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Sallen key problem stability

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daremore78

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Sallen key problem stability
« on: November 05, 2012, 06:57:43 AM »
Hello! I am developping a filtering board for analog signals (bandwidth of interest 300-5khz). I used for this purpose a lowpass Sallen key, a high pass sallen key (all butterworth configuration) and a passive RC low pass for adding a pole. I tried various Gain (3,5 , 5 ,8) on a breadboard and they all seemed ok but when I tried Gain = 10(10 for the first SK and 10 for the second. So the total is 100) I noticed a strange behaviour. (see the attachment - the comparison is with the Ltspice simulation). The strange thing is that if separated the two sallen key don't show any problem...
Could be this due to sensitivity of circuit parameters to tolerance components ?
Is maybe the impedance of the passivr RC too low ? It's only 143 ohm cause after that should be linked an ADC with impedance of 10khm
thank you

audioguru

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Re: Sallen key problem stability
« Reply #1 on: November 17, 2012, 09:29:38 AM »
A second-order Sallen and Key Butterworth filter is supposed to have a gain of 1 (a follower) when one component is double the value of the other or have a gain of about 1.6 when the components have equal values.
DO NOT try to have a gain more than that, add an amplifier instead.

There are 3rd-order Sallen and Key filters that keep the Butterworth response. The extra pole is at the input of the filter, not at the output and the other two poles are re-adjusted a little.

Opamps CANNOT drive a load as low as 143 ohms.

KevinIV

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Re: Sallen key problem stability
« Reply #2 on: November 19, 2012, 06:55:47 PM »
You may not want to use a Sallen and Key Butterworth filter unless you want this particluar response, which can be varied. An RC filter has 20db/decade attenuation. A gain dependant on frequency opamp filter circuit can produce 40db/decade ideally. More likely it will have better filtering with added distortion.

KevinIV

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Re: Sallen key problem stability
« Reply #3 on: January 23, 2013, 08:28:14 PM »
About the bandwidth of this filter, you wouldn't be able to use a resonant type. But what about a Sallen and Key filter with a gain of less than 1? A gain of 1 is okay with an input  of 20mV, maybe not for 200mV.

papagray

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Re: Sallen key problem stability
« Reply #4 on: February 03, 2013, 09:03:25 AM »
When you add gain to an active filter, the response sensitivity to errors in the component values is increased. That is why I try to keep the gain of an active filter < 3 and use a separate gain stage for gain. in your case you could add the single pole to the gain stage without any worries. Also the last opamp is seeing 220 nF through a 143 R which may be a problem. We don't know how accurate the simulation of a real part is, don't assume its perfect.