Screaming Circuits: September 2007

High Speed Layout & Parallel Caps

When I started designing things, I used a 20MHz scope to debug a 2MHz processor. Sometimes it was a cheap 5MHz Heathkit scope on a now ancient LM741 op Amp. Digikey still sells the same National Semiconductor part in DIP-8 and TO-5 metal can packages. Hard to believe.

Okay. Show of hands... Whom out there has put a 5MHz scope on an LM741 in a TO-5 package?Lm741_to5_2

Anyone? Anyone? Anyone?

Speed means something a little different now. I recently ran across an interesting article on the Analog Devices web site; "A Practical Guide to High-Speed Printed-Circuit-Board Layout." It has a lot of good layout hints (duh) but what caught my eye was the discussion on paralleling capacitors. Back in the day, for the most part, you'd just put a .01uf cap on each logic chip and be done with it. Even with power supplies, it was pretty much just "put as much as you have space and budget for. If you've got ripple, put in more". OpAmps required a little more work to determine capacitor values but in a lot of areas you could get by without much engineering when selecting capacitors.

Recent forays into motor control have opened my eyes to how different things are now. It's pretty much the same with switching power supplies - that's really what a PWM motor driver is anyway.

Parallel_caps We used to parallel up capacitors just to increase the value. Now, however, there are other reasons. Most PWM circuits really need low ESR (effective series resistance) capacitors. With too much ESR, you'll lose some volts in the caps, they heat up and maybe even explode. Other bad things can happen too, but that's enough of a representative sampling. Putting caps in parallel does the same thing with ESR that putting resistors in parallel does with ordinary R. That's the main reason switching power supplies have six or eight or more electrolytics instead of just one big one.

The other reason to put caps in parallel has to do with differing frequency responses. Again, something I didn't worry about too much. This one is important for high speed analog circuits and PWM circuits such as motor control and power as well. You can combine two or more different value capacitors, including a mix of electrolytic and ceramic to cover a range of frequency responses. Put the smallest (both in physical size and value) closest to the chip and with the shortest path to ground. The article goes into a lot more detail, but that's the gist of it.

Duane Benson
Yes, but can you parallel park?

Via in BGA pad again

Bga_w_via_in_pad_top_400Here's another example (click image to enlarge) of a via in pad situation we don't like; A large number of vias in the BGA land pads. They're pretty big via holes too.

The component manufacturer will tell you not to do this and we tend to follow that recommendation. Take a look at our white paper on the subject for guidelines on what you should do with vias in and near land pads.

We have some tricks but they will cost extra and will not be guaranteed to work. There are just too many variables.

Duane Benson
A hole lotta nothing under there

New and Tiny

I was too busy for most of the Embedded Systems conference to go out and really study the exhibit hall but I did find a few things interesting.

Sen_150 For those of you working in the sensor arena, I ran across an interesting tiny little motion sensor. It's only 6.9mm x 3.3mm and is used to detect tilt and vibration. It's really easy to use to. It basically operates as an SPST switch. Stead, it's closed, motion, it's opening and closing.

We were also told about a 1x1mm BGA package with eight balls underneath. The manufacturer will be sending us some more information and I'll write up something about it when I get the information.

Duane Benson

ESC Go time!

Show floor. day one. We're set, just waiting for all the attendees. Drop by and see us in booth 1131 if you're here at the show.

I'm not sure what else is here, but I'll look around and hopefully find something interesting to write about.


That's Jered in the red speaking with our first booth visitor today.

Duane Benson
1 if by land, 10 if by sea

Embedded Systems Conference, Boston 07

I left Portland for Boston via Newark sometime last night. I don't remember exactly when but I know I didn't sleep more than about 20 minutes during the overnight flight. Any of you that have flown coach lately know why. I'm sure we loose a couple of inches of leg room every year.

Well, I'd like to say "we have tradeshow" but it's still early in the setup so all we have is chaos.


That's pretty normal. Four of our six boxes are here. The booth is in the two missing crates. All of our cool T-shirts are here though.


