Chassis Ground layout for better ESD and EMI compliance

Ah, there's the rub. Since only one end of the shield is connected, there's no current path, so how can any signal be induced, except at, say, 1/4 wave? I think I remember hearing (reading) that that's the rationale for leaving one end flapping in the breeze in the first place.

Thanks, Rich

If the cables are non-diff that'll result in noise capacitively coupled in. With diff it can be ok. But sometimes grounding at both ends is not advised for other reasons, such as erroneous large currents. "Dude, don't that smell like pee vee cee around here?"

If the shield is not very short, it has serious inductance back to ground. And there are a zillion frequencies where the impedance spikes up. Not just 1/4 wave, but 5/4, 9/4, etc. And nearby frequencies where Z is high but not infinite. Every motor controller, light dimmer, relay contact, radio station, cell phone, ESD zap, and lightning bolt will shake the ungrounded end. And the wires inside.

Take a long cable. Ground the shield a distance away, in another room or another building. Scope the voltage on the shield at the end where it's ungrounded, against local ground.

Repeat for conductors inside the shield.

Now, ground the shield at the scope end.

John

Depends on how much common-mode noise you can stand. Seems to me that less is better than more.

But sometimes grounding at both ends is not

That would be unusual, two grounds with enough voltage diference, at a low enough impedance, to heat up a cable shield.

John

tion,

is

the

have

ng

to

=94 he

It's actually what a reporter - Christopher Onions - wrote after talking to David Viner. They were talking about English winters, not Californian winters, and it looks as if Viner was extrapolating from what had been happening over the previous few years, which - in retrospect - looks more like a local warm period driven by the Atlantic Multi-Decadal Oscillation

As a prediction about California, it's a non-event. It was never published in a peer-reviewed journal and in fact sounds more like something created by selective quotation than any kind of formal prophecy.

You've also picked up Tim Lenton's comments about the increased likelyhood of drought in California. The clip doesn't mention any dates.

What he may have had in mind might be this

"Mean changes in the number of drought months relative to the 20th century baseline for 2036-2065 along the A1B (moderate emissions) scenario."

This is actually a prediction, but you are going to have to wait a few years before you can start claiming that current weather conditions falsify it.

Bottom line is that you have set up a straw man and you don't know enough about what you are talking about to realise it.

-- Bill Sloman, Nijmegen

re

But it is likely that what he was actually talking about was this

which comes from MIT and involves predictions for the period 2036 to

It seems to be close enough to keep John Larkin happy, but he's tends to stop thinking when he sees an answer he likes.

-- Bill Sloman, Nijmegen

Oh, that. I thought you were talking about the board-edge stitching. I ran their numbers on input-to-output. Musta been put together by the same people who dreamt up AGW. MY board isn't that big and the isolated areas are less than 10% of what it would take. I'll have stitching caps at the ends of the packages.

Computer? No computer in this box. ;-)

It's a gazinta-to-twelve-gazouta at 201Hz. ;-)

I need around 50mA for each 422 driver. I could use individual isolated supplies but then I have 100KHz, and up, that I need to filter out. Second source, as long as #1 is ADI, isn't important. Cost, for this box, isn't a biggie, either. If we make a hundred of them, I'll be surprised.

Haven't built it yet (just started in layout this afternoon), so no failing EMI testing is not my problem. Yet. ;-) I am worried about it, though.

I was more interested in your take on the theories and conclusions about EMI in app note. Is the board edge fence useful? As I mentioned to Joerg in another post, the numbers for the buried input-to-output cap just don't make sense. I figure a 390pF cap on both ends should work (360MHz). A 0603 won't, though.

Which brings up a related question... Does anyone have a formula for board spacing vs. isolation voltage? There are 0805 500V caps.

ere's

or

At which point one starts thinking about wrapping the coax around a toroid a couple of times, creating a transmission line transformer and a common mode choke. If you want to block 50/60Hz, that's a biggish iron or mu-metal toroid ...

Transmission line transformers work fine with soft-iron cores.

-- Bill Sloman, Nijmegen

Not even close...

-- Les Cargill

A clients once measured tens of amps. I told them to hold a current clamp around it and that resulted in some "Oh s..t!" exclamations.

All it takes is a minor difference in chassis ground potential and a very low resistance shield.

I have measured ~50VAC difference between the "grounds" of two adjacent high-rise buildings :-( ...Jim Thompson

But if you make only 100/year it won't matter.

A formula usually isn't accepted by notified bodies. You'll have to get the standard that applies to your market and then look up creepage and clearance. I only have the standards for medical and aerospace.

For example, in the med standard basic insulation between parts of opposite polarity for 500V operation would be 4mm for creepage, basic/supplemental is 8mm and double or reinforced is 16mm.

You'll have to get special parts that are longer. And don't forget the hi-ohms bleeder resistor which is usually required so no static can accumulate up. Vishay has fairly decent prices for those.

We had some LEFs (Light Emitting Ferrites) on one of our first boards. The only way to get that much current in the circuit is if the grounds were significantly different. We never did find the problem.

OK, I'll bow out now - I was thinking of interconnects inside one box, like no longer than a few inches or a foot or two.

Thanks, Rich

Well, it's 100 * 13 per board. ...and likely not "per year". ;-)

I doubt there is any standard in our market.

Opposite polarity? So that's double the V-G voltage? 16mm kinda lets out

I'll have a look at Vishay. Thanks. None of our stuff has bleeders now, though I'd always wondered about that.

However, if this is something that repeats itself as sort of an IP block in other products then the more discrete solution could make sense.