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ADS114S06B: Thermistor resistance measurement circuit using constant current source (IDAC)

Part Number: ADS114S06B
Other Parts Discussed in Thread: ADS124S08

Hi Team

 My customer is considering thermistor resistance measurement circuit using a constant current source (IDAC).

If customer touch the wiring connected to the thermistor during resistance measurement, It is confirmed that the IDAC is turned off. (Once it turns OFF, it will not return until it is reset.)

Are there any specifications for the IDAC, such as turning it off when the voltage/current of the selected AIN line exceeds a certain threshold?

PS; Customer have confirmed that the internal voltage of 2.5V is output without any problems when the IDAC is turned off.

best regards

TATSU

  • Hi TATSU,

    Most likely if the sensor/wiring is touched and the current stops flowing there is an ESD or transient discharge that flows back into the ADC.  If the transient is large enough it is possible that the ADC is forced into an undesired state requiring the ADC to be reset.  If the transient is large enough, it is possible that the ADC can be damaged from electrical overstress (EOS).  For this reason, the IDAC output channels as well as any analog input measurement channels be properly protected against EOS events that may cause the analog input channels to exceed the Absolute Maximum Operating Conditions as specified in the datasheet.  This may include series resistance and external protection diodes such as TVS diodes.

    Is it possible for me to see the schematic to be able to make recommendations?

    Best regards,

    Bob B

  • Hi Bob

     Thank you for your response!.

    Customer ask me the additional question.

    Will the protection (OFF) work even if the IDAC output channel is shorted?

    best regards

    TATSU

  • Hi TATSU,

    I'm not sure I fully understand the question but I will try to clarify.  If an analog pin is used to deliver IDAC current, the pin must be protected from EOS events.  Turning off the IDAC through register setting is not enough to protect the ADS114S06B.  The internal ESD diodes are to protect the device during device transit and system manufacturing.  The ESD diodes are not designed to handle large transient currents from EMI/RFI or those ESD discharges beyond what is stated in the datasheet.

    There is circuit application note Circuit for Protecting ADS124S08 ADC from EOS for RTD Measurement that may be helpful regarding this topic.  The ADS124S08 is the 24-bit ADC version and would still apply to the ADS114S06B.  A current excitation thermistor circuit will be similar to a 2-wire RTD implementation.  The author has also put together a video series on EOS.  One of the videos can be found here.  On that same video page is a link to other videos in the EOS series.

    If the IDAC pin is shorted the ADS114S06B should not be negatively impacted regardless of whether the IDAC is turned on or off as long as the Absolute Maximum Ratings as stated in the datasheet are not exceeded.

    Best regards,
    Bob B