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ADS124S08: Implementation of device for 8 thermistors and an RTD measurements.

nestor Garcia
Prodigy 10 points
Part Number: ADS124S08

Hello, 

I am an Electrical Engineering undergraduate working on a class project, so safe to say I am pretty novice. 

The general idea is that my group is prototyping a Temperature measurement system. We are exploring 2 ideas for the sensors; strain gauges & thermistors. 
My role is to create a electronic interface that can extract data from both of these sensors. Not at the same time, but either 5 strain gauges, or 8 thermistors. 
As such, I am working on a PCB that can handle both sensor types & came across this ADC device. 

Sifting through the documentation, I have a general idea of how this would come together and am pretty confident this will work, but I'm not too comfortable with the implementation. 

I am using a 4-wire RTD sample as a representation for the strain gauge(SG) system and implementing a series of voltage dividers for the thermistors, yielding a single ended input to the adc. 
I will be using 3 ADC:

ADC1 - SG1 & SG2
ADC2 - SG3 & SG4
ADC3 - SG5 & 8 10k ohm thermistors

Precise resistor values are yet to be calculated, but my question is with ADC3 & how I can implement the strain gauge reading with single ended thermistor readings (if possible). 
Below is a copy of my schematic so far, please be as critical and detailed as necessary, as I have a great deal to learn.

CURR1+/- are the Strain gauge/ RTD source currents, and RTD1+/- would be the measured voltage. 
And RT1-8 are the single ended voltage inputs that come from a voltage divider. 

I'm not to clear on how the reference would work. Hopefully this all makes sense, thank you in advance. 


  • 0
    TI__Guru** 113765 points

    Hi nestor Garcia,

    There are a couple of resources available regarding temperature measurement.  They are A Basic Guide to RTD Measurements and A Basic Guide to Thermocouple Measurements.  There is also a Temperature Sensing Code Library that may be useful when making computations.  There is also example C code for the ADC itself that can be utilized as well.

    Generally you want your measurements to be ratiometric if possible.  This is important as measurement excludes the drift of the excitation source.  For thermistors the most simple approach is a voltage divider where the excitation comes from REFOUT of the ADS124S08 (which is also used as the reference for the ADC conversion).

    As far as the bridge measurement is concerned, review the material in A Basic Guide to Bridge Measurements.  I'm not sure what your intentions are for using a 4-wire RTD to represent a bridge measurement.  Can you explain in more detail your use case?  Generally your bridge measurement would be excited by the analog supply voltage which would also be the reference inputs for the bridge measurement.  The output of the bridge would then connect to the ADC inputs. 

    Review the material in the basic guides and see if the information in the guides answers your questions.

    Best regards,

    Bob B