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ADS124S08: ADS124S08IRHBR part RTD measurement accuracy query

Neel Shah
Prodigy 60 points
Part Number: ADS124S08

We are planning to use ADS124S08IRHBR part for measurement multiple PT100s. (2 PT100 in design) 

We are planning to design schematic of PT100 with ADC in four wire in parallel manner.

If cycling of IDAC and measurement from channel to channel then it will give good accuracy measurement? If yes then it will be affected in conducted emission, EMI or EMC certification?

Thanks in advance.

Best Regards,

Neel Shah

  • 0
    TI__Guru** 113765 points

    Hi Neel Shah,

    Will you just be measuring two RTDs or will you also be measuring voltage/current as well? If just measuring RTDs, it is possible to set up each 4-wire RTD separately using separate ratiometric measurements as there are two IDAC sources and two reference inputs.

    If using a single reference input, you can follow the example circuit given in A Basic Guide to RTD Measurements in section 2.11 on page 32.  One of the keys is routing current outside of the filter resistor like shown in the application circuit.  The concern when switching channels is analog settling delays of the filter components when switching input channels.  This requires that when switching channels time is allowed between the channel switching and the start of the conversion.  There is a feature in the ADS124S08 which will automatically delay the conversion start when changing configuration.  The programmable conversion DELAY settings are set in the PGA register (0x03) where various different delay times are available.  The advantage of using this method is delay time to conversion start does not require the micro to use a timer delay and greatly saves processing power and complexity.

    EMC testing and certification can be challenging.  We are in the process of testing various configurations and will have better information on circuit design in a few months.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points

    No. We won't use only two RTDs, will also be measuring voltage and current as well.

    Yes. We have already referred a basic guide RTD measurement document for designing. 

    Could you please tell us on if we do cycling of IDAC and measurement from channel to channel then it will affect in accuracy?

    We are using same design concept for two PT100 measurement which is shown in page no. 32.

    Could you please share the results and circuit design information once TI will be done on testing various configuration?

    Please note that Voltage and current measurement channels are separated ones in our design and using internal reference, if needed then another reference inputs would be use for current/voltage measurements. 

  • 0 in reply to Neel Shah
    TI__Guru** 113765 points

    Hi Neel Shah,

    Neel Shah said:
    Could you please tell us on if we do cycling of IDAC and measurement from channel to channel then it will affect in accuracy?

    The accuracy between measurements will remain the same between channels as long as you allow for the input to complete any analog settling.  The settling time will mostly depend on IDAC current and filter components used, so I cannot give you a specific delay time.

    Neel Shah said:
    Could you please share the results and circuit design information once TI will be done on testing various configuration?

    Following the completion of our EMC testing, there will be a report/application note that will be posted on TI.com.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points

    Hi Bob,

    Thanks for the response.

    Bob said:

    The accuracy between measurements will remain the same between channels as long as you allow for the input to complete any analog settling.  The settling time will mostly depend on IDAC current and filter components used, so I cannot give you a specific delay time.

    Neel said:

    We are planning to use 1mA IDAC current and ADS124S08 ADC IC. May be programmable delay time can be generated through ADC ADS1248S08 IC.

    and filter components is used as per TI four wire pt100 rtd measurement circuit with low side reference guideline document and image has been shown below.

    Is it sufficient information to give delay time for better accuracy? Could you please provide delay time for the same?

  • 0 in reply to Neel Shah
    TI__Mastermind 44851 points

    Hi Neel Shah,

    You are correct, the ADS124S08 has DELAY bits that add a programmable delay between the time you pull START high (or send the START command) and the start of conversion. This is described in section 9.4.4.3 of the ADS124S08 datasheet

    You can select the correct amount of delay based off of the settling time of your filter and the desired resolution. See section 6 in this app note for more information about analog settling and how to calculate it: https://www.ti.com/lit/an/sbaa535/sbaa535.pdf?ts=1696434782632.

    -Bryan