Cascade active filter design

Download Filterpro from TI...it works great!  Low Q sections should precede higher Q. This reduces peaking/saturation issues.

Capacitor type is important.  Use high quality dielectrics to avoid nonlinearities.

http://www.ti.com/tool/filterpro

Tim

Gayartri,

Tim is correct. Another, more capable source of filter design can be found at WEBENCH Filter Designer. You will find good functions available right now beyond FilterPro. In this software you will get help in selecting the correct amplifier for your filter as well as real R / C component suggestions.  

As for capacitive types, I recommend the NP0 or C0G types for the lower valued caps and X7R for caps above 1 uF.

Out of curiosity, what type of filter are you designing? Is it low pass, high pass, bandpass, or bandstop?

Bonnie...I'm just old school. :)   I still use FilterPro 2.0 although FilterPro Desktop does have some nice features. Beats the MathCad program I wrote for computing values.

But definitely suggesting Op Amps is helpful.  With so many amplifiers available it can be overwhelming to try and figure out the "right" one for an application.  That's why I have about 8 favorites I juggle depending on noise, bias current, bandwidth, power, or size.

Thanks!

Tim

Tim,

Actually, the Filter Designer software has FilterPro as the calculating back engine. Knowing this fact, you might think about using the web version. Think of it as an enhancement to FilterPro instead of abandment. Additionally, this software provides simulation results for closed loop, sine, and step responses. Another feature just added is the capability to export TINA-TI or Altium simulation models so you can perform more simulations beyond what is offered in the Filter desinger software.

So, to summarize. Filter Designer will offer you:

• Equivalent FilterPro calculations
• Beyond FilterPro

• single supply and dual supply circuits available
• suggestions of appropriate real amplifier(s)
• suggestions of appropriate real resistors and capacitors
• actual simulations of the real circuit that you have designed
• exportable TINA-TI or Altium models for futher simualtion

Give it a shot, play around, and tell what you think

Will do!  I guess even I can learn new things.

Thanks Bonnie!

Tim

Hi,

I am designing active low pass filter with KRC filter designing technique with equal components.

Gain is unity.

I will be developing hardware based on it.

I have already designed one 4th order filter by taking capacitor values of 10nF for first stage and 1nF for second stage.

I wanted to confirm whether the design consideration has some capacitor selection criteria.

Gayatri,

I understand. Filter Designer does not support the the making of the capacitors equal. As stated above, I would recommend the NP0 or C0G types for the lower valued caps and X7R for caps above 1 uF for your capacitor types.

For you information, there are several blogs that I have written about the active analog filters. These blogs are:

1. Trying to design an active filter?
2. Analog Filters and Specification Swimming: Selecting the right bandwidth for your filter
3. Analog filters and specifications swimming: Input bias current makes a difference
4. My capacitor isn’t perfect – say it isn’t so!
5. Analog filters and specifications swimming: What, I thought I was getting rid of the noise?
6. Where capacitance coefficients matter
7. Analog filters and specifications swimming: Selecting the right slew rate

By the way, I am not familiar with the KRC filter designing technique. Do you have any information that could help me understand this approach?

KRC appears to be a Sallen-Key filter (I had to look it up).  I had only heard of the VCVS variant.

http://www.ti.com/lit/an/snoa369b/snoa369b.pdf

Tim

Tim and Gayatri,

Thanks, I now see what you are going. Actullay, Filter Designger has both the Sallen-Key and MFB circuit topologies.

Equal R equal C Sallen-Key design is often taught because it is easy to show with the math that f = 1/(2piRC) and Q = 1/(3-K) so a gain > 3 results in infinite or negative Q.  IIRC FilterPro won't let you set that condition, but I could be mistaken.  I also don't think this approach results in the poles being located correctly for a "standard" response (Butterworth or Chebychev).  But it does make for a quick and dirty design. 

Gayatri - Sallen Key filters also suffer from the non-ideal output impedance of the amplifier and the response can rise at high frequencies.  This can be minimized with component selection but it is common to follow a Sallen-Key filter with an RC filter.

Tim

Tim,

I agree that the design you described is a nice accedemic dicussion, not practical.

You mention the limitations of Sallen-key filters are you are correct. I have attached an article that does address this issue.