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ADS62P24: voltage sign reversed

Mark Goodberlet
Prodigy 70 points
Part Number: ADS62P24

We use CMOS-Mode output of this 12-bit ADC (ADS62P24) and configure the ADC using its serial interface to produce counts data in 2s-complement format.

For years, we have used this ADC on a board without changes to the board design and without changes to the firmware that reads counts data from the ADC.

However, a recent build of the board produces counts data (from the ADC) that indicates sign of the voltage input to the ADC is reversed. For example, on previous boards, +100mV input to the ADC produced a counts output of 819 counts. However, on the current board, +100mV input to ADC produces a counts output of 3276 (or -819).  

Any ideas what could be causing the sign reversal?

  • 0
    TI__Genius 12780 points
    Hi Mark,

    We will take a closer look, and get back with you soon.

    Best Regards,

    Dan
  • 0 in reply to dBrock
    Prodigy 70 points

    Hi Dan,

    Thanks for quick reply.

    I should clarify my comments a little. 

    The ADS62P24 has differential inputs called INP and INM that must swing symmetrically about VCM=1500mV.   Range for these inputs is VCM-500mV to VCM+500mV.

    On previous boards, (INP=1600mV, INM=1400mV) gave ADC counts output of 819 counts.  On recent boards, the same inputs for INP and INM give 3276 counts.

    Mark

  • 0 in reply to Mark Goodberlet
    TI__Genius 12780 points
    Hi Mark,

    My apologies for the delay in response.

    This seems like it could be an offset binary / 2's compliment issue of how the data is being interpreted.

    If 819 counts is offset binary (0 to full scale [4095]), then the 2's compliment is 3276, as you stated. Can you check to see if this may have changed somewhere down the line?

    I know you said that nothing has changed with the design, but has it been verified that the INP and INM signals at the input of the ADC are as expected (ex. 1600mV and 1400mV)? It's possible that other components in the design may have had manufacturing variations as well, so I'd like to eliminate as many variables as possible.

    Best Regards,

    Dan
  • 0 in reply to dBrock
    Prodigy 70 points
    Hi Dan,

    Thanks for your reply. I have rechecked things as you suggested and still have not found the answer.

    We will continue to investigate and let you know if we find the cause.

    Thanks again,

    Mark
  • 0 in reply to Mark Goodberlet
    TI__Guru* 91947 points

    Mark,

    If you are using this device in parallel SPI mode, what is the voltage level on the SEN pin? This needs to be AVDD (3.3V) for the part to operate in 2's complement. If this value is around 5/8 * AVDD (2.0625V), the part will operate in straight binary mode. If you are using serial SPI, check register address 0x16 bit D4. If this is "0", the part will operate in 2's comp. If it is a "1", the part will operate in straight binary mode.

    Regards,

    Jim  

  • 0 in reply to jim s
    Prodigy 70 points
    Jim,

    Thanks for your reply. I was sure you had found the answer - but then I remembered that we use serial SPI mode. We set register 0x16 and do a read-back to verify that it is set properly as power of our power-up checks.

    We'll keep looking for an answer and let you know what we find.

    Again thanks,
    Mark
  • 0 in reply to Mark Goodberlet
    TI__Guru* 91947 points

    Mark,

    What happens to the output when you change the setting of bit D4 on add 0x16?

    Jim 

  • 0 in reply to jim s
    Prodigy 70 points

    Hi Jim,

    Again, thanks for your thoughts.   As you suggested, we made plots of output counts vs “input voltage” when using both the “straight binary” and “2s complement” setting for the ADS62P24.  Please note that “input voltage” is an upstream voltage which goes through our filters and signal conditioning before it reaches the ADS62.  So, the attached plots are a kind of total systems test.

    The plots show good linearity and expected behavior for the “straight binary” and “2s complement” settings.  Data for the plots is from a board that was fabricated a few years ago.  The board (same design) that was fabricated recently and has the sign reversal is unavailable until about 01Aug.  When this newer board returns, I will collect data from it and make the same to plots.  -and let you know what I find.

    Thanks,

    Mark

  • 0 in reply to Mark Goodberlet
    TI__Guru* 91947 points

    Mark,

    If you think there is an issue with the new parts, if you send us a couple, we can swap out the ones on our EVM and see if there is an issue. I am going to close this post and wait to here from you when you get your new boards.

    Regards,

    Jim