Decoupling caps

[zachtheterrible]

Just a quick question that ive been pondering for a while: I've been told on a few circuits to use a 100uf decoupling cap and a .1uf decoupling cap. What is the point of using the two? Wouldn't 100uf be more than enough? I would just end up with 100.1uf right?

Or is there something more to this that I'm missing?

 

[ljcox]

Zach,
100 uF caps have high ESR and thus do not shunt high frequencies very well. You need the 0.1 uF to provide a fast response whenever there is a current pulse taken from the supply. Otherwise, unwanted coupling can occur between ICs causing strange results. The 100 uF simply provides a large reserve of charge and shunts low frequiencies such as mains ripple.

 

[Nigel Goodwin]

Zach,
100 uF caps have high ESR and thus do not shunt high frequencies very well. You need the 0.1 uF to provide a fast response whenever there is a current pulse taken from the supply. Otherwise, unwanted coupling can occur between ICs causing strange results. The 100 uF simply provides a large reserve of charge and shunts low frequiencies such as mains ripple.

Actually a 100uF has a much LOWER ESR than a 0.1uF, ESR is specified at 100KHz - it's only at much higher frequencies where the internal inductance of a 100uF will start to become worse than a 0.1uF.

It's good practice to include both, a 0.1uF is completely useless at low frequencies, and a 100uF is less effective at high frequencies.

 

[audioguru]

Many analog ICs must have a 0.01uF or 0.1uF ceramic disc capacitor across their power supply pins or they oscillate at a very high frequency.

Many logic IC's go beserk without a 0.01uF or 0.1uF ceramic disc capacitor across their power supply pins.

At high RF frequencies like the 100MHz of broadcast FM, even a 0.1uF ceramic disc capacitor has far too much inductance and might even be a resonant circuit by itself. Therefore a 1000pF ceramic disc capacitor is used for supply bypassing.

Many low dropout regulator ICs oscillate without a 100uF capacitor across their output.

Battery-powered low frequency circuits work better for longer with at least 100uF across the supply, because the battery's internal resistance increases as it runs down and the resistance causes voltage fluctuations without a bypass capacitor.

 

[Dean Huster]

If you're working with digital circuitry, decoupling is mandatory, especially if you're using clocked logic such as counters, flip flops and shift registers. There are some chips out there that are very sensitive to spikes on the supply lines and will reset or clock with a power spike. The 0.1µF disc ceramics will take care of most of that. I always install one 0.1µF disc ceramic cap for each chip, as close to the supply pins as I can, preferable from the positive supply to a ground plane. 0.01µF will usually work fine also.

Dean

 

[zachtheterrible]

So the reason to use both is that 100uf will have high inductance at high frequencies and the .1uf won't?

thanks for the info :lol:

 

[audioguru]

So the reason to use both is that 100uf will have high inductance at high frequencies and the .1uf won't?

Correct.