A bypass-capacitor dialogue peels back the layers, Part 3: Continuing the discussion on layout considerations

Editor's introduction: Bypass capacitors, grounding, and decoupling are relatively low-visibility, low-glamour issues and generally not the subject of many feature stories, but are vital to a successful, reliable, error-free design. Our multipart series on the subject was the most popular articles by far we presented over the past 12 months (see references, at end )

Authors David Ritter and Tamara Schmitz of Intersil Corp. have been engaged in an on-going dialogue on the subject. Dave and Tamara believe in the value of arguing, the value of education, and getting to the heart of the problem without ego; in short, ripping apart a problem for the sake of knowledge.

Here's Part 3 of the conversation. “Listen in” and learn:

[Dr. T enters her office with a large bag of rustling papers. She dumps it on her desk as Dave comes by.]

Dave : Hey, Dr. T, watchya got there?

Tamara : It's our reader mail.

Dave : We got mail?? You mean like, “Mr. Richard Fader from Fort Lee, New Jersey writes: What's all this fuss I hear about capacitors?” kind of mail?

Tamara : Yep, actual letters.

Dave : About capacitors and layout?

Tamara : Sure! Here's one from Kyle. (all names of readers changed to protect the innocent) He used staged caps to bypass his circuits in high-amplitude RF fields.

Dave : Just like we said. Some times you need to, many times you don't.

Tamara : He's also asking about coupling caps. They don't seem as fussy as bypasses.

Dave : Yes, I've noticed that, but some folks are afraid to use a big cap for coupling because it is 'too slow'. I don't think they're thinking about it correctly.

Tamara : We're going to get to that in a later discussion. Here's a comment from Carl. He's not sure about our last conversation involving voltage drops on ground planes. He says it either needs a magnetic flux around it (inductive effect) or it's just an IR drop, which is usually pretty small.

Dave : Well, we actually are talking about pretty small voltages: 60 dB of crosstalk in a video system means millivolts of unwanted signal. In the example we showed last time, we simplified the circuit quite a bit for the sake of illustration. The actual circuit had a full DC restore (with an electromechanical relay) in each channel, and it was through-hole. Figure 1 shows what can happen when through-hole parts or vias break up a ground plane.

Figure 1. PCB layout of video mux. Fringe current lines show likelihood of crosstalk; leaded parts break up ground plane and squeeze the current lines together.
(Click on image to enlarge)

Tamara : You mean you didn't use any surface mount devices, so the ground plane was full of holes for the leaded parts?

Dave : Yes. Most of the return currents from the inputs flowed in narrow bands around the circuitry. There was a lot more resistance than in a solid plane.

Tamara : So the fringe currents were crowded together even more.

Dave : Right again. There was a lot more crosstalk than you would imagine. Surface mount parts help this a lot, since they have fewer holes, but splitting the plane judiciously is free and easy, and it eliminates the problem whether you have a lot of vias or not, Figure 2 .

Figure 2. PCB of video mux with slices in ground plane to minimize crosstalk.
(Click on image to enlarge)

Tamara : Free, easy and effective–sounds like a good general practice.

Dave : That's what I've always thought. What else have you got there?

Tamara : I've been contacted by representatives from two capacitor companies, X2Y and KEMET.

Dave : And what do they think?

Tamara : You know how you were thinking we were fussing too much about bypassing?

Dave : Well, yeah, I mean they are just capacitors.

Tamara : They say we're not fussing enough. We're only looking at two dimensions. They consider even the side view of the INSIDE of their capacitor.

Dave : Are they following the current path like we suggested?

Tamara : Yes, and they dropped the ESL by a factor of 5 from the standard value of about 2 nH by reducing the vertical enclosed area of the leads of their cap.

Dave : So even the experts follow the currents (sigh of relief). We are standing on the shoulders of giants (pauses, absently staring into the distance…).

Tamara : Dave. . . . DAVE. . . .

Dave : Oh, sorry So where are we following the currents today, Dr. T?

Tamara : I think we need to dig a little deeper and step through an example. I think our readers understand that the current path is important for placing their bypass capacitors, but might need an example. Let's look at exactly where the currents flow in a simple circuit. Let's look at the output of an op amp driving a load. Here's a simple schematic and board.

Dave : Okay, let's make it more interesting. How about a voltage reference for input bias levels?

Tamara : Figure 3 shows a single-supply op amp configuration with a gain of two.

