# Low pass filter hookup

Discussion in 'Electronic Basics' started by [email protected], May 18, 2008.

1. ### Guest

Hello,

I created a low pass filter in orcad and trying to put theory into
pratical. The circuit output is always showing the input at about one
third the pk-pk voltage even at 30kHz and more, this is suppose to be
a 3kHz lpf.. I think the wiring might be wrong.
I took the black pin of the function generator as the ground reference
point. Use two 6v batteries for positive and negative voltage and
place the middle of the batteries at the ground reference point from
the function generator.
Here is a link with my circuit and a picture:
http://www3.sympatico.ca/lerameur/LPF/
Or does any one has a specific instruction to a simple active LPF and
wirinf connection, maybe I can start from there.
I tested my OP Amp, and that is not the problem.

k

2. ### Bob EldGuest

As was mentioned, the LM339 is a comparator not an op amp. Use an LM 324, LF
347 or a TL084 or equivalent op amp. The 339 will probably oscillate if
used as an op amp and it needs a pull up resistor on the output of each amp
section. It ain't the right animal. Be sure to check the pin out, these amps
are very different than a 339.

Secondly there is no true dc path to ground for the amps except through the
generator. Place a 10K resistor from the input point on R5 to ground. This
will establish amplifier bias even when the generator is un-hooked.

Bypass the plus and minus power rails to ground with small caps, 0.1uF is
ok.

The circuit should work ok with proper amps as a six pole low pass filter
with 36db per octave cut off slope.

I calculate a cut off of 2kHz for two sections and 1364Hz for the first
section. This will be sloppy filter with a main cutoff at 2kHz. How did you
figure 3kHz?

Same here.

Graham

5. ### neon

1,325
0
Oct 21, 2006
linear filter means it must be linear a 10v bus will take the output to 5 and swing from therewant to get rid of the dc adda cap in series. and finaly an lm330 is a comparator and fast you will never get but square wave output .