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Original question:

ADS41B49 datasheet issues.

ADS41B49: ADS41B49 datasheet (INP/INM absolute max and signal voltage range at input of 1.7V + 0.375V, VCM voltage)

Simon Defayette38
Prodigy 40 points
Part Number: ADS41B49
Other Parts Discussed in Thread: ADS42B49

I saw some threads about the absolute max for INP and INM.

AVDD being between 1.7 and 1.9, 1.9V will always be an absolute max of 1.9V :   max = minimum(1.9V, AVDD + 0.3V)

But what about the input rage where it says it's VCM (1.7V) + 0.375V ? This gives a max of 2.075V.  It's over the 1.9V absolute max.  I am confuse here.

Also, on my board I measure a VCM of 1.856V at the output of the VCM pin.  Why would it be that voltage?

Thanks,

Simon

  • 0
    TI__Guru 69994 points

    Hi,

    The input range absolute max is the minimum of 1.9V and AVDD+0.3 is because if you are not powered up or in the process of powering up, there are ESD diodes at the input. You cannot exceed the minimum of 1.9V and AVDD+0.3 as AVDD is ramping up in voltage during powering up.

    VCM of 1.856V is when there are no clock conditions. When clock is running at full speed, the VCM should be within 1.7V. 

    -Kang

  • 0 in reply to Kang Hsia
    Prodigy 40 points

    Hi Kang,

      Ok, I understand for the max(INP/INM) = min(1.9V, AVDD+0.3).  But for INP/INM swing range, it says VCM (1.7V) + 0.375V which can go at 2.075V... and the absolute is 1.9V... what should I understand about this limit?  Section 8.3.1 of the datasheet states : "The input common-mode is set internally using a 5-kΩ resistor from each input pin to 1.7 V, so the input signal can be ac-coupled to the pins. Each input pin (INP,INM) must swing symmetrically between (VCM+ 0.375V) and (VCM– 0.375V), resulting in a 1.5-VPP differential input swing."

      Also, I just measured the VCM and made sure that the ADC clock input was driven by the source : it is 1.859V.  What other reason could it be? 

    Thank you very much,

    Simon

  • 0 in reply to Simon Defayette38
    TI__Guru 69994 points

    Hi,

    You may interpret the spec for the input as the following diagram. These are differential input to VINN and VINM (INP/INM)

  • 0 in reply to Kang Hsia
    Prodigy 40 points

    Hi Kang,

      But VCM + 0.375V is over the max absolute rating for analog INP/INM inputs which is 1.9V.

      In my case, VCM is 1.86V.  So 1.86V (VCM) + 0.375V will be 2.235V which is a lot more that the max absolute 1.9V for INP/INM.

      If 1.9V is max and VCM is 1.86V, then it lets me only 0.04 V. 

      Does that makes sense?

    Thank you,

    Simon

  • 0 in reply to Simon Defayette38
    Prodigy 40 points

    Hi Kang,

      Another TI ADC describes the AC and DC components on two different max absolutes ratings. 

      www.ti.com/.../ads5481.pdf

      Is that the same thing with the ADS41B49? 

      Should it be AC signal component only max rating to 1.9V and not take into accound the DC component (VCM) ?

    Thanks,

    Simon

  • 0 in reply to Simon Defayette38
    TI__Guru 69994 points

    Hi Simon,

    I believe there is a typo in the absolute max. This is an ADC with buffered input, with buffer biased at AVDD_BUFF of 3.3V.

    The absolute max should be listed as min(3.3V, AVDD_BUFF+0.3)

    I will need to double check and let you know. 

    -Kang

  • 0 in reply to Kang Hsia
    TI__Guru 69994 points

    Hi Simon,

    we are checking with design team. We will get back to you.

    -Kang

  • 0 in reply to Kang Hsia
    TI__Guru* 91947 points

    Simon,

    This is an error in the data sheet. Please see the correct description as shown in the ADS42B49 data sheet. The ADS41B49 will have this corrected in the next update of the data sheet.

    Regards,

    Jim