Caps in IC Design - RF Cafe Forums
Post subject: Caps in IC design Posted: Sun Feb 26, 2006
Joined: Sun Feb 26, 2006 1:56
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?
Post subject: Posted: Sun Feb 26, 2006 3:05 am
Joined: Mon Jun 27, 2005 2:02 pm
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:
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.
Post subject: Now, about inductorPosted: Sun Feb
26, 2006 4:26 pm
Joined: Sun Feb 26, 2006
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?
Post subject: Posted: Sun Feb 26, 2006
Joined: Mon Jun 27, 2005 2:02
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:
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.
If Rdc (DC resistance) is high then Q is low. The same applies
for capacitors only, instead of Rdc there is ESR.
subject: Posted: Sun Feb 26, 2006 5:53 pm
Joined: Sun Feb 26, 2006 1:56 am
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
Post subject: Posted:
Mon Feb 27, 2006 4:03 pm
Jun 27, 2005 2:02 pm
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.