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Subject: Re: Finding 0805 caps in UK...
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 22 Oct 2002 15:52:11 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 17:52:11 MET DST
"Mike Deblis" schrieb im Newsbeitrag
> 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?
For an 8th order BW filter you have 4 stages with the coefficients:
a1=1.9616 ; b1=1 ; Q=0.51 ; f1/fa=0.661
a2=1.6629 ; b2=1 ; Q=0.60 ; f2/fa=0.829
a3=1.1111 ; b3=1 ; Q=0.90 ; f3/fa=1.206
a4=0.3902 ; b4=1 ; Q=2.56 ; f4/fa=1.512
You can see that the last filter stage has a very high quality factor and
needs therefore a higher GBW. the last column is the individual -3dB
frequency divided by the overall -3dB frequency.
Best would be to build the individual lowpass filters with inverting
multiple feedback circuit with 3 resistors and 2 capacitors. To compensate
for the different Q you could have some gain in the first stages and less
than unity in the last. Choose first the Cs: C2>=C1*(1-Ao)*4*bx/ax^2 and
start with C1 not below 15pF. Ao is the gain and should be negative
electronic hardware designer
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