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ADS8881: Best Low pass filter for ADS8881 to achieve best dynamic performance

yong chen
Prodigy 10 points
Part Number: ADS8881

Hi, 

 I am designing a data acquisition system base on ADS8881 to capture a sine wave signal from 0 ~100kHz(differential signal),  my signal source is a little noisy,  so I need to add an external low pass filter to achieve the best dynamic  performance of ADS8881,  my question is which type of low pass filter is the best chose for ADS8881 to achieve best SNR & THD in this application?  I am thinking to use high order RLC low pass filter or MFB low pass filter,  or is there other better solution?   Is it possible to achieve the ADS8881 datasheet performance(100dB SNR, -115dB THD)  in this application ? Please help!

Below is the FFT of my signal source.

  • 0
    TI__Guru**** 295050 points

    Hello, thank you very much for your support of TI.

    About your questions, I would forward it to our engineer who will contact you and  support your question.

  • 0
    TI__Guru**** 295050 points

    Hello， about your questions, please refer to the below reply:

    Please point them to the Application Section of the ADS8881 data sheet. We have several signal chain suggestions that are optimized for various input signal types and performance targets. What does their input circuit look like today?

    In addition, noise can come from other sources besides the input signal source. How do they know that the noise in the figure above is coming from the source and not from other active components? Noise could also be caused by PCB layout, power supplies, reference voltage, etc. In general, a low-pass filter is not always the solution because it depends on how the noise is coupling into the ADC.

    The ADS8881 has a low input impedance that connects directly to a switch-capacitor sampling network. Therefore, low inductance is important when designing an input circuit for this type of ADC. I don't believe an RLC filter is a good choice for this device, so perhaps an MFB amplifier circuit is a good starting point.

  • 0 in reply to Kailyn Chen
    Prodigy 10 points

    Hi Kailyn,

      Thanks a lot for the replay, currently I am trying to using TI filter design tool for this,  and the MFB will be my choice base on your reply.  From your reply, the low-pass filter is not always the solution, so I am trying to develop two different filter, one is low pass filter, and the other is band pass filter, try to find out the best one.  I will try to put the input circuit here when I finish. 

    Some more question is TI filter design tool doesn't  support differential design, and the TI filter pro tool seem doesn't support band pass filter, any suggestion for those issue? 

  • 0 in reply to yong chen
    TI__Guru**** 295050 points

    Hello，

    please refer to this application note about "Using the infinite-gain, MFB filter topology in fully differential active filters".

    https://www.ti.com/lit/an/slyt343/slyt343.pdf

  • 0 in reply to Kailyn Chen
    Prodigy 10 points

    Hi Kailyn,

       Thanks a lot for help,  one more help needed,  I want to use FilterPro v3.1 to do the filter simulation, could you help to provide a link for me to download this software from TI? 

  • 0 in reply to yong chen
    TI__Guru**** 295050 points

    Hi，yes， the older FilterProV3.1 include the filter design about FDA, but I also couldn't find the link. 

    Now the new filter tool about Webench Tool doesn't  include the fully differential amplifier (FDA) active filter topologies.

    I am sorry  for your inconvenience that have caused to you.