• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 760 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1220: Problem in short circuiting the differential inputs of the ADC

Malik Alsalman
Prodigy 70 points
Part Number: ADS1220

Short circuiting the differential inputs doesn't sets the reading on zero or close to it. the readings starts flickering randomly.

  • 0
    TI__Guru 63885 points

    Are they shorted together and floating, or shorted together and set to a known voltage potential?

  • 0
    TI__Guru** 113765 points

    HI Malik,

    A couple of specifics on the ADS1220.  There is an internal mux register setting that will place a short at the inputs.  Where this is primarily used is for determining the offset of the device.  The returned value should be subtracted from all subsequent conversion results.

    When applying an external short to the device, you must make sure that the short lies within the common-mode input range for the ADS1220.  If the PGA is enabled, then you cannot tie the inputs directly to AVDD or AVSS, and you should not let them float as the input could potentially float outside of the common-mode input range.

    The best method of applying a short is similar to the one using the internal mux short, where the short is at (AVDD-AVSS)/2.

    Best regards,

    Bob B

  • 0 in reply to Collin Wells
    Prodigy 70 points

    Thanks, no i shorted them and connect them to ground, this is the first test. The second one, I used the load cell and tried to pass the negative region to the positive and vise versa, on both cases I was be able to capture this random reading point.

  • 0 in reply to Bob Benjamin
    Prodigy 70 points

    Yes, I saw it and agree to use it when there is a steady offset point, put in my case it is a random flickering reading between -0.4 and 1.7 volts which is approximately the half of the full scale.

  • 0 in reply to Malik Alsalman
    TI__Guru** 113765 points

    Hi Malik,

    As I mentioned previously, if you have the PGA enabled you cannot connect the inputs to ground as that is outside of the common-mode input range and the results will be random.

    With a load cell connected there may be an offset and the result may have noise.  Have you looked at the datasheet example for a bridge type measurement?  The measurement should be made ratiometric so the excitation noise cancels out of the result.  You may have other noise influences as well such as EMI/RFI.  The most common of which is power line-cycle noise picked up in the wiring.

    Can you send me your schematic and the register settings you are using?  Can you also send me the raw conversion results as a continuous series of 128 to 256 points?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points

    I did the same thing with the bridge connected and I saw the same behavior once I pass the zero where both inputs are the same.

    Yes, and can you please make it offline since I'm working on a product and there is a copy right associated with it. And I can send you a video of this behavior as well.

  • 0 in reply to Malik Alsalman
    TI__Guru** 113765 points

    Hi Malik,

    You can contact me offline at:

    pa_deltasigma_apps@ti.com

    Best regards,

    Bob B

  • 0 in reply to Collin Wells
    Prodigy 70 points

    This is what I did yesterday:

    1. Short circuit them with floating: Random reading.

    2. Short circuit them with 0V: Random reading.

    3. Short circuit them with 5V (Vcc): Correct reading (0.0002V).

    5. Short circuit them with potentiometer: Correct reading (0.0002V) at some voltages >0 and once it close to Vcc.

    Thanks.

  • 0 in reply to Malik Alsalman
    TI__Guru** 113765 points

    This question has been take offline.

    Best regards,

    Bob B