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[DPRG] Question about electronic filters

Subject: [DPRG] Question about electronic filters
From: Kenneth Maxon kmaxon at qwest.net
Date: Thu Jul 31 21:32:32 CDT 2008

The wording on the supply voltage is a little suspect there.  5V Pk-Pk tends
to indicate that AC wave forms are being applied to the power rails of the
device, which I suspect is not the case.  Possibly, the power rails are 5VDC
and return or common at 0VDC?

If the above assumption is correct, then the 2'nd wording issue is 1V Pk-Pk
which would indicate an AC waveform as the input.

The part that needs to be addressed here is the configuration of the filter
to make sure that an offset is applied such that the output is not clipped.
A DC offset can be added to the signal being filtered with only a resistor
or two in the op-amp configuration, thus stopping the clipping problem.  The
interfering frequencies generated by the clipping effect can have an impact
on the efficiency of the filter and possibly drive it close to saturation.
The other answer is that you may be supplying both +5VDC & -5VDC to the
power rails of the amplifier which can also work depending on the device.

Lastly, if the power rails are correct, and one of many standard filter
configurations are used, the voltage levels at the output can be completely
independent from those at the power rails.   One needs more information
about the configuration of the supporting components (connectivity, type,
etc) before the question can be answered.   Most simple single stage filter
designs the output *IS* independent from the supply rails, and the gain
(near DC frequencies) is set by the external components and their
arrangements as well.   One of the parts to look at on the data sheet for
the op-amp is the bandwidth-gain product.  This will specify the derating of
gain vs frequency for approximately linear operation of any given op-amp.

Likely to get a full answer out of the group, you'll need to grab paint or
some other common tool, make a drawing of the schematic in use and post it
out on the web with a link to it for people to evaluate as there are many
thousands of ways to implement op-amp filter circuits and say ~10+ are quite
common for simple filters...

-Kenneth


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On Behalf
Of ray xu
Sent: Thursday, July 31, 2008 9:14 PM
To: dprglist at dprg.org
Subject: [DPRG] Question about electonic filters


I just have a general question about electronic narrow bandpass filters.
For example I have a filter with a "pass" frequency of 200kHz.  I build it
using an op-amp with its supply 5V Pk-Pk.  I input a 200kHz frequency with a
1V Pk-Pk.  Would the output be 5V Pk-Pk or just its original 1V?



___________________
Ray Xu
rayxu at tx.rr.com
DPRG member
OOPic group member
Seattle Robotics group member
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