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Design >> Analog Design >> capacitor as a load
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Message started by skin_head on Jun 26^th, 2007, 1:22am

Title: capacitor as a load
Post by skin_head on Jun 26^th, 2007, 1:22am

hi all,

how will a capacitor work as a load on the output pin of any device??

Thanks,

skin_head

Title: Re: capacitor as a load
Post by aamar on Jun 26^th, 2007, 3:14am

Capacitor is a passive load element (consumes energy), you can look to it from different sides but all are consistent and lead to the same results at the end.

simply it is an impedance element which is frequency dependent, the higher the frequency the lower the impedance, so at very high frequencies it forms a short circuit between two nodes, and the output node of the device will be shorted.

If you study the stability of a system, together with the output rsistance of the device, it inserts a pole to the system you are designing.

In case of digital circuits, this capacitance causes a delay on the signal path, because this capacitance has to be charged, so a part of the energy will be descipated in it, instead of being used to switch the following gate or device.

These are 3 consequences of load capacitance which I have in mind at the moment.

Best regards,

aamar

Title: Re: capacitor as a load
Post by mg777 on Jun 26^th, 2007, 5:47am

If I may be permitted to pull out an old chestnut here: In a digital gate, it's not the capacitor that dissipates the energy, it is the ON resistance of the switches that charge & discharge the cap. Once this accounting perspective is kept in mind it's perfectly fine to say that a capacitor dissipates energy. Kinda like the switched cap resistor.

M.G.Rajan

Title: Re: capacitor as a load
Post by aamar on Jun 26^th, 2007, 8:03am

You have the full permition naturally.

But what I meant here is not the power dissipation in the form of currents flowing through the transistors from supply to ground or from output to ground or vise versa, what I meant here is an energy stored on the capacitance (unintentionally) because normally I would like not to have any capacitances at all, so storing this energy on the capacitance is even for me a dissipated energy.

Hope that you got what I mean. Normally if there is no capacitances the delays should be all zeros, this means that whole energy is directly subjected to the gate to turn it, as if you are pushing a box directly without inerfaces, or pushing it with a spring in between, so you have to give some energy to the spring to contract it before acting on the box, this is in this case an energy stored in the spring which will return back anywhere, but for you it is dissipated energy, which you would like to save, and it causes you only delay (but it can be used as memory too if it is required).

Best regards,

aamar