How many bypass capacitors?

Discussion in 'General Electronics Chat' started by Veracohr, Nov 14, 2013.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
    I've checked some threads here and various other sources regarding IC power supply bypass capacitors, but haven't found what I'm looking for.

    I understand the use of bypass capacitors, but can I get away with not putting them on every IC if I plan the layout well? The project I'm working on will have 16 analog IC's running on dual supplies, and if I go with the common 1 high-value & 1-low value bypass cap on each supply pin, that's a lot of capacitors. And that's not counting the other IC's I'll have.

    I plan to lay the circuit out such that the analog IC's will be sort of grouped together in clumps of 3-6. Do you think if I put them close enough, I could use one set of bypass caps for each group? They'll be SOIC packages, so they won't cover as much real estate as DIPs.

    Also, how hard is it to hand solder tiny surface mount chips? I looked at some MLCC packages and they were less than 1mm per side. I'd go with something through-hole, but from what I read, digital IC's are best bypassed with surface mount chips for lowest inductance.
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    I'll probably take a lot of flack for this...:eek:
    Unless the op amps (I assume they are op amps) are high frequency I think you can get away with protecting them from the digital circuits with say 4 sets. Modern op amps have great power supply noise rejection at lower frequencies. If they are current hogs you need to think about supplying them with some electrolytics. With the digital if you have power and ground planes (low inductance) you can also probably do with less.
    Surface mount caps are pretty easy, it's always the SO ICs that get me. I had a tech once that could solder them up like thru hole parts.

    I'm going to my room now. :rolleyes:
  3. WBahn


    Mar 31, 2012
    There's no easy answer to this question. Unless you are particularly sensitive to noise, you will likely be okay. Try to isolate the digital and analog supplies as best you can, using ferrite beads is one way to do this, but may be overkill for your application.
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    I'm a bit annal about bypass caps myself so I would be shooting for a .1 uf on each IC or pair of close grouped ones with a 1 - 10 uf low ESR covering the primary power feed traces for every few IC's.
  5. JohnInTX


    Jun 26, 2012
    Come out of your room. I was going to post essentially the same thing but it seemed like it was going to be a bunch of typing... But as indicated, for analog, you need a decent, quiet source that does not share its power with digital. So now begins the typing..

    For digital, the switching causes surges in the current requirements. A small, fast (ceramic) cap at each digital device (paying attention to loop geometries) is recommended to service the these pulse demands. Bigger caps (.1 - 1uf tantalum - something that can itself deliver some current fast) are located nearby on low-impedence (big) traces. Their function is to recharge the ceramics. A bigger cap may be located on the board to recharge the tants and so on.

    What keeps the caps from immediately delivering smooth power? Trace inductance (keep 'em wide and short, entire layers if possible) and capacitor construction - ceramics fast, AL electrolytics, not so much, tants - kind of fast with good recharge capacity.

    A good mixed design will have all of this (and sometimes more) but will have power conditioned by several stages of decoupling with low inductance traces to the source. Never share analog and digital power. Route them separately to a single point and decouple the heck out of that. Never share digital and analog power, ground or signals.
  6. crutschow


    Mar 14, 2008
    You don't need a high value and low value bypass cap at each analog IC but I would recommend a single 0.1μF ceramic cap at each power pin to ground. And of course it's preferred that the ground be an analog ground plane, separate from the digital ground plane (it could be a split ground on a single layer).
  7. tindel

    Active Member

    Sep 16, 2012
    186 capacitors

    Everything you wanted to know about bypass caps and more. Dr. Johnson is one of the best engineers on the planet. Read his stuff... there is more there than just bypass caps.

    Learn what you're doing... then you can decide if you can share bypass caps... a lot of variables factor in.
  8. MrChips


    Oct 2, 2009
    There is no simple easy answer that covers all situations. A lot depends on your specific application, IC packaging and board layout.

    You also have to look at the analog voltage levels, impedances and frequency.

    I understand that you are mixing analog and digital. That in itself calls for special consideration and board layout.

    If you are anticipating noise problems in the analog section, for example, a sensitive ADC circuitry, there are some rules to follow:

    1) Use a solid ground plane on one layer.
    2) For the most sensitive applications, use at least a 4-layer PCB and dedicate one layer for power and another for ground.
    3) Use SMD ICs and caps.
    4) If you do not have power and ground planes, make the power and ground traces as large and short as possible.
    5) Do not drive analog and digital ICs from the same power trace.
    6) Use separate analog and digital ground planes.

    As others have indicated, do not feel compelled to use low and high value caps at every supply pin. It depends on the layout and all of the above considerations. If the Vcc pins are close to each other a single 0.1μ cap may be shared between two ICs.

    A single 10μ electrolytic can be shared depending on all of the above considerations.
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Well... the first circuit I built had just two op amps, and it oscillated like a son-of-a-gun without bypass caps.

    If you are doing a PCB layout make room for each and every cap you thing you could possibly need. It is far better to have a spot with no part then to have to re-spin your board to add one.

    "1 high-value & 1-low value bypass cap on each supply pin" is on the extreme side, usually just one cap will work well. (That may be based on my work rarely getting out of the audio spectrum.)
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    Can you explain what you mean by "loop geometries?"
  11. JohnInTX


    Jun 26, 2012
    Search 'loop' and 'bypass capacitors' on the good Dr.'s page. Kennith Keenan's "Digital Design for Interference Specifications" is a good reference if you can still find it.

    In this context, it means that the circuit traces/wiring to the pins of the device from the bypass cap must be as low inductance as possible. It does no good to have a great bypass cap inches away from the chip through small traces as the trace inductance will limit the rate at which the cap can provide current to the chip. When this happens, the voltage at the pin that needs a boost will drop due to the reduced current through the trace. (We're talking nano-secs here). The solution is short, fat, preferably planar conductors for the power/bypass system. A good SMT layout will frequently have the bypass caps on the underside of the part for min. distances. Reducing the size of these 'loops' is also central to designing for minimum EMI from the board (small antennae radiate less than large loops).

    Not an expert by any means but that's the jist of it.
  12. Brownout

    Well-Known Member

    Jan 10, 2012
    Thanks. "Loop" is used in so many contexts when discussing layout, I wanted to be sure of how you were using it.
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Simple: look at the actual connections in the unit in question, keeping in mind "ground" (and any power connection in general) is not some magical zero impedance superconductor but a real wire with real resistance carrying real current. You put current thru it and one end WILL be a different voltage then the other.

    You can make your connections such that signals sneaking thru ground never appear in other circuitry by sometimes simple means like connecting all points labeled "ground" to one single point where that point is the well regulated and bypassed output of the power supply. That is what is meant by a "star" connection.

    Not every connection need be like that either (but it doesn't hurt). You can group similar items together and bring that group back to the star point. A general division into "analog" and "digital" domains is a very good place to start.
  14. Papabravo


    Feb 24, 2006
    If a trace on a PC board makes a loop it becomes an antenna that picks up incredible amounts of noise. You want to avoid loops at all costs, unless you are designing an antenna on the printed circuit board.

    oops... others beat me to it. What they said!