Question about design active filter

  • Thread starter samaaa
  • Start date
  • #1
48
0
hi:

i designed an active low-pass filter,but i don't know if it correct method or not

i want fc=200 Hz and n(order)=4. i use this method:

Active_lowpass_filter.png

by let C=100 nF and use this equation "fc=1/(2*pi*C*R1)"
i get R1= 8 Kohm,R2=R1
this is for one stage

to get filter of 4 order,i repeat that stage 4 times
http://uploadpic.org/storage/2011/M3Oveluhfm4grO0dEPhZiUv.png


so is this method correct?
 

Answers and Replies

  • #2
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073
You show four stages, each of 1st order. That will work for an overdamped system (low Q), but if you are designing for a sharp Q (lower damping and with overshoot in its step response), then you'll have to use 2nd order stages. Each comprises 2 capacitors and 2 resistors (at least), and can provide a high Q response (i.e., Q greater than 0.707). In practice, the professional designer faces a bewildering choice, each offering its own set of pros and cons. But for an introduction to the topic there are a few well-tried classics. Any general second-order filter stage will do the job nicely; you'll need two stages.
 
  • #3
AlephZero
Science Advisor
Homework Helper
7,002
293
i want fc=200 Hz and n(order)=4. i use this method:
...
so is this method correct?

No, because your first order filter has a gain of 0.5 at fc=200 Hz.

When you put the four filters in series, the gain at 200 Hz will be ##0.5^4## = 0.0625.

We don't know how much you already know about filter design, so it's hard to give much advice beyond "get a textbook and study it".

You could try Google for "design a low-pass Butterworth filter" (which seems to be what you were trying to do) but you might need to study some more basic ideas first.
 
  • #4
48
0
NascentOxygen & AlephZero thank you very much
 
  • #5
45
1

Related Threads on Question about design active filter

  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
1
Views
8K
  • Last Post
Replies
7
Views
15K
Replies
1
Views
4K
Replies
3
Views
3K
  • Last Post
Replies
5
Views
4K
  • Last Post
Replies
4
Views
5K
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
10
Views
3K
Top