From: "Tom Faloon"
Subject: Re: Finding 0805 caps in UK...
Date: Tue, 22 Oct 2002 18:57:20 +0100
NNTP-Posting-Date: Tue, 22 Oct 2002 17:58:39 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
You need to find a way of making the capacitors bigger, otherwise stray
capacitance on the board will have significant effect. I would aim, like
John suggests, to make them 5 or 10 times bigger, and make the resistors
You didn't say what topology you are using. A different topology may ease
some of the problems.
I would suggest you try a Sallen & Key (VCVS) Topology. This is a nice
simple, fairly well behaved circuit, and if memory serves me right, for a
Butterworth filter, you will find that each stage has the same cutoff
frequency. (I am assuming you want low pass.) So all caps have the same
value, and all frequency determining R's have the same values. Start with 8
off 2K resistors, and 8 off 560pF (From f= 1/(2 * PI * R * C) You then need
to set the gain of each amp, to control the damping. Look up K in a
coefficient table, and calculate the gain. If you don't have figures &
calculations, I will dig some out for you. You can put the stages in any
order, but it's best to start with low gain stages, to minimise chances of
For an eight pole filter, component values and tolerances will be getting
tight, whatever topology you use.
Mike Deblis wrote in message
> Its a 140KHz, 8th order Butterworth filter using a MAX4334ESD quad
> op-amp. I used FilterWiz LE to calculate the filter, and it seems to
> insist on low C values (you can increase them, but that changes the
> inter-stage loading). Also, FW gives a suggested GBWP of 8MHz for
> stage 3, which seems a bit high (the other stages are <= 2.5MHz GBWP).
> Any ideas?