I'm running on 20 minutes of sleep and two latte's. One from New Jersey and one from Boston. Not much to do but relax and wait...


Duane Benson
Fight Uni

One Year of Screaming and Blogging

RoseYes, that's right. Today is the one-year anniversary of the Screaming Circuits blog. Granted, that first post was nothing special. It just made some obvious statement like "Hey we have a blog". But, we did and we still do. In this last year, we have posted 138 articles and have received 15 comments. Well, actually, 17 comments but two were just spammers advertising to me. That annoys me so I deleted them.

Let's take a look at what I've learned in that year.

First, I've found that a lot of people are interesting in learning more about via in pad. The most common search term used to find the blog is "via in pad". Four of the top ten search terms are "via in pad" or some variation of that. Apparently, it's a hot topic and I really hope that my work here has helped some of you out.

Second, I've learned that keeping up a regular blog is a lot of work. I have a few months that average five posts a week. Then there is a month or two that only have five posts for the entire month. An Mr_early_layout_150average of three posts a week is probably okay, but I'll try and and pace myself a bit better in the next year.

My primary goal with this blog is simply to pass on helpful knowledge. Certainly there are secondary goals like doing a little self-promotion - but hopefully I keep that rare enough to not get in the way of the useful hints and tips - the most important goal is to help you folks out.

For next year, I'd like to make this more interactive. If you can, throw some comments my way. Let me know if a post helped you out or if it didn't. Let me know if you have other questions. The more I hear from my readership, the better I can make this tool.

So, my challenge to you blog readers, is to fire comments my way. Let's work together to make this blog a great community tool.

Thank you all for reading
Duane Benson

Dspatches from OktoberBest

All set with the show just about ready to start.

We've got one of our sponsored organizations here with us, The Portland State Aerospace Society (PSAS). That's them in the photo getting ready to show their rocket guidance system to our booth visitors. Check them out at


Duane Benson

OctoberBest 2007

If you live in the Portland, Oregon area and would enjoy an early Oktoberfest with beer, brats and maybe learn a new thing or two, come to the OktoberBest trade show on September 12th. We will be there in booth 29 and Duane will be speaking about how to avoid layout and assembly got'chas with advanced component packages. Show hours are noon to 5:00pm and the advanced packaging talk is at 4:15

Our friends from Portland State will also be there with us. The Portland State Aerospace Society was recently added to the Screaming Sponsorship Program and they will be there to hang out and chat about active guidance, live telemetry and Mach-3 WiFi. The team is great and is really advancing the field of amateur  rockets.

The show is located at the Portland Community College Rock Creek Campus [17705 NW Springville Rd. Portland, OR 97229 - go our Hwy 26 and take 185th North. Then turn right on Springville Rd.]. The cost is $10, or a simple donation of food for the Oregon Food Bank.

For more information, you can check out their website at

By the way, the real Octoberfest is September 13 - 16 in Mt. Angel Oregon, about 30 minutes South of Portland Well, the Real one is in Germany, but close enough.

Duane Benson
Bad wolf


Bent SMT pins

Protect those pins!

It's nice to be flexible, right? We know that most of you like to order parts in cut strips to save money, or you have a short strip left over from a prior job. That's cool. We're all about saving money and not wasting parts. In fact, we like that a lot.

Bent_pins_800Sometimes, though the old parts drawer doesn't do us (or you) any favors. We recently ran across a small strip of QFPs in a kit. Normally, a set of three in the original strip wouldn't be a problem. In this case, though, they had likely gotten just a bit too friendly with some of the other parts. Or maybe they had gotten into a fight or something.

All three parts had a substantial number of bent pins. In cases like this, we'll look at the chips and see if we can straighten them, but we can't always do that. Sometimes we have to call and ask you to send more parts and delay the job, which is a bummer all around.

When kitting up a job, give components like this a quick look over. When you're storing extras for future use, make sure to keep the sensitive bendy parts like this away from big and heavy things.

Duane Benson
Always one more thing to check, huh?

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