Javascript® Electronic Notebook Third Order Sallen & Key Lowpass Actve Filter by .
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The intent of Low-Pass filters are very much the same. And that is to pass signals that are below the defined cutoff frequency and suppress signals that are above the defined cutoff frequency. How well it does this, is a function of the Filter-Order.
With Low-Pass filters, the tradeoff between using a low order and high order filter is in the leakage from the stop band and the time domain artifacts (e.g. ringing, which is a oscillation that occurs in response to a step input). Low order filters (first and second) don't exhibit a lot of ringing, but can have leakage from large signals in the stop band. On the other hand, higher order filters (third and fourth) will have much less leakage from the stop band, but will exhibit more ringing.
Below is the diagram of a Third Order Sallen & Key Lowpass Actve Filter. I don't intend to provide a indepth explanation of the filter. There are plenty of other web sites that describe filters indepth. But here is a brief overview. The first Op-Amp stage (U1A) is a buffer (Gain = 1) that isolates the input audio source from the filter section. Resistors R6 and R7 set the bias level to half the supply voltage, at the "Positive" input for the Op-Amp. The Op-Amp output is fed back to the "Negative" input. Since the Op-Amp output will always adjust and make the "Negative" input follow the "Positive" input, the output will also be at half the supply voltage.
The second Op-Amp (U1B) is onfigured as a VCVS (Voltage Controlled Voltage Source) two pole filter made up of R2, R3, R4, R5, CB, and CC. Between the input buffer and the VCVS two pole filter is a Single Pole RC filter made up of R1A and CA. R1B effectively sets the input impedance of the filter stage and reduces the input voltage. Input capacitor (C1) and output capacitor (C2) serve as DC isolators for the overall filter and form a low frequency High-Pass filter. With the components specified (1 uF), the the low frequency response will start rolling off at around 16 Hz.
The values on the schematic are for a filter with Butterworth response that is 3db down at 750 Hz, and uses 0.001 uF capacitors for CA, CB, and CC. Using the entry area above, adjust the values to suite your requirements. The resistors listed on the schematic are standard resistor values based on the Resistor Tolerance selected. The lower the tolerance selected, the closer the filter will be to your design goals.
The chart is actually created from a series of straight lines. So it may look a little lumpy with some design values. But it gets the idea across.
