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Design >> Analog Design >> Gm-C Filter design ..
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Message started by sanredrose on Mar 5^th, 2008, 2:20pm

Title: Gm-C Filter design ..
Post by sanredrose on Mar 5^th, 2008, 2:20pm

The following are my requirements to design a Low Pass Filter using Gm-C / OTA-C Topology. Any ideas on how to go ahead with this ?

Low-Pass Filter:
50Hz corner frequency,
100Hz stop-band frequency
-50dB distortion
SNR = 50dB
Maximum total capacitance = 2pF

Saravanan T K

Title: Re: Gm-C Filter design ..
Post by topquark on Mar 7^th, 2008, 3:10am

Hello!
Ok, I see you're a new member.
This forum is to share ideas & exchange know-how and not a platter for getting your designs done!!

I've seen this happen too often when people just post their problems without giving it much thought on their own, worse even just posting specs to be met for the final design! :o

I'd recommend you (and this goes for all newbies) first come up with your design approach and the roadblocks you face in meeting some spec or simulation issues. Then, search with keywords over here (oftentimes, your question may have been raised by someone else before). If you still have some queries, post the topic addressing the specific question in mind not throwing around your whole problem to the forum!

Please bear in mind several talented designers take out precious time from their schedule to answer queries here.

I hope you apprecitae this.

-topquark

Title: Re: Gm-C Filter design ..
Post by loose-electron on Mar 14^th, 2008, 9:59pm

Time to start doing some research on gmC and filter topology issues! LOts of books on filters out there, and lots of JSSC papers on gmC filter circuits.

Otherwise, time to hire a consultant! ;D
I really dont think you are going to make the capacitance number, but total C plays off of noise so that is TBD.

Seriously, if you need help, let me know.

sanredrose wrote on Mar 5^th, 2008, 2:20pm:

Title: Re: Gm-C Filter design ..
Post by Saravanan T K on Mar 17^th, 2008, 7:43am

I did find papers relevant to Gm-C filter design but i could not find any material that covers the design aspect of Gm-C filters. I previously worked on Switched Caps and found Allen & Holberg to be an useful text ... But i couldn't find any for Gm-C filters ...

I have another question ... .I found people in my class using the Filter Design Toolbox of Matlab to obtain transfer functions for the filters. From my observation i find that its a toolbox for Digital filters like IIR / FIR ... Can that be used to design an analog filter ? Though it sound silly ... i am just curious to know what it is ...

Title: Re: Gm-C Filter design ..
Post by panditabupesh on Mar 17^th, 2008, 12:55pm

Matlab, even for a digital filter design, transforms the specifications to analog domain, designs an analog filter and then using bilinear transformation designs a discrete version. A Matlab discrete filter always has zeros at z=-1 (a consequence of bilinear transformation).

Bupesh

Title: Re: Gm-C Filter design ..
Post by loose-electron on Mar 17^th, 2008, 3:20pm

just getting the poles and zeros right, you can do that in Matlab or even on an Excel spreadsheet (phasor method is the easiest math i think) as for books on the topic? Not too much that I am aware of, some journal papers are about it. You got to develop a transconductor (think differential OTA or similar) and then a bi-quad (4 transconductors and 2 capacitors) and then you need a tuning method (I tune the -6db point using a stepping and comparison method), and you are done.

Title: Re: Gm-C Filter design ..
Post by pancho_hideboo on Mar 18^th, 2008, 7:16am

See doc/signal in MATLAB.
Then move to Category of Analog Filters.

In Signal Processing Toolbox, there are many functions useful for analog filters.
And I'm using optimization tool box.

You can build complex continuous-time integrator model in simulink.
Also you can build model for emulation of OTA-C in simulink.