# help of design of active band reject filter

Design a circuit using op-amp that will reject the 60kHz power line noise and also reject high signal frequency above 800Khz. The stop-band width around the 60kHz centre frequency is 10kHz. Use a cascade of 1st order high pass and 1st order low pass filter to implement this circuit. The voltage gain at passband frequencies is 10.

As mentioned in the title, can sum1 tell me this should be a band reject or band pass filter?
i try to construct a band reject filter but the result seem like not correct.

here is my circuit using multisim
http://i325.photobucket.com/albums/k...23/circuit.jpg
here is the result using AC analysis in decibel
http://i325.photobucket.com/albums/k...123/result.jpg
bode plot

can sum1 help me out of this?
thanks

### 12 Replies

• Kok you do not have the filters in cascade.

You have them both going to the summing junction of U3

Try a 60Hz notch filter and then the 800KHz low pass in CASCADE and then to U3 of gain -2 it looks like.

There are many other ways to do this.

A multiple feedback bandpass, a high pass low pass Sallen Key, state variable etc.

Hope this helps

Dana

• In reply to Dana Tremblay:

Kok,

I cannot see that the topology that you have shown can do what you want. Here is a possible implementation. It is a 60kHz Twin-T with a single pole at 800kHz.

Your circuit appears to have the input signal biased at 0V with the op amps powered from a single power supply. This will not perform properly. I've shown a dual power supply to bias the input within the common-mode range of the op amp. I recommend an OPA132 or OPA140 op amp. The AD op amp you have shown emits UHF radiation that will make your hair fall out.

Regards, Bruce.

• In reply to Bruce Trump:

thank you all for help

Bruce: do you have any reference link about the method u suggest me?

thank you

• In reply to Bruce Trump:

could you plz help me about the calculation ,i tried so many time but  cant get it ,its urgent

• In reply to niloo eh:

Niloo:

The venerable T-notch filter has been around since the days of vacuum tubes from which I come from.

There are very many links on the web for it's theory and design, not like the old slide rule days.

Many sites have Java design tools such as this one for example.

http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

Bruce: nice tip on the AD8671 being a UHF chatterbox.

I'll remember that.

I see by the color of your hair, same as mine, you may go back to our days when

real radios glowed in the dark ;)

Dana

• In reply to Dana Tremblay:

Dana,

I should have broadened my observation that all op amps starting with "AD" in their part number can cause a wide range of maladies and should be avoided.  Extended use of op amps with prefix of OPA, TLC, TLV, TLE, THS will eventually cause graying of the hair but, as can be seen, no extensive hair loss.

Regards, Bruce.

• In reply to Bruce Trump:

Bruce:

Now that you mention it I do distantly recall taking a fancy new fangled spectrum analyzer up

into that neck of the woods with a AD part and seeing sum funny fuzz.

Wonder what AD is doing that Burr-Brown aka TI isnt?

Not to worry about the hair.

I still got mine.

Dana

• In reply to Dana Tremblay:

thank you very much for your help ,im not sure if im doing correct or not i set the fo=219k and get the capacitors and resistors ,im doing correct ???

:S

• In reply to Dana Tremblay:

Mr Bruce

im trying to find the value of your resistors and capacitors hours but i still couldnt get them ,plz help me i have to finish it by tonight

• In reply to niloo eh:

by using the web site that dana gave me u get ur answers but by calculation on my own i cant