Filter Design Example

Design a LPF that has the following circuit characteristics:
Overall gain = 10
Pass-band frequency = 1 kHz
Stop-band frequency = 10 kHz
Pass-band attenuation = -3dB
Stop-band attenuation. = -100dB


The bode plot above demonstrates the location of the pass-band and stop-band. For all filters, apass and astop represent the pass-band and stop-band attenuation. For low-pass filters, fpass and
fstop represent the pass-band and stop-band frequencies.

Filter Design Solution
The above LPF can be designed using Microchip FilterLab® filter design software, the figures below show the various design steps.

In the Filter menu, select Design.


Filter Design dialog enables the user to create a filter by specifying all aspects of the filter. Filter Specification tab enables the user to specify the approximation type, the selectivity and the gain. Select any approximation, the selectivity for the approximation, and the overall filter gain. Overall Filter Gain must be a number between 1 and 10.


Bessel approximations only support low-pass filters. Therefore, when the Bessel approximation is selected, the only available option will be low-pass.


To input filter Parameters, refer to the figure below. For further details, refer to FilterLab® 2.0 User’s Guide Chapter 1.
The pass-band attenuation must be in the range: -0.01 to -3
The stop-band attenuation must be in the range: -100 to -10
Frequency values must be in the range: 0.1Hz to 10MHz


The Force Filter Order option enables the user to specify the filter order or have the program calculate the filter order based on the dialog entries. Filter Order can be from 1 to 8. For the Butterworth filter, the attenuation above the cut-off frequency fc is a moderately steep -20dB/dec/order, which means 1st order: -20dB/dec, 2nd order: -40dB/dec, 3rd: -60dB/dec, etc.

For the Chebychev filter, it has steeper roll off above the cutoff frequency than Butterworth. This means it needs a lower filter order than Butterworth filter for the same attenuation above fc.

For a Bessel filter, the attenuation above the cut-off frequency fc is not as steep as Butterworth. This means it needs a higher order filter than the Butterworth filter for the same attenuation above fc. Bessel approximations only support forced filter orders.


Circuit Tab enables the user to modify the circuit topology and component values. See figure below. Resistor selection enables the user to change from 1% resistors to the exact calculated value. Changing the Resistor Selection affects all stages. Topology enables the user to change the topology for Low Pass selections. Changing the topology only affects the stage for the active tab. Band-pass selections only support Multiple Feedback (MFB) topologies, while the high-pass selections only support Sallen Key topologies.


The Capacitor Selection pull down enables the user to change the value of a capacitor from the default value calculated by FilterLab® 2.0. FilterLab® 2.0 automatically scales the other resistors and capacitors of the filter selection to maintain the desired filter specifications. Select “Automatic” from the combo box to automatically calculate the capacitor value. Select a value to force the capacitor to that value.


The Frequency View displays the filter response. The left axis displays the attenuation of the filter. The default left axis scale is +10 dB to -80 dB. The right axis displays either the phase (in degrees or radians), or the group delay. The frequency range is automatically set to three decades when the filter order is forced.


The Circuit View displays the current circuit for the specified filter, which consists of op-amps, resistors, and capacitors. For the Microchip op-amp selection, please refer to FilterLab® 2.0 User’s Guide Appendix E.


The Spice Listing View generates a spice model of the designed filter allowing time domain analysis in spice simulations.
FilterLab® 2.0 provides a net list of the filter circuit that can be imported to a SPICE simulator. The SPICE output of FilterLab® 2.0 and the Microchip op-amp macromodels are designed to be compatible with PSPICETM or other SPICE 2G6 circuit simulators. Other simulators may require translations.

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