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ADS1232: Noise coming into the reading when reading load cell data using ADS 1232 and other ADCs.

Tony Philip45
Intellectual 755 points
Part Number: ADS1232
Other Parts Discussed in Thread: ADS1247, ADS122U04,

Hi,

We were evaluating a couple of ADCs for measuring weights using a load cell set up, We have evaluated a couple of ADCS such as the ADS1232 ,ADS1247 and ADS122U04 24 bit ads, We have built custom boards for all this with excitation voltage of around 5v to the load cell and Analog inputs.

One issues we have face is that the data values keep jumping when we are touching the laptop to large extend (more like an offset error) for the ADS1232.

when we connect the charger this becomes offset becomes less but there is more of noise now due to 50/60hz (when we touch the laptop). Do you have any solutions to this problem ? Will it affect us when we move this into mass production?.What should we do to reduce this behavior?

We set the ADC to the lowest sample rate and highest ENOB setting for the all readings and tests .

  • 0
    TI__Guru** 113765 points

    Hi Tony,

    Welcome to the E2E forum!  Unfortunately this is a bit difficult to answer as I really don't know anything about your system and how it will be used.  Generally the biggest issues with load cell measurements is poor PCB layout and issues with external noise induced from cabling and improper shielding of the cable and connector to the board.  It also sounds like you may be also having a ground loop issue.

    It would be helpful for me to see your schematic, PCB layout, pictures of your setup and raw data sequences of 128 consecutive results or more.  It would also be helpful to know what your expectations are for the load cell you are using and the load cell parameters.

    You can post that information here or send it to pa_deltasigma_apps@ti.com.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Guru** 113765 points
    Hi Tony,

    Were you able to make any progress? Is there anything more I can do to help?

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Intellectual 755 points

    Hi Bob,

    We were able to troubleshoot and find out it was a noisy reference that caused the issues. We put in an LT1207 with 20mA (max) output but we found that it was not able to power the chips Analog side due to high current demand.

    We put it back on our precision power supply and found that it draws 70mAs just the analog side alone!  The readings were stable to an extend (because the smps is noisy still). Do you have any suggestion for high power precision references?

    What confuses me is that it doesnt match the datasheet Specs too. Do you think there are leakages within the chip?

    I am sending you the schematic and the board layout .

    Thank you so much.

    Tony Philip

  • 0 in reply to Tony Philip45
    TI__Guru** 113765 points
    Hi Tony,

    How are you measuring the analog current? Does the 70mA also include all other connections to the analog supply? Is DVDD also powered when determining the analog current?

    If the current measured is the total current for all devices connected to the analog supply, then you should take that into account for all devices connected. The maximum analog current for the ADS1232 is specified as 2.5mA and this is tested at our device final test. When powering up the ADS1232, PDWN should be held low until both AVDD and DVDD are fully established, and AVDD should be powered at least 10us before PDWN goes high. This is shown in Figure 39 and discussed on page 23 of the ADS1232 datasheet.

    If you have load cells connected when measuring the analog current, this may be a larger portion of the current draw.

    As fart as the supply is concerned, usually a linear low-noise LDO regulator is sufficient for powering the ADS1232, the reference and excitation for load cells.

    I will also respond to your email.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Intellectual 755 points

    Hi  Bob,

    I am supplying  power to the Analog pins and REF pins together using the same supply. The current is as shown by the precision power suply's display. The digital power is of another suply for now to isolate it further from noise. I ill look into this and let you know thank you.