Thanks Dave D. your reply is very helpfull.

Joe

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dave Dilatush
Sent: Monday, July 30, 2001 11:43 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Bandpass filters


Joe Denehan wrote...

>Can anyone suggest a site to learn more about bandpass filters (don't need
>to be very accurate, just for lighting effect). I'm trying to build an
audio
>spectrum analyzer and will be using Op-Amp based bandpass filters and
LM3915
>dot/bar display IC. Any help would be gratefully accepted.

Hi Joe,

If you want a decent, fairly compact introduction to active filters,
take a look at National Semiconductor's application note AN-779, "A
Basic Introduction To Filters...", which you can get at

  http://www.national.com/an/AN/AN-779.pdf

You've got several possibilities available for filters that might be
useful in what you want to do:

The so-called "Sallen-Key" filter design uses one opamp, two resistors
and two capacitors, which I think is about the minimum component count
you'll be able to achieve; however, this is basically a low-pass or
high-pass filter (which can be made to have a resonant peak at the
cutoff frequency by proper choice of components), and center frequency
and bandwidth cannot be adjusted independently.

The "multiple feedback" filter configuration uses one opamp, three
resistors and two capacitors and gives a true bandpass filter, unlike
Sallen-Key; but again, the center frequency and bandwidth adjustments
are interactive.

A third choice is the "state variable" filter, which takes three
opamps, two capacitors and seven resistors.  This has independent
adjustment of center frequency and bandwidth, as well as high-pass,
bandpass, and low-pass outputs.

A reasonable compromise might be the circuit below (set your viewer to
use a fixed-pitch font to view it, otherwise it'll look like
nonsense).  This arrangement uses one opamp, two capacitors and four
resistors, and its center frequency and bandwidth can be set
independently.

The opamp shown is one that I frequently use; it's a CMOS low-voltage
medium-bandwidth amp for general-purpose applications.  An LM324 would
do just as well if you have enough supply voltage, and it's a lot
cheaper.


                       +----[R2]----+
                       |    16K     |
                       |            |
                       +----[C2]----+
                       |   .01uF    |
                       |            |
                       |  |\LMC6484 |
                       |  | \1/4 U1 |
   IN ----[C1]---[R1]--+--|- \      |
         .01uF   16K      |   \     |
                          |    >----+---- OUT
                          |   /     |
                       +--|+ /      |
                       |  | /       |
                       |  |/        |
                       |            |
           +---[R3]----+----[R4]----+
           |   10K          6.2K
           |
          GND

   Center frequency is:

   Fc = 1 / (2 * pi * R * C), where:

   R1 = R2 = R and C1 = C2 = C.

   Bandwidth between -3db rolloff points is:

   BW = Fc * (2 - (R3/R4))

   Voltage gain at Fc is:

                 1
   Avc =  ---------------
             3 * R3
           --------- - 2
            R3 + R4

Hope this helps a bit...

Dave D.

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