2015-01-15 – Windoze 10 no longer plays well with TI FilterPro 🙁 The program will run but then lock up toward the results. TI WeBench filter design tool is available on-line but still not as nice as the original.
Want to play with op amp filters? The TI FilterPro Desktop (Version 126.96.36.19946) is a nice tool to quickly design filters with. Note: You will need to register with TI in order to download the executable.
(Thank you Roy, K1LKY, for sending these updated links)
But below is a “cheat sheet” to quickly calculate values for the CALF. Clicking on the screen shots will show a larger view.
In this example we will make a 4 pole 2.2KHz low pass filter using 2 op amp stages and an overall gain of 1:
From the File menu select New for a new design (by default it will start with a new design)
Select your filter type: Click on Low Pass then click on the Next button.
Enter the following values:
Gain: 1 V/v 0dB (unless you need amplification or attenuation)
Passband: 2200 Hz – This is your cutoff frequency
Stopband: 5000 Hz
Stopband Attenuation: -45dB
Filter Order (AKA poles):
Check the Set Fixed box
Select 4 in the drop down. This will set the design to a 4th order filter using 2 op amp stages (2 poles / stage).
Select the filter type in the list at the bottom. I usually like Bessel low Q type designs. In this case select Bessel with a Q=.81 and click Next.
Select the Feedback type:
Select Multiple Feedback – Inverting (CALF) – my personal preference for designs
Other types for different designs:
Sallen Key – Non-inverting type
Multiple Feedback Fully Differential – Just as the complex name it describes.
You will be presented with a schematic, characteristic curves, BOM, etc.
At this point I will typically play with the standard tolerance values found in the upper right hand corner based on what is in my junk box. Select 5% or 10% in the drop down for resistors and 20% for capacitors to begin with. You can always narrow your results from there.
Adjust the R/C values. For example, maybe I do not have 22nF & 56nF capacitors but plenty of 10nF. Click on the “22nF” and 56nF values of capacitor C1, change it to 10nF, and hit ENTER. The other R/C values in that stage will change as needed.
Finally from the File menu select Save Design. Just in case you need to go back!
Try out your values on Spice or a breadboard, tweak as needed and enjoy your new filter!