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ADS1247: Stability Issue

Nimit Vachhani
Prodigy 60 points
Part Number: ADS1247
Other Parts Discussed in Thread: ADS124S06

Greetings Everybody,

I have made a pcb using ADS1247. I am connecting 4 wire load cell to ADS1247. One thing i have noticed is that ADS1247 output varies a lot. I have also cross checked with OSCILLOSCOPE but it shows okay. I am setting up ADS1247 on startup and to make sure all the settings are correct i am also reading back those values.

Please guide me what to do ? I can upload my schematic and also send the pseudo code for ads1247 by tomorrow.

Thanks.

  • 0
    TI__Mastermind 48661 points

    Hi Nimit Vachhani,

    Can you please explain what you mean by "ADS1247 output varies a lot"? The ADS1247 is not a 24-bit noise-free ADC at any data rate or gain setting, so the output is going to vary within the RMS noise specified by the ADC datasheet.

    Have you connected a known good signal to the ADC to benchmark your system's performance? For example, a small battery e.g. AA or AAA whose output voltage is 1.5V. This will help you characterize the behavior of your system. You can also perform a shorted input test to make sure the noise of the ADC or system is within the expected value.

    Please send us the schematic, the register settings you are using, and the specifications of the sensor (sensitivity, excitation voltage, etc.) so we can determine what signals you are trying to measure. Please do not send the pseudo code yet, let's review the hardware first

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Greetings Bryan,

    I did prepare the schematic but i am unable to post here. My schematic is PDF file. FYI, i am not from embedded background so my schematic will be full of bugs. I am connecting load cell to ADS1247.

  • 0 in reply to Nimit Vachhani
    TI__Mastermind 48661 points

    Hi Nimit Vachhani,

    You can post a file to e2e by hitting the "Insert" button at the bottom of the text entry box, then selecting "Image/video/file". This will allow you to post the schematic into this thread.

    I would also try my suggestion from the previous post:

    Bryan Lizon86 said:
    Have you connected a known good signal to the ADC to benchmark your system's performance? For example, a small battery e.g. AA or AAA whose output voltage is 1.5V. This will help you characterize the behavior of your system. You can also perform a shorted input test to make sure the noise of the ADC or system is within the expected value.

    You always want to benchmark your system performance before you apply real signals so you understand the actual accuracy + resolution of your system, as well as how applying the sensor changes these parameters

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Now I am feeling that my connections might be wrong. Tomorrow morning I will try to post my schematic of ads1247 with Stm32 micro-controller.

  • 0 in reply to Nimit Vachhani
    TI__Mastermind 48661 points

    Hi Nimit Vachhani,

    Ok, I will review the schematic tomorrow then

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Hi i have uploaded the schematic in 8712.ads1247.pdf. It is not that tidy but let me know if you find any problem in understanding it. I have checked all the connections and all is fine. I am trying to connect 6 wire load cell to ADS1247. 

  • +1 in reply to Nimit Vachhani
    TI__Mastermind 48661 points

    Hi Nimit Vachhani,

    A couple of questions / comments about the schematic:

    • You show both a GND net and an AVSS net - are these connected somewhere?
    • You have a lowpass filter on your inputs (S+ / S- pins) but not on the SENSE lines. Instead you only have inductors in series with these pins. Why did you design the circuit this way?
    • The RESET pin appears to be floating, because you show pull-up resistors on all of the other digital pins but not on RESET. I would include a pull-up to DVDD on RESET as well
    • I would not include a pull-up on SCLK, DIN, DOUT, or DRDY. Importantly, SCLK should idle low, not high

    One thing to keep in mind is that the ADS1247 is not the best ADC for ratiometric bridge measurements because the reference input range is limited. See Table 7.3 in the datasheet, which I have copied below. Note how the maximum differential reference voltage is AVDD-AVSS-1V, or 4V in your case because AVDD = 5V and AVSS = 0V. Unless there is a large amount of lead resistance in your 6-wire bridge the voltage between REFP and REFN will be much greater than 4V, and the ADC output will be unstable.

    You will need to correct this issue by reducing the VREF voltage for this ADC. Or you can consider using the next-generation version of this ADC, which is the ADS124S06. The ADS124S06 does not have this limitation on the VREF inputs, and will therefore work well for ratiometric bridge measurements.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Greetings Bryan,

    • You are right GND net and AVSS net are connected at one point through 0 Ohms resistor.
    • Yes i have kept low pass filter on S+ and S- pins. Sense + and Sense - lines i have tried making Pi filter using an inductor and capacitors, but if you say i can make low pass filter. Will low pass filter present on S+ and S- lines do the work on Sense + and Sense - ? Actually i was trying to copy the circuit from another make device and also i am not from embedded or electronics background.
    • The RESET pin pull-up resistor was missed by me so i have used axial 10kOhms resistor to pull it up.
    • I will also keep SCKL, DIN, DOUT and DRDY pins floating by removing 10kOhms pull up resistors. Do you say that SCLK should be pulled down ?
    • I didn't know much about REFP and REFN limitations. Should i add voltage divider on SENSE + and SENSE - in such a way that it is in acceptable range ?
  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Thanks a lot buddy. The problem was with RefP and RefN. I stepped down the reference and it started to work as expected. Near future i will consider ADS124S06....

  • 0 in reply to Nimit Vachhani
    TI__Mastermind 48661 points

    Hi Nimit Vachhani,

    I am glad we could help resolve this issue!

    -Bryan