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Original question:

LMP90100: Use multichannel mode with burnout currents

LMP90100: Input signal changes if Output data rate is changed while Using multichannel mode with burnout currents

Naman Kumar
Intellectual 430 points
Part Number: LMP90100
Other Parts Discussed in Thread: ADS124S08

I am using ScanMode3: One or more channels Continuous Scan with Burnout Currents, and using 4 channels for conversion. When i switch ON burnout currents and change ODR from 13 sps to 26 sps, the voltage between sig+ and sig- increases by almost 0.5mV and hence the test counts change(they remain stable at that level). When ODR is increased from 53 to 241 sps, the signal voltage and test count increased by 3 times. So the signal voltage increases with increase in output data rate and hence the test counts increase. When i switch off burnout current it works fine. Can this issue be resolved or is this the way it works?

Regards,

Naman

  • 0
    TI__Guru 56442 points
    Hi Naman,

    I didn't have time to get to your question today - I'll take a look first thing next week.
  • 0 in reply to Alexander Smith
    Intellectual 430 points
    Is there something wrong with circuit design or burnout currents?
  • 0 in reply to Naman Kumar
    TI__Guru 56442 points
    Hi Naman,

    Did you have a chance to evaluate the ADS124S08?

    I believe that the burnout current is flowing through an unexpected path when the ODR is changed and the digital filter's reset which may lead to this offset.

    9.3.7.1 Background Calibration
    Background calibration is the process of continuously determining and applying the offset and gain calibration coefficients to the output codes to minimize the LMP90xxx’s offset and gain errors. Background calibration is a feature built into the LMP90xxx and is automatically done by the hardware without interrupting the input signal. Four differential channels, CH0-CH3, each with its own gain and ODRs, can be calibrated to improve the accuracy.

    9.3.7.1.2 Using Background Calibration
    .....

    Does the problem persist after calibrations have been performed?

    Alternatively, does the issue persist if the Burnout currents are disabled before switching the ODR, then re-enabling the burnout currents?
  • 0 in reply to Alexander Smith
    Intellectual 430 points
    Hi Alexander,

    I am not using background calibration. But I am checking the offset and gain registers continuously while reading ADC conversion data and they are not changing when i change the ODR. Also I am not changing the ODR on the go, I keep it fixed. Its just that I change ODR by flashing code with new value and I am enabling burnout currents after writing all the ADC settings. It is actually the input signal which is changing as I checked it with multimeter. This is weird as it never happens and it is working fine with burnout currents disabled.
  • 0 in reply to Naman Kumar
    TI__Guru 56442 points

    Hi Naman,

    So you re-write the registers to change the ODR and the input signal is output as expected. Then you enable the burnout currents and the offset between the signal generator output increases based on the ODR?

    This sounds like an issue with the signal generator being affected by the burnout current. Adding buffer amplifiers to the input may help. 

  • 0 in reply to Alexander Smith
    Intellectual 430 points
    I was using a single strain gauge for 2 channels, due to which i was facing issues. The issue is resolved now.