Lorick, sorry for the late reply but I've been busy. I looked at your web
site to check out the circuit and the first obvious problem is that filter
is NOT a `band-pass' filter, but a `band-reject' filter!... Also, there is
no need to have three pots for offset. Just use one and a voltage follower.
Normally you would use a voltage reference and a follower.

   Out of curiosity, I ran a SPICE simulation with a Bode plot on the first
stage of the filter. Unfortunately, TI does not seem to provide text
versions of their SPICE models like their competitors, but they did include
it in the TLV2774 data sheet in PDF format. Lacking time, I decided to use
an LMC662. Sure enough, it had a `notch' around 300Hz... I was using 2Vrms
for an input centered at 330Hz with a +5Vcc.

   I would recommend getting rid of the current `analog glue' with the 10K
pots, and use a switched capacitor filter. Check National Semi, Linear Tech,
and Maxim for more info. You can easily replace your circuit with a filter
in an 8-pin package that has better specifications in all parameters. It's
been years since I last designed a classic Sallen and Key Op-Amp filter...
The current generation of switched capacitor filters come in a variety of
`flavors' and several vendors provide free software to help you design them.
I would say, in general, if you go beyond a 2'nd order filter, you should
look at an alternative to Op-Amps unless you have spares in a given package
and/or find a price advantage.

   - Tom

At 10:27 AM 3/8/00 -0500, Lorick wrote:
>I threw this message OT now that it's branched to the filter problem.
>
>0 to 3.7v common mode range, does that mean the difference between the 2
>inputs can't be greater than 3.7v?  I just woke up so I thought I'd ask
>before assuming wrong.
>
>What I'm putting in, according to the below schematic link, is a 2.5v offset
>on the non-inv input, and the inv input sees from -2.5v to 2.5v in an
>attempt to swing both ways in the single supply circuit.  Does that violate
>the 3.7v common mode range?
>
>I threw together a temporary page with the general form of the schematic,
>the calculations for the parameters, and the
>final circuit with the calculated values and cascaded general stages, and a
>SPICE bode plot.  So that's the circuit I simulated and built and it
>performed the exact same in the real world and computer, but I can't figure
>why the output is 0.5v with the TLV2774 when it is otherwise a calculated
>2.3v  with other general purpose op amps.
>
>http://rivendell.fortunecity.com/perilith/831/filter.html


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)