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ADS52J90: LPF

toshi-e2e
Genius 4735 points
Other Parts Discussed in Thread: ADS52J90, THS4509, OPA2300

Hi all,
My customer plans to use ADS52J90 for a photo sensor input application.

A signal path is as follows;

Photo sensors  -> I/V conversion of a photo diode input (OPA2300) -> single to differential conversion (THS4509) -> ADS52J90  

Input channels are 32, so that the sampling rate is 50MHz. 
All signal path is DC coupling.
The photo sensors signal band width is from DC to >10MHz (or 20MHz; not fixed yet). 


Questions are;

1.  For considering LPF,  what is the differential input resistance and the differential input capacitance of ADS52J90?

2. Could you give me your advice about the LPF  design?
       -  According to Anti-Aliasing Calculation Tool for A to D Converters , the required attenuation is -61.96dB and the number of Filter Poles is 14.
          I feel it's hard to design for realizing the LPF by analog method.  Is my understanding right? 
 
          If no, could you tell me  the recommended analog LPF for 50MSPS? 
          If yes, should be done by digital LPF after conversion?  In this case,  what is the recommended LPF for ADC?   

 Regards,
Toshi

  • 0
    TI__Mastermind 49636 points
    Hi Toshi,

    Please refer to Figure 86 of the ADS52J90 datasheet for the differential input resistance and capacitance. Assume the switches are all on.

    Was the required attenuation of -62dB imposed by the customer or was this a setting that you chose in the AA calc tool?
    The attenuation of -62 dB is difficult to achieve in analog domain. Hence it should be done by digital LPF.
    The recommended LPF is typically Nyquist 3rd Order.
  • 0 in reply to Praveen Aroul
    Genius 4735 points
    Hi Praveen-san,
    thanks for your reply.

    About the input impedance, I understood that the "S" of switches are switching at the same time.
    About the -62dB attenuation, I choosed it by using AA calc tool.
    About the LPF design, I will discuss with customer.

    Thanks again,
    Best regards,
    Toshi