth is better than a solid ground plane... I understand that fast switchin digital circuits can cause a ground plane to ring, inducing noise in other parts of the circuit... this will not be the case here... I can also appreciate why a "series" earth track might cause less than ideal performance... however, a solid ground plane in an all audio circuit should be relatively quiet...

Has anyone seen a quantitative (or even an audio-qualitative) analysis of the topic (plane versus star)?

Quote:
Take care to keep the feedback routes (power and ground) of your regs completely clean.
I don't understand what this means... can you elaborate?

Regards & Thanks

Mark
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Old 20th April 2005, 12:53 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I wish I could refer you to a schematic, but I have not learned how and I would need to search umpteen threads to find them.
Lets try a thousand words instead.
The schematic will show the tapping sensing the output voltage anywhere along the output rail. Instead it must come from the last point on the board before it finally supplies the load. The ground reference must also pick up the last point before the load. These 2 sensor tracks must not share current with any other loading. The sensors could ultimately be taken off board to measure the voltage at point of use ( as done in commercial power supplies).
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