[SI-LIST] Re: clocks,caps&hspice!
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Sun, 8 Jul 2007 19:06:09 -0400
General answers to very general questions:
> Is the 'clock signal' driven by a buffer?
Yes, clock signals are driven by some sort of buffer. If there were no
buffer, there would be no signal. Every oscillator uses an amplifier or
buffer to work.
If you mean, is there another buffer beyond the one in the oscillator?
Maybe. Maybe not.
> Why do we use capacitors as loads, wont they load the driving ckt like
> resistors?
I don't normally use capacitors as loads, but occasionally (rarely) find
that they help eliminate a problem. Sometimes they can help filter high
frequency components from a signal, and slow down the edges; but this can
backfire too so do it with caution.
Simple answer: no, capacitors don't act like resistors, but they do increase
output current from the buffer, like resistor loads do. The current tends
to be phase shifted compared to the current into a resistive load.
Data sheets specify capacitor loads on many of their test circuits, because
it's easy to make test circuits that way. The data sheet is often written
to replicate what is easily testable (using ATE, automatic test equipment),
not how the part should be used in practice.
> And in hspice does the simulator automatically connect an infinite
> resistor
> to open ends?
If it adds a resistor, it is probably not an infinite one, it is probably
one determined by GMIN or some other parameter. Some SPICE programs add
them, some don't. Try it, see what messages Hspice gives you.
Andy
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