Problem making an active bandpass from this .PDF link

[mramos1]

I have spent a couple hours and give up. The link below page 4 and 5 explain how to get the values for the circuit on the last page (bandpass filter, and I was leaving the .01uF in there).

**broken link removed**


The sample is freq =1K and bandwidth=100Hz. I dropped the circuit in circuitmaker and it works pretty good.

I have tried 3 times to get the values for frew=2500Hz and bandwidth=500Hz. I get R1=625, R2=32ohms, and R3=6.25K (roughy on the 3 tries).

I drop it in circuit maker and it passed 100HZ - 5Khz.. And probably more.

Does anyone have a better filter? Formula? I was using 1vp-p and was not railing the opamp at all.

Thanks

 

[Analog]

use filtercad from linear tech. These are switched capacitor filters, and they work very well.

 

[JimB]

I think that you are making an error in your maths.

I knocked up* a speadsheet to do the calculation, and it agreed with the PDF on the values for 1000hz and 100hz BW.

My values for 2500hz and 500hz BW are:

R1 = 3183ohm, Req = 637ohm, R2 = 796ohm, R3 = 63662ohm
C1 and C2 = 0.01uF

JimB

* = the English meaning of course!

 

[mramos1]

I think that you are making an error in your math.

I knocked up* a speadsheet to do the calculation, and it agreed with the PDF on the values for 1000hz and 100hz BW.

My values for 2500hz and 500hz BW are:

R1 = 3183ohm, Req = 637ohm, R2 = 796ohm, R3 = 63662ohm
C1 and C2 = 0.01uF

I still have the circuit loaded in circuitmaker, will try your values now. I like yours numbers better than my numbers for sure. Thanks JimB.

Ran your numbers. Mind doing 100hz on the BW or send me the spread sheet (hope Excel).

100Hz input was still passing through with your values. 5KHz was OK.

Not sure what I did wrong, but I did it 3 times (the same way it appears).

Same answer 3 times means ask for help.

 

[JimB]

Yes, the spreadsheet is Excel.
PM me with your e-mail address and I will send it to you.

JimB

 

[Ayne]

http://www.captain.at/electronics/active-filter/
u can easely caculate the values of components

Now please solve my problem, I want to design a filter that can pass the frequency 0Hz to 6Khz.. I want to use op-amp, it should be active filter... Thanks.

 

[mramos1]

An active lowpass fiter?

**broken link removed** has one and calcs

 

[mramos1]

JimB, thank you sir. That gave me almost what I needed (thanks to both you and Ayne). Almost as it, for a single op amp filter I found the op amp will cut off the high pretty darn good. Not the lows though. So looks like I will do a highpass into the bandpass to fix that.

Ayne, try the website you sent with a TL082. It might get you want you want as it did not filter low freq well at all, but did the high very well. Now this is in CircuitMaker, not on the breadboard.

Off to look for a couple op amps now.

Thanks again for the help.

 

[JimB]

JimB, thank you sir.

You are welcome.
JimB

 

[audioguru]

Ayne,
You hijacked this thread. You should start your own thread.

You need to pass the frequencies from 0Hz so the filter is not a bandpass filter. You need it to cut high frequencies so it needs to be a lowpass filter.

 

[mramos1]

Get him Audioguru.

I wanted to tell him that as well.

I read his link to a webpage I have been too before and it matched JimB's calcs 100%.

I guess the op amp bandpass filter really does not work well in the low freq range. Mine was 2500Hz center with width of 500Hz. According to circuit maker anyway, it rolled off at the right spot on the high but the low did not roll off down to 200Hz when I stopped looking.

I did actually build it. And now I need a high pass filter (for my lower freq) to route into the bandpass filter to fix that.

Then I guess I will need a preamp for my mic as well

 

[audioguru]

Why are you connecting a simple, poor bandpass filter to a mic?

 

[mramos1]

I am trying to detect a police siren to light an LED without learning the DSPic.