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DDC114: Anti-aliasing Filter

Mitchell
Expert 4915 points
Part Number: DDC114

Hi Team,

Is an anti-alias filter necessary for the DDC114 in the case of frequencies above the Nyquist limit? I assume with a max sample rate of 3.125khz that if there are faster signals they will fold down and alias. Correct? Do you have any suggestions for an anti-alias filter for a photodiode that normally might be connected direct to the DDC114?

The issue my customer and I are struggling with is that it is charge integrating, and the amount of charge is so small, we are concerned about putting anything additionally in the signal chain. Not sure about filtering the current vs voltage if this was a standard ADC.

Thanks,
Mitchell

  • 0
    TI__Genius 12025 points
    Hi Mitchell,

    How are you?
    I will forward your question to our system engineer.
    We will reply to you in 2-3 days.

    Thank you!

    Best regards,
    Chen
  • 0 in reply to Chen Kung
    TI__Expert 4915 points
    Hi Chen,

    Do you have any update on this?

    Thanks,
    Mitchell
  • 0 in reply to Mitchell
    TI__Genius 12025 points
    Hi Mitchell,

    How are you?
    Could you please send us your email address?
    Thank you and Have a nice day!

    Best regards,
    Chen
  • 0 in reply to Mitchell
    TI__Expert 4590 points
    Hi Mitchell,

    Not sure if you got in contact with Chen already but just some thoughts on this (sorry for late answer...)

    I have never seen anyone try using a filter in front between the detector and the device (well, once but made the circuit not work). Part of the reason is that this is not "just" an ADC sampling an input current. There is the "gated" integrator in front. There is a need to keep the input capacitance low as it affects the transfer function for the amplifier noise sources increasing their effects. Big series resistors add noise plus there is also the need to keep the input impedance low as you want the sensor current to flow into the device, etc... So, folks simply focus on keeping the signal from the detector as clean as possible to begin with.

    If I think of this in a different way, basically it would be like adding an antialias filter in front of the transimpedance amplifier... Note: It is a different thing to think what happens between the integrator or TIA and the ADC...

    But back to the problem... What is the frequency and the level of the signal they want to measure? If the signal is very fast and customer wants to capture its time domain shape, you may want to look into transimpedance amplifiers which will preserve it (as long as you are in the BW...). The DDC will basically integrate everything in one period (except for non-idealities) and that information will be lost. But if slow changing, customer just needs to worry about avoiding coupling from any noise sources at the input. Then the noise numbers of the datasheet will give an idea if they will be able to measure what they want... Also, when signals are very small, they should pay attention also to drifts, offsets, etc...

    Regards,
    Edu