Searching \ for '[EE] Filter question' in subject line. ()
Help us get a faster server
FAQ page: www.piclist.com/techref/logic/dsps.htm?key=filter
Search entire site for: 'Filter question'.

Exact match. Not showing close matches.
'[EE] Filter question'
2005\10\11@022400 by

Hello,

I need to build an amplifier very similar to this one i found in a book
(Webster, 1995)
http://img111.imagevenue.com/img.php?loc=loc256&image=c55_webster.jpg

Mine needs to have a BW (-3dB) from 0.05 to 500Hz.

According to this book design, the 1uF and 3,3M resistor forms a high pass
filter at 0.05Hz, but 1/RC = 0.3Hz.

Also, the low pass filter after that (150K and 0.01uF) is said to be at
100Hz, but 1/RC = 667Hz.

Am I getting something wrong?

Thanks
Julio
I need to build an amplifier very similar to this one i found in a
book
(Webster, 1995)
http://img111.imagevenue.com/img.php?loc=loc256&image=c55_webster.jpg

Mine needs to have a BW (-3dB) from 0.05 to 500Hz.

According to this book design, the 1uF and 3,3M resistor forms a high
pass
filter at 0.05Hz, but 1/RC = 0.3Hz.

/ He says "passes all fequencies above 0.05 Hz". "Passes" is a rather
loose term. A single pole filter has a "roll off" of 6 dB per octave
and 0.33/0.05 ~=6 or about 2^2.5 so loss at 0.05 hz would be *about*
15 dB (voltage).
/

Also, the low pass filter after that (150K and 0.01uF) is said to be
at
100Hz, but 1/RC = 667Hz.

/
At the top end he seems to have hedged his bets in the opposite
direction and has placed the cutoff frequency a factor of 667/100 ~=6
times outside the passband.
/

Am I getting something wrong?

/
Probably not - I suspect his words and his circuit don't match. He may
be an order of magnitude out (or two) in his words.

Question: WHY do you meed 0.05 Hz lower frequency? - that's 20 seconds
per cycle.
His original circuit is in the context of an ECG amplifier. It seems
very unlikel;y that any 1/20th of a Hz components would be significant
there.

What is you application.

It's easy enough to build the circuit shown and then experiment.

I must say [IMHO of course :-) ] that that's a nasty looking webpage
hoster you've got there :-(.

RM

>His original circuit is in the context of an ECG amplifier.
>It seems very unlikel;y that any 1/20th of a Hz components
>would be significant there.

It is significant, in that you need to limit the DC response to minimise the
muscle artefact offsets you see in the signal, but you also need to have a
close to DC response to get the required heartbeat signal, and see problems
with it.

>-----Original Message-----
>From: piclist-bouncesmit.edu [piclist-bouncesmit.edu]
>Sent: 11 October 2005 07:24
>To: piclistmit.edu
>Subject: [EE] Filter question
>
>
>Hello,
>
>I need to build an amplifier very similar to this one i found
>in a book (Webster, 1995)
>http://img111.imagevenue.com/img.php?loc=loc256&image=c55_webster.jpg
>
>Mine needs to have a BW (-3dB) from 0.05 to 500Hz.
>
>According to this book design, the 1uF and 3,3M resistor forms
>a high pass filter at 0.05Hz, but 1/RC = 0.3Hz.
>

1/( 2* PI * 1uF * 3.3M ) = 0.05Hz

1/( 1uF * 3.3M ) = 0.3 Radians/second

Regards

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

Thanks.

I had completely forgotten about the 2pi factor...It's been quite a few time
since the last time I saw poles and zeroes

Regards
Julio

2005/10/11, Michael Rigby-Jones <Michael.Rigby-Jonesbookham.com>:
{Quote hidden}

>
1/2PiRC = 0.159/RC  Try it for the hipass!

----- Original Message -----
From: "Julio Jumpa" <julio.jumpagmail.com>
To: <piclistmit.edu>
Sent: Monday, October 10, 2005 11:24 PM
Subject: [EE] Filter question

Hello,

I need to build an amplifier very similar to this one i found in a book
(Webster, 1995)
http://img111.imagevenue.com/img.php?loc=loc256&image=c55_webster.jpg

Mine needs to have a BW (-3dB) from 0.05 to 500Hz.

According to this book design, the 1uF and 3,3M resistor forms a high pass
filter at 0.05Hz, but 1/RC = 0.3Hz.

Also, the low pass filter after that (150K and 0.01uF) is said to be at
100Hz, but 1/RC = 667Hz.

Am I getting something wrong?

Thanks
Julio
I heard a Feynman anecdote (this one is supposedly true) that has Dr.
Feynman teaching an intro physics class and he writes an equation on the
board. He leaves off the 2*Pi factor but a student catches his mistake. Dr.
Feynman does this 3 or 4 times and each time the same kid corrects him. He
is rewriting the equation and as soon as he hears "Dr. Feynman....." from
the class he says "F*ck the 2 Pi" without looking.
Maybe that should be OT? But I just physics class anecdotes.
James

On 10/11/05, Michael Hagen <picdogdslextreme.com> wrote:
>
> 1/2PiRC = 0.159/RC Try it for the hipass!
>
> {Original Message removed}

More... (looser matching)
- Last day of these posts
- In 2005 , 2006 only
- Today
- New search...