analogfilterlow passoperational-amplifier
Having trouble to tell what kind of filter (Low-Pass, High-Pass, Band-Pass, Band-Reject(Notch) are present given a topology in cascade.
In the following picture I see 3 stages, is this ok? (2 active stages, 1 passive stage) but cannot tell if LP, HP, BP, or Notch filter is which one and why.
/!\Note: the values of resistors and capacitors showed are of no importance right now and are the values by default in circuit lab.
Yes, as long as the parallel capacitances sum to the capacitance of the replaced capacitor there is no difference in the circuit. This is typically done in order to get a non-standard capacitance value out of standard capacitance values. For example, if you need \$8.6\mu\$F then you can put two standard \$4.3\mu\$F capacitors in parallel to get it since \$8.6\mu\$F is not a standard value.
You need to take account of filter source impedance and filter load impedance: -
At high frequencies, the loading impedance of a filter on the signal source can be onerous. Typicaly for an RC low-pass filter, the impedance seen by the source will be "R" at high frequencies so, make sure "R" is big enough to not cause errors due to excessive loading. If an MCU pin is the source then it will be typically around 10 to 100 ohms output impedance. This means you should use a value of "R" that doesn't create a problem. Possibly 1 kohm to 10 kohm.
Given that the load might be 100 kohm, and "R" might be 1 kohm, there would be a potential-divider error of 1% and this may not be a problem. However, if "R" is 10 kohm then there will be a potential-divider error of 10% and this could easily be a problem.
So, choose "R" first in order to match the source and load impedances and if this cannot be adequately met then you need to consider an output buffer.
Clearly if a Sallen-key design is used you get an output buffer for free and, assuming the load is 1 kohm or greater, the op-amp will handle it. You still have to consider the input loading effects though because like the simple RC filter, at high frequencies the input impedance is the first resistor seen by the source (R1 below): -
Best Answer
In a purely academic sense, a bandpass or notch filter by itself must have an even number of stages so the roll off rate is the same on each side of the filter. This isn't always the case in the real world, but that is beyond the scope of this question.
A low or high pass filter can have an odd or even number of stages. The easiest way to determine filter type is by looking at the reactive components. Typically, in active filters capacitors are used exclusively. They are easier and cheaper to manufacture, and with tighter tolerances than inductors. Since the impedance of a capacitor goes to zero as frequency goes to infinity, a capacitor to ground is a low pass filter. In the case of the Sallen-Key topology, a capacitor to ground and in the feedback loop will form a low pass filter. As the others have mentioned, your circuit is a 5th order low pass.
A high pass filter is formed by switching the Rs and Cs.
There is also a Sallen-Key bandpass filter, but it's only first order roll off on each side.
Another common topology that is pretty handy to know about is the state variable. It isn't as dense as the Sallen-Key, but one circuit contains a high pass, low pass, and bandpass with the same critical frequency.