[sdiy] Transfer function troubles

[Tim Stinchcombe]

Hi all,
	Before I delve into the depths of "The Circuit and Filters Handbook"
in search of an answer, I'm wondering whether anyone here can provide me
with a simple explanation of the following.

When trying to work out the transfer function for a flter I sometimes end up
with terms in my expression which are not factors of either 'Vout' or 'Vin',
and thus I cannot re-arrange to obtain 'Vout/Vin'. The example I'm currently
looking at is the 2nd order VCF at the end of the PA381/PA382 datasheet (fig
2b): if I assume that the input nodes of the 382 are virtual grounds I get
an expression for Vout/Vin from which I can derive the values of w and Q
that are given, and all is well; however if I work it with the nodes at the
proper levels of -2.1V, I get a couple of extra terms in 's', and a constant
term, which prevents me from forming Vout/Vin. What I want to know is what
is the physical significance of these extra terms in the time domain: are
they connected with the DC levels, some sort of 'initial conditions', or
what? And is there some reasonable 'excuse' for simply ignoring them?

Thus far I haven't found an explanation in even the 'intermediate level'
books I have, so any ideas or pointers/references will be greatly
appreciated!

Thanks,
	Tim

__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk