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AMC1350: Random AMC1350 Offset at startup

MOHAMMAD RASOOL
Prodigy 30 points

Part Number: AMC1350
Other Parts Discussed in Thread: ADS1220, , AMC3302

Greeting,

Our product implements AMC1350 and ADS1220 to measure voltages upto 200Vpk. We see a small offset that is random at startup but is consistant when the device is kept running. This offset is around 1mV at AMC1350 output pins as stated in data sheet but it moves slightly everytime the device is powered up. This produces errors in our readings. We also see the same behaviour in our current measurement setup with AMC3302.

Please, share any rectification methods and causes for this issue. If there is anything else needed from my end please let me know.

Schematics: DLV003_V2_VI_BOARD.pdf 

Best regards,

Rasool

  • 0
    Prodigy 30 points

    Additionally, here is the test code for reference. The issue is prominent in some units than others.

    Best regards,

    Rasool

    adc_ads1220.hadc_ads1220.cpp

  • 0
    TI__Expert 4281 points

    Hello Rasool, let me please check with the design team. 

    Meanwhile, what is the value of R2, R3, R8, R9 ?

    The maximum loading capacitance of the AMC device is 500pF or 250pF differentially. Also, I would remove the common mode caps on the output. This typically makes things worse. The output is differential. Do you have any strong source of common mode noise between the AMC device and the rest of the circuitry ? (MCU?)

    Best regards, Jiri

  • 0
    TI__Expert 4281 points

    Rasool, would it be possible to measure outputs of the AMC1350 and AMC3302 with a precise bench multimeter? 

    I would like to know if the offset comes from the AMC devices or the ADS device. 

    ADS devices are a different group and my colleagues would need to take over.

    Best regards, Jiri

  • 0 in reply to Jiri Panacek85
    Prodigy 30 points

    Hi Jiri,

    Thanks for replying. The resistors are 10 Ohm with 1% tolerance. Thanks for the recommendation of removing the differential caps. That was missed in our reviews. ADC has its own high PSRR LDO fixed at 3.3V and AMC has a 5.3V from a switch mode power supply on the LOW side. On the isolated HIGH side another high PSRR LDO powers the AMC with 5V. I did not see any noise issue in the oscilloscope for the SMSP I use.

    I see 0.259 mV at the output of AMC1350 pins with input pins shorted. I have removed the differential capacitor while measuring. Do let me know your thoughts.

    Best regards,

    M G Rasool

  • 0 in reply to MOHAMMAD RASOOL
    Prodigy 30 points

    I made some more measurements with inputs shorted. This time I measured after a powerup sequence at 100 PLC on my DMM. Here are the results:

    a. 0.327mV

    b. 0.316mV

    c. 0.304mV

    and d. 0.319mV

    It changes every time I power cycle the board. This is also what we read with the on-board ADC. A slightly different offset. 

  • 0 in reply to MOHAMMAD RASOOL
    TI__Expert 4281 points

    Thank you for additional measurements. I asked my colleague to test it out on our EVM. Please allow a few hours for the update.

    Best regards, Jiri

  • 0 in reply to Jiri Panacek85
    Prodigy 30 points
    Hi Jiri,
    Any update to share?
    Best Regards,
    M G Rasool
  • +1 in reply to MOHAMMAD RASOOL
    TI__Prodigy 170 points

    Hello Rasool,
    I apologize for the late response. We have checked the offset error of the AMC1350 and found similar behavior to what you are describing, though we also found that letting the device sit for a short time after power up decreases the spread of the offset error between power cycles. We have some questions about your setup that would help us:
    Can you tell us how soon after power up of the AMC1350 you are measuring the offset error?
    How close to the outputs of the AMC1350 are you measuring when evaluating the offset error? Are you measuring before or after the filter?
    What does your test setup look like when you measured the offset with your DMM and what DMM did you use? And does the offset the ADC measures match the value you measure with your DMM?
    How critical is the precision of the offset error in your application? Does the ~25uV change of the offset error you measured cause problems in your application or does this mainly concern your expectation of the device behavior?
    Thank you!
    Best regards,
    Simon

  • +1 in reply to Simon Nielebock
    Prodigy 30 points

    Hello Simon,

    I hope you are doing well. And thanks for verifying the issue. Here is the reply in order:

    1. 1000ms after power up. It takes a few hundred milliseconds to initially settle, and we wait a few more in the time we configure our adc.

    2. I measure at the filter output that adc sees. Please share your email so that I can share my layout with you.

    3. I use HDM3055 Series DMM from Hantek. Offset measured by DMM is the same as seen by the ADC. 

    4. Yes, the issue is critical because at lower end our errors are very high when we scale that to the applied signal range. That was not expected in our design.

    Let me know your thoughts.
    Best regards,
    M G Rasool
  • 0 in reply to MOHAMMAD RASOOL
    TI__Prodigy 170 points

    Hello Rasool,

    thank you for your answer! It helps us understand your setup better.

    Please allow a couple days for me to test a few more things. I will contact you via email once I have more results for you.

    Thank you for your patience.

    Best regards,

    Simon