A Conversation About Dead Bugs
Sometimes the best way to learn about a subject is to be part of or listening to a conversation regarding the topic. This discussion between Chris and Mateja discusses low noise techniques for prototyping.
Mateja: Hey, did you see my tweet about breadboards?
Chris: Yeah, what happened?
Mateja: 60 Hz. It’s just everywhere!!! It won’t leave me alone!!! As soon as I take it off the breadboard though it works. I’m using an Olimex development board with the half-perfboard. So now I’ve got it on the perf with jumpers and it works well.
Chris: You try turning off the lights?
Mateja: I’ve tried that before, but not with this project … what do you think is worse, fluorescent or incandescent lights for noise?
Chris: Fluorescent hands down,those things are nasty. But they throw off a lot more than 60 Hz. What are you working on?
Mateja: I’m writing a driver for an RTC module for our sensor platform, but I’m using a dev board because we have some hardware revision issues that we have to work out with the sensor board. So I’m using an Olimex dev board and a handmade breakout board for the RTC. I couldn’t get the clock output of the rtc to stabilize on the breadboard because it was picking up the 60hz noise and trying to trigger off that at the crystal input. The crystal would never stabilize.
Chris: Oh yeah, that’s a stinker of a problem. Putting a crystal on a breadboard in general probably isn’t advisable.
Mateja: Really, breadboards are useless for anything above the audible spectrum. I guess low duty cycle digital works too but only for blinkenlights, not communication stuff.
Chris: Have you ever heard of the dead bug method?
Mateja: What’s that?
Chris: Not sure who came up with it, but Bob Pease wrote about it a bit. It’s how he creates circuits to reduce noise and preserve signals in high frequency systems. He flips chips over and then puts the plastic part down on a grounded piece of copper; then the leads are sticking up in the air like a dead bug.
Mateja: How does he connect to the leads?
Chris: He skywires them.
Mateja: I’m not familiar with the term skywire.
Chris: Oh, just running a wire from one component to another; so they’re “wires in the sky”.
Mateja: Like wire wrapping or with female to female jumpers?
Chris: Solder.
Mateja: That’s probably the safest but it’s not that flexible.
Chris: Flexible, like “easily changeable”?
Mateja: Right, it’s not easy to modify.
Chris: True, that’s one of the drawbacks,but when you’re a solder cowboy like myself, it’s not too big a problem. Plus you can’t beat the connection between two soldered components. If you do stuff on a bread board, pressing down on a component can literally change the circuit because of how it’s contacting the ground plane.
Mateja: You’ve probably shaved a couple years off your life with all the flux fumes.
Chris: Haha, don’t I know it. Again, worth it.
Mateja: OK, fine. You convinced me I’ll solder it.
Chris: Yay! You’ll be better off, especially for a crystal.
Mateja: Oh the crystal is soldered down, I have it on a tiny little single sided PCB that has pins like a DIP package.
Chris: Oh that’s good. Before I forget, here’s a good explanation of the dead bug technique:http://www.lpkfusa.com/articles/prototyping/edn_2_96.pdf. It starts on page 2.
Mateja: Yea I see it, I’m going to have to look around the shop for a copper clad board. How do you adhere the IC to the copper?
Chris: Basically every IC has a ground pin, so you bend those down and solder them directly to the copper plane.
Mateja: Oh dear.
Chris: Oh yeah man, welcome to my world! But soldering them should hold them on there and provide a really short path to ground too.
Mateja: You only get to bend those things once…
Chris: Yessir. Twice if you’re really careful. Expensive components you’re using?
Mateja: Shoestring budget more like.
Chris: I mean, it was easier back in the days of DIP packages, easier to bend those leads down. But you don’t really need to bend them. What kinds of packages are you using?
Mateja: This one is TSSOP.
Chris: Well, you could always just solder in a jumper wire or a 0 ohm resistor, that’ll do in a pinch and no bending involved.
Mateja: OK, thanks for the advice, I’ll break out the hot iron.
Chris: Sounds good!
