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ADS61B49: over range inputs do not always make the output code of 0x3FFF

user1817395
Prodigy 40 points
Part Number: ADS61B49

Hi,

My ADS61B49 sometimes make output code of 0x3FFE (11111111111110) even if the input differential voltage is around 3V. Sometimes the same chip output code goes 0x3FFF.

I got such devices on my 2-3 boards (chips) out of 8 boards.

All those chips seems to be within the spec because the data sheet specifies input differential of -1dBFS. And the devices are not tested at over range inputs.

But for me this is curious because this doesn't happen for SAR ADC devices in my experience.

Are those devices, which make max code of 0x3FFE, not defective?

Kind Regards,

  • 0
    TI__Genius 12780 points
    Hi User1817395,

    Just so I understand clearly, you have a couple of devices that are not outputting the full scale output code when the input should be exceeding full scale (data sheet says 2 Vpp for differential input).

    Are you using LVDS (serial) or CMOS (parallel) data output? Are you able to share your schematic?

    Best Regards,

    Dan
  • 0 in reply to dBrock
    Prodigy 40 points

    ear Dan,

    Thank you for your quick response.ADCdrive.pdf

    Please find the drive circut schematic. I am afraid I can not check if the schematic pdf file has uploaded properly.

    The ADC is driven by LHM6553 fully differential amplifier. The common mode voltage for LHM6553 is provided by ADS61B49. I gave an input voltages for ADS61B49 exceeding full scale 2 Vpp(say 3Vpp) through the drive circuit of LTH6553 of which gain is 1.

    Data outputs of ADC is LVDS. The data was captured by our Zync based FPGA board and TI's capture board TSW1405.

    Kind Regards,

    K. Kobayashi

  • 0 in reply to user1817395
    TI__Expert 4880 points
    K. Kobayashi,

    Can you check, all on-board voltages (Vcm, etc.)and compare between the working board and the defective boards to see if you catch a bug in your design?

    Thanks

    Yusuf