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ADS1248: Problems with measuring current settings (IDAC)

Markus Hofbauer
Prodigy 80 points
Other Parts Discussed in Thread: ADS1248

Hi all!

I use one ADS1248 and have connected 2 Pt100x resistors (2-wire) on AIN0 / AIN1 and AIN2 / AIN3. The measuring current (IDAC) for the Pt100 is connected with IOUT1 resp. IOUT2. I use the following IDAC currents: 0.75mA, 1mA, 1.5mA - all now problem during normal operating conditions (I use the ADS1248 since about 1.5 years for my projects).

Now the problem: Under burst-test conditions (2kV burst voltage) we measure (reproducible) measuring-current (IDAC) values at the Pt100 resistors, that exceeds the specified tolerance limits! For example the following 3 measured values per nominal IDAC-value (the first value in each line is the setup nominal IDAC0 value, the other values are measured):

0mA:          0                   0                 0

0,75mA:    0,545mA    0,713mA     0,5627mA

1mA:          0,721mA    0,944mA    0,829mA

1,5mA:       1,087mA    1,423mA    1,251mA.

Each measuring-current value is constant for about 1 - 5 seconds, then change to another (wrong) value. After burst-test, the measuring-current keeps at a constant but wrong value! How is it possible, that the ADC produce such wrong IDAC values? The only way (we have found) that the ADC works correct after such burst-conditions, is to reset (or power-up) the ADC by RESET\ pin. Just recovering all ADC-registers with there nominal values don't work.

Do you have any idea what's wrong here?

My problem is, that we have already a big amount of units in the field. So we cannot just change the hardware circuit (better EMC protection), i need a firmware solution at this time!

Best regards,

Markus.

  • 0
    TI__Guru** 113765 points

    Hi Markus,

    Can you tell me more about how the 2kV burst is being applied so we can understand a little more about problem?  2kV is usually handled by the ESD cell, but there are other considerations relative to sharp rise times across any inductance.  This is often related to layout issues and in particular the ground.  If the burst causes a situation where one of the absolute maximum conditions is exceeded, the device register contents may be corrupted or the device might be placed in an invalid state.  It would be helpful to see the schematic and PCB layout to determine potential issues.

    I would suggest trying to read back the register contents to see if they have changed after the burst event.  This might give us some more detail on what is going on.  At this point, the only firmware solution I can think of is to reset the part.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 80 points

    Hi Bob,

    here a schematic of our grounding/shilding:

  • 0 in reply to Markus Hofbauer
    TI__Guru** 113765 points

    Hi Markus,

    Nothing was inserted as an image or attached as a file.  You can insert multiple images, but you can only attach one file.  If you want to use a zip file you can group more than one file that way.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 80 points

    Hi Bob,

    another try:

    7723.Potenzialtrennung DIDO Burst.pdf

    Best regards,

    Markus.

  • 0 in reply to Markus Hofbauer
    TI__Guru** 113765 points

    Hi Markus,

    Do you see the problem in both indirect and direct bursts or just one in particular?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Guru** 113765 points

    Markus,

    A couple of other things to analyze.  Does the ADS1248 recover if the Start pin is taken low, then returned high?  This should reset the internal analog circuitry.

    Another thing to look at is the Vrefout pin to see if the value has changed.  The IDACs derive the current output based on the internal reference.  It would be interesting to know if the reference voltage changes with the burst test and if this is the result of the erratic current readings.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 80 points

    Hi Bob,

    1. The direct bursts onto the binary I/Os are the problem.

    2. Start-pin is always high. Only after power-up, i let the ADC goto sleep mode (Start-pin = low) and then set (and let) Start-pin = high.

    Vrefout-pin is wired over a 1uF capacitor to analog-ground.

    Best regards,

    Markus.

  • 0 in reply to Markus Hofbauer
    TI__Guru** 113765 points

    Hi Markus,

    Is it possible to check the reference voltage on the Vrefout pin after the burst test to see if it has changed in value?  Is it possible to try and see if instead of a Reset changing the state of the Start pin will reset the device to the correct current output?  The answer to these questions can help in narrowing down the problem.

    Best regards,

    Bob B