What kinds of caps

[Electronman]

Hi Folks,

I want to design an active high pass filter to pass signals above 40kHz. It has to have a very high sharpness

What kinds of caps are suitable to do so?

Thanks

 

[Sceadwian]

Tantalum I believe would be a decent choice.

 

[Speakerguy]

Tantalum would actually be bad; they exhibit very high distortion when used in signal applications.

For signal applications, polypropylene film (excellent), mylar (very good) and NP0/C0G ceramics (very good) are what you want to use. Tantalum bad, high-K ceramics also bad (X7R, X5R, Y5V, Z5U, etc). Aluminum electrolytics are OK for DC blocking, but make sure they are a larger value than what the application actually calls for.

The high sharpness means you will need to use a high-order filter or a specialty type filter like an elliptical or chebychev.

 

[Sceadwian]

How do they exhibit high distortion when used in signal applications? I see them used frequently in audio frequency apps, what's the distortion you're talking about?

 

[Speakerguy]

The distortion is just that - distortion

**broken link removed**

https://www.electro-tech-online.com/custompdfs/2010/01/tantbench.pdf

http://greygum.net/sbench/sbench102/caps.html

http://greygum.net/sbench/sbench102/caps1.html

If -60dB SNR is OK in your app, then tants are fine. But that's a only 10 bits resolution. Polyester are much much cheaper and perform better. Of course you can't get them very large in value, but for a 40kHz high pass they will be fine.

 

[crutschow]

Tantalums may be ok for coupling in AC applications but they are a poor choice for deterimining filter characteristics since they typcially have a wide tolerance and are polarized. Speakerguy79's capacitor suggestions are good.

It you don't yet have a filter design you might try Filterpro from TI Active Filter Design Application - FILTERPRO - TI Software Folder.