Post by thread:[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 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: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@spam@mit.edu] >Sent: 11 October 2005 07:24 >To: piclistKILLspammit.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 person. Please contact us immediately to tell us that you have 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-JonesKILLspam.....bookham.com>: {Quote hidden}> > > > >-----Original Message----- > >From: EraseMEpiclist-bouncesspam_OUTTakeThisOuTmit.edu [piclist-bouncesspam_OUTmit.edu] > >Sent: 11 October 2005 07:24 > >To: @spam@piclistKILLspammit.edu > >Subject: [EE] Filter question > > > > > > > 1/( 2* PI * 1uF * 3.3M ) = 0.05Hz > > 1/( 1uF * 3.3M ) = 0.3 Radians/second > > Regards > > Mike > > >

1/2PiRC = 0.159/RC Try it for the hipass! ----- Original Message ----- From: "Julio Jumpa" <KILLspamjulio.jumpaKILLspamgmail.com> To: <RemoveMEpiclistTakeThisOuTmit.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 <spamBeGonepicdogspamBeGonedslextreme.com> wrote: > > 1/2PiRC = 0.159/RC Try it for the hipass! > > {Original Message removed}