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THS4304 active filter BPF, TINA-TI result is different from WEBENCH.

Shinichi Inoue
Guru 19701 points
Other Parts Discussed in Thread: TINA-TI, THS4304

Dear Specialists

My customer is considering Bandpass filter.

At first, The circuit is chosen by Webench filter desiner.

And then transferred to TINA-TI design.

He was selected THS4304.

The frequency response is so far from expected.

A bandwidth is more than 10MHz and gain is less than -180dB.

I would like to know what is the problem.

I appreciate your great help.

Shinichi

WEBENCH® Design Report.pdf

Active filter frequency response of THS4304.pptx

1MHz BPF THS4304 webench_design_1198841_309.TSC

  • 0
    TI__Genius 13935 points

    Hi Shinichi,

    It looks to me like the DC operating points of the amplifiers are not optimal.  

    I added resistors between the non inverting inputs and ground equal to the feedback resistors, this gives the same offset voltage on each input due to the bias currents and seems to cancel them out.

    In a real circuit you may need a way to trim these to make up for the Vos and the bias current offset.  

    Regards,

    Loren

    1MHz BPF THS4304_reviesd.TSC

  • 0 in reply to Loren Siebert 1
    Guru 19701 points
    Dear Loren

    Thank you for your reply.

    Great!

    I confirmed your circuit and frequency response is OK.
    I understand it needs to consider Vos and bias current in an actual circuit.

    I have one more question.
    Could you please advise.

    According to the result, a gain(insertion loss) is about -40dB.

    Is this normal value?
    I'm not sure because I'm unfamiliar with active filter.

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Genius 13935 points

    Hi Shinichi,

    I am not familiar with the Webbench filter tool.  Looking at the schematic I see that R1_S1, R1_s2 and R1_S3 are set to very large values.  This, combined with the resistors R3_S1, R3_S2 and R3_S3 set up very high loss attenuators.  Each stage has a lot of loss.   I am not sure why this is the case, but it appears to be necessary to get the sharp roll off in frequency response.  

    I put capacitors to ground on the non inverting inputs - this simulates the direct ground of the original circuit while leaving the resistors to offset the DC currents.  I still get weird DC values, so the resistors may not be correct yet. 

    Regards,

    Loren

    6253.1MHz BPF THS4304_reviesd.TSC

  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points
    Hi Sinichi,

    Are you still working with this circuit? I have looked at it more and the feedback resistors are too large for this op amp. The bias currents set up a large DC offset across the feedback resistors. Can you use a different op amp? Maybe a FET input amp would work better at this frequency.

    Another option is to make a passive filter and then buffer it on the input and output sides. This gives most of the advantagnes of an active filter.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Guru 19701 points

    Dear Loren

    Thank you for your reply.

    I am still considering the difference.

    THS4304 is selected tentatively, it can be changed.

    In this case, do you have any recommend device?

    On the other hand, I confirmed low pass filter and high pass filter, there is no the insertion loss.

    It is different from band pass filter rejection spec and amp.

    Could you please see attached file. 

    I don't know why the result caused, (I think it may be a difference of formula). 

    Anyhow, I am going to introduce low and high combination filter the customer.   

    If you have any information, could you please let me know. 

    I appreciate your great help.

    Best regards,

    Shinichi

    1MHz HPF 2stage + LPF 2stage THS4304 webench_design_1198841_337 - autosave 16-04-18 18_04.TSC

  • 0 in reply to Shinichi Inoue
    TI__Genius 13935 points

    Hello Sinichi,

    I am out of the office on business travel.  I looked at your new circuit briefly.  It looks much better, the component values are reasonable. 

    I did run some quick simulations, and everything looked ok.  For these component values the amplifier you selected is fine.

    Regards,

    Loren