Bypass capacitor

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jrcfg
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Bypass capacitor

Post by jrcfg » Sun Oct 10, 2004 1:25 pm

Is it always necessary to have a bypass capacitor across Vcc & Vss when using IC's? I have a circuit which includes op amps, buffers, touch sensor IC's, and various passive components supplied by a 78M05 regulator (500mA, +5v) which has the recommended in & out capacitors (.33uF in, .1uF out). The touch sensor IC data sheet recommends bypass caps as close as possible to the pins but the buffer (CD4050BE) and the op amp (UA741IN) data sheets don't mention bypass caps. I guess it would be good engineering practice to go ahead and use them. Obviously, I am not an engineer, just a newbie learning a new hobby. Thanks

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haklesup
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Re: Bypass capacitor

Post by haklesup » Sun Oct 10, 2004 4:12 pm

In general, Yes. Depending on the chip and where it is in the circuit, the cap either helps to prevent noise in a chip from getting on a supply line or from being disturbed by noise already on that line.<p>For either of these purposes, it is best to place the cap as close to the supply pins as construction permits. If a particular digital signal is disturbed by noise, you may also decouple that line to ground (close to the input, and only if signal is not too high a frequency or it will be filtered)<p>For most digital chips, a .1uF cap (ceramic) works fine. On larger boards, I also like to put a few 47uF caps (polarized electrolytic or tantalum) near where the power comes on the board for larger surges and lower frequencies (caps in series act as high pass filters with the value roughly indicating the cutoff frequency).

Dean Huster
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Re: Bypass capacitor

Post by Dean Huster » Mon Oct 11, 2004 4:04 am

Excellent reply. Our eyes tell us that the cap that's located just four inches from the IC ought to do the trick, but that four inches is a lot of inductance and resistance that is in series between that cap and the IC and it keeps the supply line too "soft" to catch all those fast transients. My worst offender was always the 74192/3 counter chips. If those things weren't bypassed, they'd do all sorts of crazy resets and parallel loads that you didn't want them to, not allowing them to roll over to zero or complete a count sequence.<p>You'll find 0.01µF used a lot on commercial boards. They're smaller than the 0.1µF although I tend to use the 0.1µF myself -- it's just habit.<p>You'll also often see commercial designs insert small resistors (4.7 to 10 ohms) in series between a supply and a section of sensitive circuitry with a 10µF and/or 0.1µF bypass cap on the load-side of the resistor. The resistor makes for a really steep low-pass filter the keeps circuit noises isolated to their respective circuits.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

jrcfg
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Re: Bypass capacitor

Post by jrcfg » Mon Oct 11, 2004 5:28 am

Great! Thanks very much for the good info guys.

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