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TLV9054: Schematic review and issue review

Nam, Dino
Expert 6285 points
Part Number: TLV9054

Dear team, 

My customer would like to use TLV9054 as active filter and buffer. but there is some delay issue. 

Please review below customer schematic and issue waveform and let me know your opinion. 

- input signal : blue , 60Hz 5Vp-p sine wave. 

- output signal : Red, 3Vp-p waveform 

There is delay issue(about 300us) between input and output signal. The customer can't control input signal due to signal delay(300us) so they want to reduce this delay within 50us. 

- Is it possible to reduce this delay as changing RC component value? If not, Do we have to change other device not TLV9054?

- Is this delay from slew rate of TLV9054 or is there other factor? please let me know you opinion. 

Thank you. 

  • 0
    TI__Mastermind 23820 points

    Hi Dino,

    I did a little testing in the webench filter design tool to see what the expected delay is with different filter values. It was interesting that most filter architectures that use a similar sallen-key topology will give about 200us of delay at 60Hz.

    I checked the passband frequency that the customer's circuit was at and found it to be close to the 1.11kHz cutoff. 

    One way to lower the delay, is the increase the cutoff frequency. In the image below I put  3kHz as the cutoff frequency.

    I found that selecting the Chebyshev filter design I get the following values and results:

    I see a delay just a little larger than 50us. If you push out the passband frequency to be 3.5kHz, then you get about 45us of delay. 

    Let me know what you think. 

    Best Regards, 

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    TI__Expert 6285 points

    Hello Robert, 

    Thank you for your strong support. 

    I discussed your recommendation with customer. I am sorry but  the customer requirement is within 1us not 50us. 

    - Is this delay unique to the sallen-key topology? Is there any way to reduce delay time to 1us? 

    - If there is no way reducing delay time using the sallen-key topology, Will using other topology meet customer requirements?

     Please review it again and let me know your opinion. 

    Thank you. 

  • +1 in reply to Nam, Dino
    TI__Mastermind 23820 points

    Hi Dino,

    That certainly changes things. 

    Nam, Dino said:
    - Is this delay unique to the sallen-key topology? Is there any way to reduce delay time to 1us? 

    This delay isn't unique to sallen-key topology as all filters create delay. There is a way to reduce the delay to be within the 1us but it requires pushing the low pass filter's cutoff far out to 200kHz. Basically you are trading effective filtering for minimizing delays. In this case I created this: 

    The delay came out to be about 1.22us which is within range of the 1us that the customer is looking for.

    Nam, Dino said:
    - If there is no way reducing delay time using the sallen-key topology, Will using other topology meet customer requirements?

    A lower order filter should be able to help reduce delays. The following is a simple single order filter that goes before the feedback of the op amp. 

    Please talk to the customer to see if they can allow a low pass filter much further out then their original intent. 

    Best Regards,

    Robert Clifton