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FilterPro design : Min GBW required

Julien Calve
Prodigy 10 points
Other Parts Discussed in Thread: LMP7717, LMV793, TINA-TI

Dear members,

I'm designing a low-pass filter with FilterPro for a photomultiplier tube application. I was planning to design a butterworth filter as an anti-alisaing filter used in front of an high-speed ADC.

I have seen that the question about the Min GBW is already asked on the forum. (http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/164098.aspx).

The fact is that we need a filter with a cutoff frequency of 5MHz with a gain of 1 and the supply voltage is 1.8V. It means that the min GBW required is higher than 200MHz... Amplifiers with the highest BW are LMV793 or LMP7717 (88MHz). Is it an acceptable compromise or will we have to reconsider our board design?

Thanks a lot in advance!

  • 0
    TI__Guru* 93105 points

    Hi Julien,

    The main effect of using an operational amplifier having a lower GBW than FilterPro recommends is the gain and phase response will be compromised. The amount they are compromised may be acceptable, or not, depending on the particular application. I find that low-order, smooth roll-off filter having a Butterworth or Bessel response may deviate little from their ideal response when using an operational amplifier having a fraction of the recommended GBW. That, however, doesn't hold with a more aggressive, high-order Chebyshev response where specific pass-band ripple is indicative of the response. Also, the filter topology makes some difference with regard to minimal GBW and the Sallen-Key is often more forgiving.

    My recommendation is that synthesize your filter using FilterPro. Then, set up the filter schematic in a PSpice simulator and run the gain/phase response. If you don't have a PSpice based simulator, you can download TINA-TI Spice for free at the TI website. I see that the LMV793 and LMP7717 have simulation models available on their web pages. Once you have completed the simulation, determine if the response is what is needed to support your application.

    Regards, Thomas

    PA - Linear Applications Engineering