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[ADS8350] Antialiasing Filter

Kuramochi Tadahiko
Guru 28715 points

Hi,

 "DAQ Circuit" is mentioned in the figure 56 on the datasheet.

This antialiasing filter is structured with 10Ω*2 and 3.9nF.

Could you tell me that how is this filter designed?

Or is this filter the charge injection filter not antialiasing filter ? 

Regards,

Kuramochi

  • 0
    TI__Expert 8975 points
    Hello Kuramochi-san,

    The filter capacitor helps reduce the error due to charge injection during sampling & acts as charge bucket.
    The filter R helps with stability of opamp when it has to drive the filter cap.

    The RC thus formed helps reduce the high frequency content which will get aliased back after sampling. Hence it is also referred as anti-aliasing filter.

    You can calculate the R & C values by following some of TI's precision designs.
    One such precision design is www.ti.com/.../slau515

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Guru 28715 points

    Shridhar-san,

    Thank you for your reply.
    I apologize for the lack of information.

    The cut-off frequency of the filter is 2MHz (1/2π*10Ω*2 *3.9nF) .
    I think that this value too high for the sampling rate@750kHz.

    Why is the cut-off frequency high for the sampling rate?

    Thanks and Regards,
    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Expert 8975 points
    Hello Kuramochi-san,

    The primary responsibility for the RC filter is for charge injection & stability of the driver amplifier.

    The RC values are designed based on system specifications of THD & SNR. You can have different RC values based on these specifications, driver chosen etc.

    In the datasheet design, the component values were calculated for certain SNR & THD specification to meet. The captured waveform shows the best possible performance achieved which is close to specifications.

    Anti-aliasing principle is applicable for any high frequency above Fs/2. In this case due to the design based on specifications, the anti aliasing will take effect for frequencies above 2MHz.

    Thanks & Regards,
    Shridhar.