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isolated DC/DC converter

OK, Thanks.

You mentioned it was 2 inches away. That makes it difficult, to say the least!

Tough to do with several sources - transistors and diode rectifiers inject noise into the ground plane

That can make life more difficult by shunting other noise sources into the area you are trying to protect

Standard stuff

Also tough to do, especially with a wideband preamp

Williams used slew rate limiting in the switching elements for his ultra low noise switching converter. But he was looking at the ripple on the output signal, and as far as I remember, he did not show the noise injected into the ground plane. You can do the same thing to slow the switching speed down, and reduce the fast edges injected into the ground plane.

Cutting the ground plane won't help there. It will make the heat problem worse.

I usually figure the first 40dB is free. The next 20 dB is harder, and anything past that is very time-consuming and gives less and less return for the time and effort.

From your photo at

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you are running from 24V.

60dB down gives around 24mV or so of switching spikes, depending on how fast you switch and where the spikes go.

Most 250MHz 12 bit ADC's use 2V p-p or less input swing. The LSB is

2/2^12 = 0.000488, or 488 uV. If you add 20dB gain from an input op amp, you are looking at 48uV for the LSB. So you want the spikes to be less than that.

Then you need another 20 * log(24e-3/48e-6) = 53.97 dB of isolation on top of the free stuff. That will take some time:)

Of course, the estimates are very rough ballpark. But when you end up 60 dB short, you know you are in for some fun:)

There are some other simple PCB tricks than can be very effective, but they are proprietary and I can't divulge them at the current time.

So I'd go for reducing the switching spikes at the source with slew rate limiting, and use soft turnon diodes in the rectifier instead of schottky.

Mike

Reply to
Mike
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Lots of people say stuff like that. Makes no sense to me: how is sharing stuff like that going to hurt you? And if it does a little, so what?

John

Reply to
John Larkin

On a sunny day (Sun, 06 Nov 2011 09:43:21 -0800) it happened John Larkin wrote in :

Somebody I know bought a rather expensive digitiser pad. If I had one that worked in Linux maybe that would be nice to make quick sketches. There is also something in the opera browser that allows one to share sketches live, tried it some years ago, but even on an extremely fast link it was very slow. Maybe I should check ebay.

Reply to
Jan Panteltje

Why on earth would you say the following?

That is unbelievable. Why should I risk anything for someone who could care less how much damage it would cause.

You also use the same thing, John. Proprietary is exactly what it means.

Mike

Reply to
Mike

Here's a recent example. Not even proprietary:

Subject: Re: Fast power LED driving Date: Sat, 20 Aug 2011 05:38:35 -0700 From: John Larkin Newsgroups: sci.electronics.design Xref: z sci.electronics.design:846558

[...]

I have a cute and cheap way to drive mosfet gates really fast and hard. I'd post it publically, but Thompson would steal it...

[...]

John

There are many others.

Mike

Reply to
Mike

I was ragging JT. He won't post anything for fear that I might benefit from it. As someone else noted, I had already posted using NL37WZ16s as fet gate drivers.

John

Reply to
John Larkin

I guess it depends on how close to 100% selfish you can get.

John

Reply to
John Larkin

And why would I "steal" it? More likely that I'd note that Larkin infringes one of my patents or those of my clients. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

In the US, these guys are great:

but specifying and qualifying custom magnetics is still a nuisance.

John

Reply to
John Larkin

Sure is, great fun, textbook stuff. I just noticed, there seem to be some Gibbs Ears on the spread-spectrum plateau.

John

Reply to
John Larkin

What gets me is why is someone who is so afraid of divulging such proprietary information, talking about it at all?

Reply to
krw

Nah. I was just interested to find out if you knew the trick, or if there was something else you were hiding. From your response, it is clear you do not, and you obviously do not care how much damage you could cause. Is that not being selfish?

So it will stay that way.

Mike

Reply to
Mike

Anyone who trusts a Larkin "design" is more than a wee-bit the fool. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

100%!

John

Reply to
John Larkin

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John

Reply to
John Larkin

He has accused me of not knowing the tricks, but won't reveal the tricks. So how can he say I don't know it?

And if it's so proprietary, why is he encouraging me to reveal it if I know it?

Makes no sense.

It's probably common and trivial, as most supposedly proprietary tricks are.

John

Reply to
John Larkin

sketches.

sketches live,

Actually most Wacoms (and their not rebaged knock offs) work pretty well. You could try one, taint expensive now. I have one and it works fine in linux.

?-)

Reply to
josephkk

Surely there are lots of "tricks" you can use (to reduce the effect of noise from a SMPS). How are we to know which particular one you are talking about?

Limiting transition speeds. Use a shielded inductor. Minimise area of high dI/dt loops. Cascaded ferrite beads/low R + chip ceramic filter sections. Physical separation.

In general I find high switching frequencies much easier to filter than low frequency ones. So say using a 500khz rather than 100khz regulator makes things easier. (This was for relatively low frequency "victim" circuits, so YMMV here).

Less obvious ideas - put SMPS on opposite side of a board to everything else, with a groundplane layer in between.

Try injecting an opposite polarity signal to the interfering one. Hmm, doesn't sound very practical..

Dither the switching frequency (already mentioned here).

--

John Devereux
Reply to
John Devereux

And don't forget to patent 'making republican babies'. May make you so rich you can finally hire somebody to fix your home appliances. :-)

Reply to
Jan Panteltje

And a decorator. He should hire a decorator.

John

Reply to
John Larkin

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