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ADS5527: Wrong differential analog input

emilio arnieri69
Prodigy 110 points

Part Number: ADS5527

We realized a board that integrates several ADS5527. At the moment the board does not work well (it seems that the output of the ADCs is always zero). We suspect that the problem is related to the analog input at pins INP and INM. In fact we are feeding the ADC with a differential signal with no DC component (there are DC blocking capacitors on the input analog lines just before the ADC inputs INP and INM). 

The ADS5527 datasheet at page 32 say "The INP and INM pins have to be externally biased around a common mode voltage of 1.5V available on VCM pin 13. For a full-scale differential input, each input pin INP, INM has to swing symmetrically between VCM+0.5V and VCM–0.5V, resulting in a 2-VPP differential input swing".

Unfortunately in our design we are not using the pin VCM (it is connected to ground with two capacitance).

Do you think that our problem is related to the wrong input signal (no DC component)?

If yes, do you have a suggestion to solve the problem? 

PS. I've added an image of the connection scheme used to integrate the ADS5527.

  • 0
    TI__Mastermind 20541 points
    Hi Emilio,

    We are looking into your question and will get back to you soon.

    Regards,
    Neeraj
  • 0 in reply to Neeraj Gill
    TI__Guru* 91947 points

    Emilio,

    Yes, this is your problem. Is there a way you can use a couple of bias-T, one for each input, and use an external power supply to provide the proper bias voltage?

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 110 points

    Dear Jim,

    thank you for your answer.

    We are thinking to solder an external wire after the DC blocking capacitors as shown in the attached figure (see red lines) to provide the proper bias voltage. Do you think that this could be a solution? The input signal is low frequency (75MHz).

  • 0 in reply to emilio arnieri69
    TI__Guru* 91947 points

    Emilio,

    I think this should work for you. Just keep the amplitude of the 75MHz IF input within spec of the data sheet.

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 110 points

    Dear Jim,

    thank you very much for your useful help.

    At the moment we are also thinking about another possible solution, then we need the following information:

    1. What dc current do the inputs draw? We assume it is extremely small.

    2. What happens if both the inputs have the same bias but it is not exactly 1.5V but 1.61V or 1.89V.

    3. What happens when there is difference in bias levels on the positive and negative input of 50mV or 100mV.

    Regards,

    Emilio

  • 0 in reply to emilio arnieri69
    TI__Guru* 91947 points

    Emilio,

    1. It is very low. See the input impedance vs frequency Figure on page 33 of the data sheet.

    2. The data sheet limits are 1.45V to 1.55V. If you exceed this in either direction, you will no longer be able to achieve full scale operation. This will be reduced and offset in one direction depending if you are below the VCM min or above the max.

    3. I am not 100 percent sure but I am guessing there will be an offset in the output and the full scale swing may be reduced as well.

    Regards,

    Jim