Notch filter using KHN biquad filter

[CaptainSpock]

Hi
Would like to ask if I can implement a notch filter using a KHN biquad filter (state variable), something like this **broken link removed**, at page 8. Meaning this would use 4 op-amps right? And what would the formula for the centre frequency be?

I'm actually designing a filter to eliminate 50hz ambient noise from a pulse oximeter system, open to any suggestions =)

Thank you.

 

[LvW]

As written in the paper you have to use a 4th opamp to sum all 3 outputs.
As the formulas show the center frequency is wo=1/RC.