## PSN-L Email List Message

Subject: Re: Filter input
From: ChrisAtUpw@.......
Date: Wed, 10 Mar 2010 06:52:41 EST

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In a message dated 10/03/2010, barry_lotz@............. writes:

Hi  All
I was using multipole LP filters with my seismographs. They use the  opamp
non-inverting input. The note with the filter design says that the input  "
must return to ground via a low-impedance dc path". I wanted to reduce in
half the input to the filter. I know the output of an opamp buffer would work
but if I used a voltage divider w/o the buffer how low of resistor would
be  considered "low impedance".

Hi Barry,

(http://www.ti.com)  called FILTERPRO.

The 'return to ground' is either the earth end of  the coil or the zero
level of the previous opamp. If you are using a  Sallen&Key filter and want
to reduce the input sensitivity, you can connect  a resistor 1/10 the input
filter resistor value to earth and put a >x10  resistor onto the junction.

Regards,

Chris Chapman

In a message dated 10/03/2010, barry_lotz@............. writes:
Hi=20
AllI was using multipole LP filters with my seismographs. They use=
the=20
opamp non-inverting input. The note with the filter design says that the=
input=20
" must return to ground via a low-impedance dc path". I wanted to reduce=
in=20
half the input to the filter. I know the output of an opamp buffer would=
work=20
but if I used a voltage divider w/o the buffer how low of resistor would=
be=20
considered "low impedance".

Hi Barry,

from=20
www.ti.com called FILTERPRO.

d of=20
the coil or the zero level of the previous opamp. If you are using a=20
Sallen&Key filter and want to reduce the input sensitivity, you can co=
nnect=20
a resistor 1/10 the input filter resistor value to earth and put a >x10=
=20
resistor onto the junction.

Regards,

Chris Chapman
```