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Original question:

THS4541: Required details for Simulation of Bandpass and Glitch Filters

THS4541: ths4541

saritha nalluri
Prodigy 60 points
Part Number: THS4541
Other Parts Discussed in Thread: ADC3644

Hi Nick,

we are getting input of 1-45MHz frequency of I data to our unit and then we are connecting to FDA and then to ADC . final our application is to process a signal in 1-45mhz range .

here i want to achieve flat response amplitude response for 1-45Mhz frequencies what values i have to change in my design.here DNP in circuit are not connected and we are giving input to inverting terminal and using in unity gain configuration.after 20MhZ my LPF is response is too worst.

for example my VGA output is 800mv and in my FDA input it is going to half and after glitch filter it is going to Quarter what is the reason and in ADC sampling we are observed some offset.is there any possibility of -ve offset in my design.? Please clarify.

  • 0
    TI__Expert 6050 points

    Hi Saritha,

    I will look into this behavior you are talking about further. Are you able to provide screenshots of the measurements you are observing and share how you are measuring at the different points you mentioned in your post?

    Also, it appears that you are using different passive component values than the ones I showed in simulation. Can you share why you have selected these new component values. Most specifically, R159, R161, R165, and R166 as these set the gain of this stage.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 60 points

    THS4541_lowpass,Glitch_E2E.TSC

    Dear Nick,

    Here i attached my simulation file. according to my design my input is from VGA and it is single ended. it is coming with a -40mv negative offset 

    i feeding it to differential amplifier.after that i am giving to ADC3644.

    in between i using a lowpass filter and glitch filter. here in my circuit something is went wrong and falling output voltage according to increase of frequency. from 10-40MHz.

    i want the amplitude is to be fixed for all voltages.here i changed some modifications in my circuit after that also iam not getting proper output.

  • 0 in reply to saritha nalluri
    TI__Expert 6050 points

    Hi Saritha,

    I see that you have R157 and R160 as DNP. If you have nothing connected in these positions, does that mean you have C120 floating on the IN+ signal path?

    Also, the parallel combination of R167 and R165 forms an Rg resistor value that is not equivalent to R161 which will cause a mismatch in gain setting. It is possible this could be part of the issue as well.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 60 points

     R157 i shorted  and R160 is open ,c120 i shorted in my board .in the above message i attached a TSC file in that i modified the design according to my board.

    Nick Nauman said:
    Also, the parallel combination of R167 and R165 forms an Rg resistor value that is not equivalent to R161 which will cause a mismatch in gain setting. It is possible this could be part of the issue as well.

     this i didn't understood can u explain?

    Please simulate the above TSC file and make the changes in that lowpass filter and glitch filter sections amplitude is constant from 1-40MHz .

  • 0 in reply to saritha nalluri
    TI__Expert 6050 points

    Hi Saritha,

    Please see the simulation below:

    If you simulate with the original values in your simulation, you will see that the the gain is higher because of the resistor mismatch:

    In order to keep a consistent magnitude for the frequency response you will need to adjust the values of the lowpass filter and glitch filter passive components. You can try changing these values in simulation to see the effect that they have.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 60 points

    Hi,

    That feedback resistors i was placed according to datasheets only.