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Power Supply Decoupleing

aka Proper Bypass Techniques

David VanHorn says:

Question: I've noticed over the years that the 'standard' has shifted. 25 years ago, you were supposed to put a 0.01 uF ceramic across power on TTL packages that generated incredible noise and used incredible power. Then the recommendation shifted to 0.1 uF ceramic or tantalum. Then in 2001 the recommendation shifted to 1 uF. Now we should have a combination of a high value and a small value. People are including large filter capacitors on the outputs of regulators.

If board space and cost are key issues, why include multiple capacitors? If package noise and power usage have dropped dramatically, why require more bypassing than 25 years ago? A 7805 regulator was only good to +/- 5% 25 years ago, but now its +/- 1%. Why do we then need more, not less, bypassing?

Answer: Its the logic type familys. CMOS switches from rail to rail very quickly, requiring a short pulse of current. CMOS inputs are also referenced to the supply voltage so if the supply drops because of something switching, so does the logic0/1 threshold. TTL on the other hand, has thresholds set by transistor junctions so it is less susceptable to this.

As the switching speeds increase, the need to get that current pulse out of the cap becomes more important. Big caps just can't do it so a compromise is required. The little caps can supply the needs of the chip but also need to be charged up again and fairly quickly. The next capacitor up the chain can be larger and a bit slower to provide that function. Those caps in turn, need to be topped up and if the regulator isn't fast enough, yet another cap can bridge the gap.

If space is at a premium, you can replace the large, low frequency electolytic caps and the small high frequency ceramic caps with a single (or fewer) Tantalium cap(s). They are able to react faster than the big caps and have more capacity than the small caps. Be aware though, Tantaliums are not nice components...see Capacitors

You want the caps to absorb well at the third harmonic of the clock. 0.1uF does well at 3 MHz, 0.01 at 30. 0.001 at 300. It's a broad response, so dont think that there's one specific value. However, if you use 0.1uF on a 20 MHz part, you won't get the supression that you could if you used 0.047uF.

Russell McMahon says

Practical capacitors are unfortunately not purely capacitive nature. They also have inductive and resistive components. Larger value capacitors generally have higher internal inductance and lead inductance than smaller valued capacitors of the same type. Consequently the impedance of a given capacitor will generally have a a minimum at a certain frequency and the impedance will be higher and both lower and higher frequencies.

The rule of thumb that was mentioned above is in the order of correct for ceramic capacitors. ie The "correct" value with short leads for decoupling in the MHz plus region is around 0.1uF. As the frequency rises a smaller capacitor will be optimum. The excessively enthused can even specify capacitors based on the series resonant combination of lead lengths and capacitance.

[the] ARRL handbook gives these figures for series resonance (optimum bypassing) for disk ceramics with total lead lengths of 0.5 inch.

Cap uF  Freq MHz

0.01      15
0.0047    22
0.002     38
0.001     55
0.0005    80
0.0001    165

Sounds like we should be using 100 pF decoupling caps with 100 MHz Scenix's ! :-)

RF practice (and serious microprocessor practice in some cases) is to group several capacitors of different values together to combine the characteristics of each = effectively a rather broad bandpass filter. Use of small ceramics and larger valued distributed electrolytics (tantalum for the brave, solid aluminium for the wise, wet electrolytic for the adventurous) can be useful.

Place the cap at the ground pin, and route power to the cap first, and from there to the chip. Otherwise, you compromise the bypass. Murata and panasonic make through-hole 3 leaded caps which help eliminate this by having ground, in and out leads (I and O are reversible)

Motors Solonoids

Peter Cousens says

A trick I use on RF boards is to have non plated through holes included for small decoupling caps of around 100pf. I put surface mount caps (805) through the hole and solder it on the top and bottom, one of which will be the ground plane.

see Capacitors, Gotchas@, Noise@

see also:




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