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ADS1248 internal VREF to bias a thermistor

TonyMCC
Intellectual 770 points
Other Parts Discussed in Thread: ADS1248

I would like to use the internal voltage reference option of the ADS1248 ADC.

My question is...would it be possible to use the internal voltage reference to bias a thermistor without affecting the ADC performance ?  I could add an op amp buffer between the ADS1248 VREFOUT and the thermistor sensor if you think it is necessary.

Thanks.

  • 0
    TI__Guru* 98215 points
    Tony,


    The ADS1248 internal reference has a load regulation of about 50uV/mA (typical). The reference should be able to sink or source 10mA max. Based on that, you might be able to add some loading to the reference pin with a minimal change to the reference voltage. However, I don't like tampering with the reference. If you think about it, it's an input like the other analog inputs and any changes to that will add error to your measurements.

    Out of curiosity, can you use the IDAC currents to drive the thermistor? You should be able to set up a ratiometric measurement similar to an RTD measurement. You would still have to use a precision resistor for the reference input, but it's common to make thermistor measurements this way.

    If you're doing something different, post back with a schematic.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 770 points
    Thanks Joseph...I will reconsider the options.
  • 0 in reply to TonyMCC
    TI__Guru* 98215 points
    Tony,


    If you look in the ADS1248 datasheet, Figure 115 shows the topology of a ratiometric measurement. It's a basic figure, but the idea is that with the current excitation, you get a code out that is the ratio between the thermistor value and a known reference resistor.

    With this measurement, you'll want a precision reference resistor. The accuracy of your resistor (along with the drift) directly affects the measurement error.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 98215 points
    Tony,


    I want to add just another comment. The way you have described the original measurement (using the internal reference to bias the thermistor and another resistor) isn't unheard of. I'm just offering another alternate topology.


    Joseph Wu