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ADS1241: Unclear value for Vref in the datasheet

Anthony Papageorgiou
Prodigy 30 points
Part Number: ADS1241
Other Parts Discussed in Thread: ADS124S08

Hi !

I want to measure a temperature with a voltage divider and a NTC thermistor. The range of the measure is 0-5V and I use a voltage reference chip (5V to supply the ADS, the voltage divider and for Vref+, Vref- set to GND).

I'll use PGA = 1, as for the range as far as I understood the datasheet if I want a full scale measure I should set Range = 0.

What I understand from the first line is that I can't have a greater difference than 2.6V between Vref+ and Vref-. Should I need a 2.5V regulator for Vref then ?

Thanks in advance,

Anthony

  • 0
    Prodigy 30 points

    Sorry the image didn't get in the message, I'm talking about the first line.

    Anthony

  • 0 in reply to Anthony Papageorgiou
    TI__Mastermind 49931 points
    Hi Anthony,

    Thanks for your post and welcome to the forum!

    I would recommend using a regulator as a resistor divider would subject your reference voltage to unnecessary thermal noise from the resistor divider circuit. Keeping the reference voltage as clean as possible helps ensure accurate output data.
  • 0 in reply to Anthony Papageorgiou
    TI__Guru** 113765 points

    Hi Anthony,

    The ADS1241 is an older device, so if you want to increase performance you could take a look at the ADS124S08.  The ADS124S08 has a number of other options and data rate settings as well as including an internal reference and internal oscillator.  With the ADS124S08 you can also apply a reference voltage equal to the supply where the positive full-scale range is equal to the reference voltage making it easier to make single-ended measurements.  This device is probably better suited for your application, and an EVM is available.  In the EVM user's guide is an example of a voltage excited thermistor measurement.  The ADS124S08 also includes current DACs (IDACs) that will also allow current excitation for the measurement.

    As to the ADS1241, you can apply a 5V reference input if you use Range 1 setting in the ACR register.  Using the setting of Range 1 will divide and reduce the full-scale range to 1/2 of Vref.  In the end, the full-scale range will be the same for the 2.5V Vref at Range 0 as it will be with 5V Vref at Range 1.  The advantage is you will not need a 2.5V reference source in addition to what you have planned.

    Where your issue will be is you cannot directly measure 0 to 5V as the full-scale range is +/-2.5V.  So the voltage you are measuring must stay within the input range for the temperature range you wish to measure.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points
    Hi Alexander & Bob,

    Thanks a lot for your answer, I'll look into it.

    Have a great day,

    Anthony