PSN-L Email List Message

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


 
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,
 
    You can download a FREE filter design program from  _www.ti.com_ 
(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 All
I 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,
 
    You can download a FREE filter design program= from=20 www.ti.com called FILTERPRO.
 
    The 'return to ground' is either the earth en= 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

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