# Caps in IC Design - RF Cafe Forums

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marko
Post subject: Caps in IC design Posted: Sun Feb 26, 2006 2:00 am

Lieutenant

Joined: Sun Feb 26, 2006 1:56 am
Posts: 3
hello....someone asked me this question and i wasn't sure about the answer. for a dc block why not pick the smallest possible capacitance value because at dc no matter what the value is the cap will be an open. the only answer i could think of was that if you pick a very small cap value and depending on your frequency of operation you have unwanted ac voltage drop across the blocking cap because small cap. value will give rise to higher impedance. is there any other reason?

marko

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IR
Post subject: Posted: Sun Feb 26, 2006 3:05 am

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello marko,

For DC block capacitors, the desired thing would be a low insertion loss for the AC signal. Therefore if you will choose the lowest possible value you will get the exact opposite.

The reactance value is given by:

Xc=1/(2*pi*f*C)

For most applications a reasonable value of reactacne is between 3 to 5 ohms. When you choose the capacitor value, it is of course most important to consider the frequency of operation, ESR and the SRF of the capacitor. You should be below the SRF of the capacitor, or else the capacitor might act as inductor.

There are many technologies of capacitors: Y5V, X7R... They differ from each other by temperature range, ESR value, SRF etc.

_________________
Best regards,

- IR

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marko
Post subject: Now, about inductorPosted: Sun Feb 26, 2006 4:26 pm

Lieutenant

Joined: Sun Feb 26, 2006 1:56 am
Posts: 3
thanks IR for your reply....one more question about passives in IC design. i know the definition of Q in English but don't completely understand the concept, especially this point: lower Q means more lossy inductor--does this mean that if you have a very low Q then the current entering one port of inductor won't equal current at the output because the lower Q means higher resistance and capacitive coupling to substrate (current going into the substrate)? is that essence of Q: that there would loss of current?

marko

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IR
Post subject: Posted: Sun Feb 26, 2006 4:41 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello marko,

Just a quick overview about Q Quality Factor:

There are 2 definitions:

1. Q defined as the ratio between the energy dissipated as heat to the total energy stored in the sine wave.

2. The more modern definition, which is valid for band-pass and band-stop filters:

Q=Fo/BW

The first definition is more of the unloaded Q and can be used to answer your question:

Very low Q means that the ratio of the energy being transferred through the inductor to the energy being dissipated as heat in the inductor is high. That menas that the inductor dissipates the AC energy in the form of heat due to the internal DC resistrance of the wire.

Q=Xl/Rdc

If Rdc (DC resistance) is high then Q is low. The same applies for capacitors only, instead of Rdc there is ESR.

_________________
Best regards,

- IR

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marko
Post subject: Posted: Sun Feb 26, 2006 5:53 pm

Lieutenant

Joined: Sun Feb 26, 2006 1:56 am
Posts: 3
hi IR...sorry i'm bugging you...so would you say there's loss of current? i mean the heat dissipated has to be because current is turned into heat from the series resistance and hence there will loss of current...is that right?

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IR
Post subject: Posted: Mon Feb 27, 2006 4:03 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello marko,

There is no such thing loss of current. There is loss of Enrgy and power. Energy is being dissipated in the coil because of its associated Rdc.

By adding this resistance ot the circuit, the current through the circuit is being reduced. If the Q is high, then the power dissipation through the coil will be reduced and more current will flow through it.

_________________
Best regards,

- IR

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