Rank: Member
Groups: Member
Joined: 4/7/1999
Posts: 11
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Hi all,
I want to implement a 3pole Butterworth Sallen-Key filter. This I can achieve without too many problems.
Is it possible to combine this filter with a differential-to-single-ended op-amp to create a "differential sallen-key filter"?
I'm not sure how the feedback structures would work since one side would be positive feedback and the other negative.
Anyone heard of such an animal?
Max.
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Rank: Elite Member
Groups: Member
Joined: 7/13/1998
Posts: 347
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Hi Max,
You are attacking a problem that gave me many headaches some time ago.
From your problem description I gather that you want to make a diff-amp with one opAmp. This will not be a very good one, in terms of Common Mode Rejection. I know of no structure to make this a three-pole filter.
Anyhow, you have to ask yourself what you want to filter: the dif-signal, the common mode signal or both.
In the well known three opAmp realisation of an Instrumentation Amplifier it is easy to insert one pole (caps over both feedback resistors of the input stage), but this only filters the dif signal, the common mode signal is unaffected.
Do not make the mistake to 'mirror' the Sallen-Key filter around both positive and negative input: this is not stable. (see HP journal october 1994)
Common mode filtering can be done with RC or LC elements at the input. However, placing (filter)components at the input of an Instrumentation Amplifier causes almost always a degredation of the common mode rejection, because of component tolerances.
There are some ways to work around this, but this depends on the frequency and level of the dif-signal and the common mode signal and the required amplification and common mode rejection.
bye, Hugo.
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