which opamp is good for 100MHz high pass sallen key filter ?

[whitewiz]

related:www.national.com/an/oa/oa-26.pdf

I need to design 2nd order 100MHz sallen key filter and looking for good opamp.

The cut-off frequency of the filter is 100MHz. I used LM324 opamp but this kills the

gain before 100MHz. My simulator is PSPICE and they have lots of library.

Could you please recommend any good opamp using in PSPICE ?

Chris

 

[LvW]

Re: which opamp is good for 100MHz high pass sallen key filt

The answer is simple - but it will not satisfy you:
1.) In principle it is not possible to build a real active high pass since the output of each circuit drops down with rising frequencies
2.) When you need an opamp high pass circuit with a corner frequency of 100 MHz the opamp transit frequency should be higher at least by a factor of 50 - resulting in a GBW of at least 5 GHz. Hard to find.
3.) Consequence: Use a passive filter (or use gm-C-topology in integrated form)

 

[eem2am]

There is no opamp for this high frequency.

you need something like a microwave filter.....kind of bits of microstrip...like you see in FM receiver

 

[whitewiz]

Re: which opamp is good for 100MHz high pass sallen key filt

The answer is simple - but it will not satisfy you:
1.) In principle it is not possible to build a real active high pass since the output of each circuit drops down with rising frequencies
2.) When you need an opamp high pass circuit with a corner frequency of 100 MHz the opamp transit frequency should be higher at least by a factor of 50 - resulting in a GBW of at least 5 GHz. Hard to find.
3.) Consequence: Use a passive filter (or use gm-C-topology in integrated form)

I think the cut-off frequency need to decreased to at least 10MHz because the output should filter out in the low frequency and then, I need to find the wideband opamp.

Thank you so much for your answer

Added after 1 minutes:

There is no opamp for this high frequency.

you need something like a microwave filter.....kind of bits of microstrip...like you see in FM receiver

I think it is impossible to find the kind of opamp which has GHz range. I guess the

operating frequency should be decreased to at least 10MHz. Could you please

recommend any broadband opamp to work this kind of range ?

Thanks

 

[kender]

Look into AD8039, it can operate up to 350MHz.

 

[whitewiz]

Re: which opamp is good for 100MHz high pass sallen key filt

Look into AD8039, it can operate up to 350MHz.

Thank you for your answer.

Is this Opamp(AD8039) library which I can use in PSPICE 9.1 or Agilent ADS 2005A software ? I can not find this opamp in the library of PSPICE or ADS.

Your help is much appreciated if you recommend if you recommend the library using in these software.

Chris

 

[LvW]

Re: which opamp is good for 100MHz high pass sallen key filt

It is best to have a look on the opamp manufacturers homepage (analog.com or linear.com or ti.com or national.com). Here you can choose the part which best suits your needs.

 

[kender]

Re: which opamp is good for 100MHz high pass sallen key filt

Look into AD8039, it can operate up to 350MHz.

Is this Opamp(AD8039) library which I can use in PSPICE 9.1 or Agilent ADS 2005A software ?

Chris,

If you're looking specifically for a high-speed OpAmp which has a PSpice library, post a request here **broken link removed**

- Nick

 

[FvM]

Re: which opamp is good for 100MHz high pass sallen key filt

Voltage and current-feedback OPs with 1 to 3 GHz GBW are available (e.g. from TI and Analog) and allow to design usable 100 MHz active filters. Depending on the bandwidth reserve, you will observe more or less deviation from the ideal transfer function. The said factor 50 may be regarded as a rule-of-thumb for frequency charecteristics almost unaffected by OP GBW. But the result also depends on the intended filter Q. Low Q filters may be designed with a factor of 5 or 10, but the filter parameters needs to be adjusted for limited GBW.

 

[LvW]

Re: which opamp is good for 100MHz high pass sallen key filt

Hi whitewiz,

I think some additional comments are appropriate - independent on the final value for your filter pole (10 MHz or 100 MHz):

A factor of 10 between opamp transit frequency and filter cut-off (as proposed in the last reply) may be sufficient in order to meet the required corner frequency roughly. However, don´t forget you are going to design a HIGHPASS which must not attenuate frequencies ABOVE the corner frequency (until which maximum ??). I doubt if the mentioned factor of 10 can do this with a sufficient accuracy for frequencies higher than two times the corner frequency.

As far as the mentioned Sallen-Key-topology is concerned - I recommend to use no other gain values than 1 or 2, because these values are easy to design.

Added after 5 minutes: During selection of an opamp don´t forget to look for a suitable slew rate.

 

[whitewiz]

Re: which opamp is good for 100MHz high pass sallen key filt

As far as I know, slew rate is proportional to bandwidth. So high slew rate can satisfy the broad bandwidth. How much is good slew rate for this kind of high pass filter ?

Thank you for your answer and your comment is really helpful.

Hi whitewiz,

I think some additional comments are appropriate - independent on the final value for your filter pole (10 MHz or 100 MHz):

A factor of 10 between opamp transit frequency and filter cut-off (as proposed in the last reply) may be sufficient in order to meet the required corner frequency roughly. However, don´t forget you are going to design a HIGHPASS which must not attenuate frequencies ABOVE the corner frequency (until which maximum ??). I doubt if the mentioned factor of 10 can do this with a sufficient accuracy for frequencies higher than two times the corner frequency.

As far as the mentioned Sallen-Key-topology is concerned - I recommend to use no other gain values than 1 or 2, because these values are easy to design.

Added after 5 minutes: During selection of an opamp don´t forget to look for a suitable slew rate.

Added after 4 minutes:

I forgot to mention that.

I selected CLC425(Wideband opamp). Its bandwidth is 75MHz and slew rate is 250V/us. Is this good candidate to use ?

Hi whitewiz,

I think some additional comments are appropriate - independent on the final value for your filter pole (10 MHz or 100 MHz):

A factor of 10 between opamp transit frequency and filter cut-off (as proposed in the last reply) may be sufficient in order to meet the required corner frequency roughly. However, don´t forget you are going to design a HIGHPASS which must not attenuate frequencies ABOVE the corner frequency (until which maximum ??). I doubt if the mentioned factor of 10 can do this with a sufficient accuracy for frequencies higher than two times the corner frequency.

As far as the mentioned Sallen-Key-topology is concerned - I recommend to use no other gain values than 1 or 2, because these values are easy to design.

Added after 5 minutes: During selection of an opamp don´t forget to look for a suitable slew rate.

 

[LvW]

Re: which opamp is good for 100MHz high pass sallen key filt

Quote/ Nov. 17th, 6:55 :

I think it is impossible to find the kind of opamp which has GHz range. I guess the
operating frequency should be decreased to at least 10MHz. Could you please
recommend any broadband opamp to work this kind of range ?

I thought you are going to design a HIGHPASS !
In this case, it is not sufficient that the opamp works fine up to 10 MHz.
Instead, the maximum operating frequency has to be much larger !
But how large ? YOU must know which frequencies have to pass your filter !
(What is the purpose of the filter ?)
To answer your question it is really necessary to know:
1) What is the corner frequency of the highpass ?
2.) what are your attenuation requirements ?

Regards

 

[FvM]

SR of 250 V/us is rather low and implies less than 1 Vpeak at 50 MHz. However, you didn't tell much about the application and bandwidth or voltage levels. We can't know about.