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ADS8519 - Is Need Buffer input required or not

Mohamed Sidiq
Prodigy 155 points
Other Parts Discussed in Thread: ADS8519

Hi,


I have a  differential bipolar signal with the range of (+10v to -10v) 20vp-p signal, This signal is connected to the input of pre-amplifier stage to convert the differential bipolar signal into single ended bipolar signal (+10v to -10v). The output of the preamplifier stage is connected to the PGA to varying the gain of the signal.

My question is Shall I connect directly the output of PGA to ADS8519 ADC or I need to buffer the PGA output signal and then fed to the ADC (ADS8519). I am attaching the block diagram for your reference, Please advice me.

Regards,
H.Mohamed

  • 0
    TI__Expert 8975 points
    Hello Mohamed,

    Thanks for sharing the details.

    Since I am not sure which PGA you are planning to use & its specification, I can not provide you conclusive answer.

    You can perform transient simulation & stability of the PGA block (for all gain settings) & check if the output settles to 16 bit accuracy in the acquisition window of the ADS8519 (1.8us).

    If you are settling well within the specification for all gain settings, you need not have buffer. Otherwise please introduce a buffer stage as suggested in datasheet.

    I have few additional queries with regards to your application. Based on your application need, I may be able to suggest another device which can help reduce complexity.

    I will send a E2E friend request. Please accept that & we can have separate communication to discuss further.

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Prodigy 155 points
    Hi Shridhar,


    I have replied in our previous discussion about our ADC specifications.

    I think ADS8519 is meeting our full requirement.


    Once we have done the schematics, please verify the schematics and give your feed back.

    Then, Please refer our previous discussion about the specification requirement.



    Thank you,

    Regards,

    Mohamed Sidiq.
  • 0 in reply to Mohamed Sidiq
    Prodigy 155 points

    Hello Sridhar,

    Sorry for the delayed reply !

    I changed My filter Design as compare with previous design. I am attaching the file of TINA filter simulation, and Schematics for verification. Please find the attachments.

    My requirement is

    I am receiving the Continuous differential bipolar signal as input. This input is given to the PGA (AD8253ARMZ) . The output of PGA is given to the 8th order Butterworth filter (Using OPA4778UA) which allows the 0-30Khz range of input signal. The output of filter is directly given to the ADS8519 without buffer.

    I need a clarification regarding this design in below:

    1. Does the input to ADS8519 need additional buffer?
    2. How to use the control pin of SYNC, R/C#, TAG?
    3. In this Butterworth low pass filter section, How to avoid the Phase shift of input signal with unity gain?

    Please let me know if any change to be take care in schematics based on my requirements. 

    Thanks in Advance,

    Warm Regards,

    H.Mohamed Sidiq.ADC_ADS8519_WITH FILTER.pdfnew_LPF_45COFF_8TH_ORDER_OPTIMIZE2.TSCLPF_45CUT_8THORDER_FINAL.pdf

  • 0 in reply to Mohamed Sidiq
    TI__Expert 8975 points
    Hello Mohamed,

    Please find my comments below.

    1. Additional buffer - I simulated your filter circuit with ADS8519 input structure as load with sampling cap switching at 250ksps. The sampling caps are not getting settled even with 2us acquisition time. Please have buffer stage to drive the inputs.

    2. Comments regarding the schematic
    a. Vana supply max is 5.25V. You have connected 12V supply. Please have 5V supply drive the analog supply.
    b. I think you plan to operate in +/-10V range. This needs the R3IN pin to connect to CAP pin. This connection should be decoupled to GND with 2.2uF cap. Currently we have 2 different 2.2uF to GND. The connection between R3IN and CAP pin is not present.
    c. DGND has no connection to AGND. Please short them. Also, Digital supply & IO lines are referenced to AGND.

    3. Control pins
    a. SYNC - This pin is useful for synchronisation with external dataclk. Since you are using INT dataclk, this may not affect you.
    b. R/C# - This pin controls the ADC conversion timing along with CS pin.
    c. TAG - This can be used during daisy chaining of the devices.

    For the filter query getting in touch with WEBENCH forum is the best alternative. I will try to get more info & share with you once available.

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Prodigy 155 points

    Hi Sridhar ,

    Thank you for your response!

    I am updating your comments and also attaching the schematics. Please find the attachments. If any change to be made in schematics please let me know. Based on your feedback I will process the schematics to next level.

    I used the opamp opa228ua for input buffer of ADC is this Ok or I need to find the different OPAMP.

    Note: I will short both analog and digital GND by ferrite bead finally. Because we are using 32 ADC Channel.

    Thanks in advance !

    Warm Regards,

    Mohamed Sidiq.

    ADC_ADS8519_WITH FILTER_UPDATED.pdf