Would this work?

JoeBob · 18th July 2003, 12:31 AM

would this circuit work for a low-pass circuit? Or would the cap after the input impedance setting resistor screw things up?

I ask because if I use a "normal" style PLLXO with a resistor followed by the cap in front of the input impedance setting resistor than the two resistors would make a voltage devider, I would obviously like to avoid that.

JoeBob · 18th July 2003, 02:06 AM

Oh common, no one even looked at it...

Konrad · 18th July 2003, 02:58 AM

Put the capacitor in paralell with the feedback resistor!

AndyM · 18th July 2003, 01:32 PM

The -input works as a virtual ground in this configuration so there is no voltage swing there (at least with an ideal opamp). So the capacitor won´t contribute anything and could be left out.
As Konrad said, put the capacitor in parallel to the feedback resistor and you´ll be fine.
For calculating the capacitor use the feedback resistor value, not the input resitor value (unless you make them equal for a gain of 1)

Ciao
Andy

JoeBob · 18th July 2003, 02:47 PM

Ah, I should have thought of that, thanks that helps alot.

millwood · 18th July 2003, 03:40 PM

JoeBob, it is one of those "active filter" designs that you can find pretty much in any op amp cook book.

18th July 2003, 01:32 PM	#4
AndyM diyAudio Member Join Date: Jan 2002 Location: Ingolstadt, Germany	No, it won´t... The -input works as a virtual ground in this configuration so there is no voltage swing there (at least with an ideal opamp). So the capacitor won´t contribute anything and could be left out. As Konrad said, put the capacitor in parallel to the feedback resistor and you´ll be fine. For calculating the capacitor use the feedback resistor value, not the input resitor value (unless you make them equal for a gain of 1) Ciao Andy

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18th July 2003, 02:06 AM	#2
JoeBob diyAudio Member Join Date: Nov 2001 Location: Montreal, Canada	Oh common, no one even looked at it...

18th July 2003, 02:58 AM	#3
Konrad diyAudio Member Join Date: Jun 2003 Location: way up north	Put the capacitor in paralell with the feedback resistor!

18th July 2003, 02:47 PM	#5
JoeBob diyAudio Member Join Date: Nov 2001 Location: Montreal, Canada	Ah, I should have thought of that, thanks that helps alot.

18th July 2003, 03:40 PM	#6
millwood Account Disabled Join Date: Apr 2003 Location: US	JoeBob, it is one of those "active filter" designs that you can find pretty much in any op amp cook book.

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