Active 3-pole single stage filters need only one op-amp instead
of two. But they are difficult to dimension because mathematics
is by far not that straight-forward as for the usual double-stage
ones. It took me a couple of days or even weeks to understand
can quickly dimension active 3-pole single stage Sallen-Key low-pass
filters. The explicit intention of this
page is to prevent semiconductor companies from selling millions
Restrictions: In contrast to my Active
Low-Pass Filter Design and Dimensioning this utility is limited
to filters with a stage gain of 1 (with one exception, see below), Chebychev filters are available in few
standard ripple values only and the Sallen-Key topologie is available
Filter orders > 3: It is possible to dimension here
the "entrance stages", i. e. stages 1 and 2 of filters
with up to 9 poles (or 5 stages respectively). The remaining stages
may be dimensioned using Active
Low-Pass Filter Design and Dimensioning.
Method of evaluation: The evaluation of the component
values is based on a set of several hundred of pre-evaluated resistors
and capacitors values normalized to unity frequency. They are
scaled at run-time to the desired frequency and to the users impedance
requrements. All capacitor values in the table are selected from
the E6-series and so that they are as close as possible to each
other. If C2 and C4 differ from each other,
C2 was chosen to be the smaller one so that the input
impedance is kept higher. (Note: C2 and C4
may always be permuted, but different resistor value sets, not
shown here, result in these cases.)
Filters with 3 equal capacitors:
Often it is desirable to use 3 equal capacitors. For gains = 1
this is not possible, but for gains = 2 it is. You may enter your
desired capacitor value and get three resulting resistur values.
If you like to learn to know a mathematical way to evaluate
the component values have a look at John-Paul Bedinger's "3-Pole Sallen-Key Butterworth Active Lowpass Filter:
How to use: Enter the desired filter parameters and
find the resulting component values below. Increase or decrease
"Resistor Scaling" if the resultant resistor values
shall be higher or lower. Adopt either the primary or the alternative
resistor value set. Both result in the same frequency response.
The advantage of the primary resistor value set is its higher
input impedance and all 3 resistor values are closer to each other.
Disclaimer: This utility is based on a table of several
hundred of more or less manually modified and edited values. Though
I successfully tested some filters with random parameters and
I believe all other values to be correct, this whole process is
rather prone to error, and it would take several days for an appropriate
test. Therefore I recommend to do simulations prior to trust the
results evaluated here. No responsibility for any kind of errors
or bugs is taken from anybody.
This page has been tested with MSIE 6 and Netscape 6.2.
Back to Active Filter Design and Dimensioning
To abbreviate atto, femto, pico, nano, micro,
milli, kilo, mega, giga and tera use a, f, p, n,
u, m, k, M, G and T
Look at Active
Low-Pass Filter Design and Dimensioning in order to dimension
the remaining stages of filters with orders > 3.
Enter the desired value for C in the Filter
Parameter Table above