Need help with sallen key bandpass filter

Thread Starter

alialiali

Joined Dec 12, 2008
26
Hi every one
I'm working on bandpass secound order filter and I have to have band pass between 23k and 53k using sallen key filters whaen I use the formola to dsign my high pass which should gave me 23k fc its working and I used the folwing calculation

R1 = mR, R2= R, C1=C , C2=nc FSF = 1 from the table
FSF * fc = 1/ 2pi RC squrt mn
m= 0.229 from the table
n= 3.3 from the table
I'm looking for LP for 53k
1*23k=1 / 2pi * (C=0.01u from th table) * (R which we need 2 find) * squrt 0.229*3.3
R = 1 / 2pi * (C=0.01u ) * (23k) * squrt 0.229*3.3 = 707 ohms
Since R = R2 so my R2 value is 707 ohms
R1 = R*m = 0.229 * 707 = 161.9 = 162 ohms
C1 = C which is 0.01u so the value of my C1 is 0.01u
C2 = n * C which is 3.3 * 0.01u = 33n
so my C2= 33n



and my LP filter which should gave me the fc=53k as

R1 = mR, R2= R, C1=C , C2=nc FSF = 1 from the table
FSF * fc = 1/ 2pi RC squrt mn
m= 0.229 from the table
n= 3.3 from the table
I'm looking for LP for 53k
1*53k=1 / 2pi * (C=0.01u from th table) * (R which we need 2 find) * squrt 0.229*3.3
R = 1 / 2pi * (C=0.01u ) * (53k) * squrt 0.229*3.3 = 345
Since R = R2 so my R2 value is 345 ohms
R1 = R*m = 0.229 * 345 = 79.005 = 80 ohms
C1 = C which is 0.01u so the value of my C1 is 0.01u
C2 = n * C which is 3.3 * 0.01u = 33n
so my C2= 33n


the main problem when I contact the output from my HP to the input of my LP filter the over all fc is not maching the fcs I looking for

can any one advice me how to conact the two filters together this can be using athered filter but my quation is do I need to change the holl calcultion and how?

many thanks
 

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bertus

Joined Apr 5, 2008
20,446
Hello,

Your resistors have a very low value.
What happens when you divide the capacitors by 10 and multiply the resistors by 10 ?
The frequency will be the same but the impedance will be changed.

Greetings,
Bertus
 

bertus

Joined Apr 5, 2008
20,446
Hello,

What I am trying to say is : recalculate the filters with capacitors of 1 nF and n* 1nF in stead of 10 nF and n* 10nF.

Greetings,
Bertus
 

Thread Starter

alialiali

Joined Dec 12, 2008
26
Ok I did that divide the capacitors by 10 and multiply the resistors by 10 I can see some difrernt but the problem now is I am getting 57.71kHz for the LP which it should be 53kHz and the HP= 20.84kHz which should be = 23kHz
please can you help me more
 

bertus

Joined Apr 5, 2008
20,446
Hello,

What kind of filter are you making ?
Where is the table you are talking about ?
Most sallen-key filters I know have equal resistors for the low pass and equal capacitors for the high pass.

Greetings,
Bertus
 

Thread Starter

alialiali

Joined Dec 12, 2008
26
Many thanks for your king reply

I enclod copy of the table so you can have look and gave me some advice

Table 1. Butterworth Filter Table
FILTER FILTER
ORDER
Stage 2
FSF Q
1.000 0.7071
1.000 1.0000 there is full table in the picture below
 

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Last edited:

bertus

Joined Apr 5, 2008
20,446
Hello,

The table does not look familiar to me.
Here is a link to a PDF from TI about reference designs.

focus.ti.com/lit/an/slod006b/slod006b.pdf

From chapter 16 filters are discussed, sallen-key from 16.3.

Greetings,
Bertus
 

Thread Starter

alialiali

Joined Dec 12, 2008
26
Thx a lot for the Pdf file but the problem it's very difcult 2 understand it can you please help me bit mor how 2 apply the formlu to my filter as I need band pass betwen 23k and 53k by uding 2 secound order sallen keys filter only
 
Last edited:

bertus

Joined Apr 5, 2008
20,446
Hello,

Here ia a PDF special on sallen key filters.
Chapter 3 is for low pass
Chapter 4 is for high pass
Each chapter has simplifications in it.

I used the calculator on the web to have the 23 kHz highpass filter
.http://www.pronine.ca/acthpf.htm



R1 = 4.89 k Ohm, R2 = 9.78 k Ohm and C1 = C2 = 1 nF

The same I did with the 53 kHz lowpass filter.
http://www.pronine.ca/actlpf.htm



R1 = R2 = 2.12 k Ohm , C1 = 2 nF , C2 = 1 nF


Greetings,
Bertus
 

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Thread Starter

alialiali

Joined Dec 12, 2008
26
This two circuits is great just I’d like to ask how to connect them together so I get my final out put from one port instead to have tow deferent out puts port ?
Do I need to implement a there’d filter to feed this two in it ?

Please gave me some ideas
Many thx
 

bertus

Joined Apr 5, 2008
20,446
Hello,

You can connect the highpass filter at the output of the lowpass filter.
This will create the bandpass filter.



Greetings,
Bertus
 

Thread Starter

alialiali

Joined Dec 12, 2008
26
Thats great the high pass signal is ok as it's gaving me 23kHz but when I test the output for the 53k it's gaving me 61KHz so please can you gave more advice.

I think we need to add another filter such as adder or first order filter
 

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Last edited:

bertus

Joined Apr 5, 2008
20,446
Hello,

A buffer is an opamp with the + as input and the output conected to the - input.




When you place this between the two filters they will not "see" each other.

Greetings,
bertus
 

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Thread Starter

alialiali

Joined Dec 12, 2008
26
Do you know what I learned hear from you I wouldn’t learn it in one year
I greatly appreciate your kind help

The result of the test now still negative as nothing changed do you think we need to add some resistors to the buffer ?
 

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bertus

Joined Apr 5, 2008
20,446
Hello,

What happens when you change the TL072 (wich has a bandwidth of 4 Mhz)
to a LF357 ( wich has a bandwidth of 20 Mhz) ?
Perhaps the limited bandwidth is giving some problems.
(you are using rather high frequencies).

Greetings,
Bertus
 
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