Figure 3. Simple op amp and voltage reference circuit.
(Click on image to enlarge)

Dave : The voltage reference biases both inputs at half-supply to allow for optimal input range.

Tamara : This time we've chosen to use a 2-layer board for the layout. (The last one was 4-layer.) The second layer is an almost complete ground plane, just two jumps over the input and output lines, Figure 4 .

Figure 4. PCB layout of simple op amp and voltage reference circuit
(Click on image to enlarge)

.Dave : Let's trace the current paths. (Figure 5a and Figure 5b )

Figure 5a and 5b. AC and DC current paths in the reference voltage.
(Click on image to enlarge)

Tamara : People sometimes confuse the AC and DC paths, so let's put the AC high frequency paths in Blue and the DC paths in Green.

Dave : I'm going to go one step further. I'll identify the drive currents with solid lines since they travel mostly on the top of the board and the return currents with dotted lines since they predominantly travel on the ground plane.

Tamara : You da man!

Dave : You might think the reference supply was just about DC, but it is also part of the AC circuit in the amplifier. Check out the high frequency current path in the reference circuit.

Tamara : I especially like how the stack of passive components allowed you to cleanly bring in the input line and share a small ground pad among U2, R4, C3 and C5.

Dave : And it didn't keep me from constructing a close (well-placed) feedback path through R3 to that input network.

Tamara : The high frequency path is short and tight with loops through both the output bypass capacitor (C5) and the reference bypass capacitor (C3). I'm guessing that's why you put C3 down near the amplifier (U2) instead of up by the reference chip (U1). They even share an extra ground connection on the top layer.

Dave : That's it! We want to keep the high frequency currents contained small enclosed area means small inductance. For contrast check out the DC currents.

Tamara : They spread out all over the board and even seem to leave the top of the board.

Dave : Yep! The DC has to come from the power supply, which means it enters and exits through connectors or finds its way to a local supply regulator. In either case the paths are large.

Tamara : Which is why we use bypass capacitors in the first place: to keep the high frequency currents local and shunt the inductive and resistive paths that would cause a lot of unwanted voltage drops.

Dave : Now take a look at the output currents from the amplifier. (Figure 6a and Figure 6b )

Figure 6a and 6b. AC and DC current paths in the op amp.
(Click on image to enlarge)

Tamara : Once again it looks like the DC currents spread out toward the top of the board (where the supplies are connected) but the AC currents are in tight loops very local to the output amplifier.

Dave : The AC loop also doesn't loop back upon itself or cross itself except as it is spreading out on the ground. That is a good practice to minimize crosstalk.

Tamara : Dave, there are none of your famous cuts in the ground plane this time. Why not?

Dave : The signals don't really have a chance to interact. The signal flow is directly from left to right, input to output. We didn't draw the input current paths, so that could be an exercise left to the reader.

Tamara : But a cut in the ground plane, like in Figure 2, is most useful at corralling signals and keeping the fringes from interacting with each other.

Dave : Absolutely. But don't forget that this whole conversation started by discussing bypass capacitors.

Tamara : Yes, indeed. We can choose the right size, type and package of capacitor, but it won't be as effective if we don't optimize the layout.

Dave : It's probably the most important thing we can say: questions about placing bypass capacitors can almost always be answered by following the currents and minimizing the current loop areas. There's not much else to it.


About the authors
Dave Ritter grew up outside of Philadelphia in a house that was constantly being embellished with various antennas and random wiring. By the age of 12, his parents refused to enter the basement anymore, for fear of lethal electric shock. He attended Drexel University back when programming required intimate knowledge of keypunch machines. His checkered career wandered through NASA where he developed video-effects machines and real-time disk drives. Finally seeing the light, he entered the semiconductor industry in the early 90's. Dave has about 20 patents, some of which are actually useful. He has found a home at Intersil Corporation as a principal applications engineer. Eternally youthful and bright of spirit, Dave feels privileged to commit his ideas to paper for the entertainment and education of his soon to be massive readership.

Tamara Schmitz grew up in the Midwest, finding her way west with an acceptance letter to Stanford University. After collecting three EE degrees (BS, MS, and PhD), she taught analog circuits and test-development engineering as an assistant professor at San Jose State University. With 8 years of part-time experience in applications engineering, she joined industry full-time this past August at Intersil Corporation as a principal applications engineer. In twenty years, she hopes to be as eternally youthful as Dave.

1 comment on “A bypass-capacitor dialogue peels back the layers, Part 3: Continuing the discussion on layout considerations

  1. jsodifjoeif
    September 27, 2015

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