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ADS124S08EVM: Drifting signals

Alessandro Schaer
Prodigy 30 points
Part Number: ADS124S08EVM
Other Parts Discussed in Thread: ADS124S08

Dear community

we are building a test platform to measure forces applied on spring elements. We decided to use strain gauges (SG) for this purpose. For evaluation of the signal we are using a ADS124S08EVM.

We have the majority of the issues solved, but for some reason we encounter big drift while nothing is happening. We initially supposed it may be due to temperature fluctuations, however as we are using two strain gauges in half bridge configuration, temperature should not have any influence on the measured signal. Furthermore, we observed a similar behaviour without SG.

Attached you can find a summary of the latest plots with a summary of the used parameters.

Please let us know if you have any suggestions on parameters of the EVM or hint on the measurement with SG.

Thank you in advance and best regards

Alessandro

drift.pdf

  • 0
    TI__Guru** 113765 points

    Hi Alessandro,

    The EVM was never tested for long term drift performance and is not necessarily optimized for drift.  As far as your configuration, what is the actual resistance of your strain elements?  At one point you used 120 ohm resistors which will actually exceed the normal operating current of the reference output of the ADS124S08 (10mA max).

    How are you actually collecting the data? Are you just taking periodic points?  How often?

    There are a number of potential things that can affect drift performance.  To narrow down the drift issues I would first create a baseline measurement by creating a voltage divider from J8 pins 1 and 4 to create a bias voltage at mid-AVDD supply for the analog inputs (make sure JP11 has pins 1 and 2 jumpered to AGND).  I would then short the pins J8 pins 2 and 3 together and connect to the mid-supply junction of the resistor dividers.  You may find some improvements using the global chop feature. 

    Now that you have a baseline, you can try different options to see what is affecting the drift the most.  One additional thing you could try is to make a Kelvin connection to the bridge as a sense point for the reference inputs which essentially creates a 6-wire bridge connection.  Often times there are losses in the cabling which is enough to cause drift due to temperature.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points
    Hi Bob

    Thank you for your reply! We are collecting data with your (TI) software at 2.5 Hz.

    We will follow your advice and see if we can get any better results. Good to know though that the EVM itself (as a non final design) might be the problem.

    Best
    Alessandro
  • 0 in reply to Alessandro Schaer
    TI__Mastermind 23785 points
    Hi Alessandro,

    I was wondering about one thing in your measurement report. You are talking about an 'internal or external 2.5V power supply'
    The ADS124S08 does not operate down to power supplies of 2.5V. The minimum analog (AVDD) and digital (DVDD) power supply voltages are 2.7V.
    But I guess you are referring to the internal or external voltage reference instead here. Is that correct?

    Regards,
  • 0 in reply to Alessandro Schaer
    TI__Guru** 113765 points

    Hi Alessandro,

    Just to be clear, the EVM was not checked for long term drift performance, but that in no way means you cannot get good drift performance with the EVM.  Based on your data, it would appear that your drift is worse with the strain gauges as compared to the resistors alone.  This drift can come from a number of sources. 

    In theory, with a ratiometric measurement, drift will cancel out of the measurement due to drift in the excitation source if both the reference and excitation are the same.  But what about self-heating of the stain gauges or issues as they relate to the cabling?  What about the stability of the environment?

    As I stated in my previous post, the 120 ohm resistances will draw significant current which exceeds the normal operating condition of the ADS124S08 internal and onboard reference when using the REF5025.  Also, with the heating of the resistive bridge, I calculate the resistors will dissipate about 13mW of power.  Depending on the tempco of the resistors, there could easily be a measureable drift of the resistors. 

    So I don't think you can make any strong conclusions about the EVM drift based on your experimentation so far.

    Best regards,

    Bob B 

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob

    Thank you for your reply an suggestions.
    We were able to perform some more measurement. Please see the attached document for updated plots, etc.
    Thanks to your suggestions we managed to reduce drift to approximately 3uV. This is not a bad result, but if possible we would like to reduce it further.
    We also tried measuring the behaviour of the Voltage supply of the EVM with and without connection to SG.  We saw big drift/fluctuation over time in the measured signal. May this be a source of the problem?
    We are collecting data with Delta-Sigma ADC EvaluaTIon Software. Regarding parameters, you can find the register map settings in the second slide of attached document. 

    Best 
    Michael 

    2870.drift.pdf

  • 0 in reply to Michael Mattmann
    TI__Guru** 113765 points

    Hi Michael,

    If the excitation and reference are the same voltage source, then you should not see the drift of the source within the measurement.  This assumes that the components do not drift.  We often will use a large ESD bag or isolation box to limit the air currents that surround the components.  We have seen significant drift due to air currents from fans, and adjacent equipment.

    There will be some self-heating of the components in the wiring, strain gauge and any resistors connected.  I created a bridge using low drift, 499 ohm precision resistors excited from the internal reference connected to J8 and saw the same output as I did when I had shorted inputs biased to mid-supply.  As the output was close to zero from the bridge, I would expect the results to be similar.  I will attempt the experiment again using a different resistor in one of the legs to increase the output.  The whole EVM was placed into a large ESD bag to isolate from external air currents.

    Best regards,

    Bob B