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ADS124S08EVM: Measure Thermistor with ADS124S08EVM

Jeff Zhang22
Prodigy 170 points
Part Number: ADS124S08EVM
Other Parts Discussed in Thread: ADS124S08

Hi,

I am using ADS124S08EVM board to measure a thermistor. 

I use 2-wire RTD measurement circuit for this thermistor. The circuit is using Figure 15 of UM of EVM. The thermistor is connected to J7 pin 1 and 4. I use Evaluation software to setup the registers and get the reading. but I can not get correct reading somehow. All readings are 2.5V. 

The registers setting from 0x02 to 0x07 are: 0x10, 0x08, 0x14, 0x14, 0x03, 0xF5. 

What do I miss?

  • +1
    TI__Guru** 112635 points

    Hi Jeff,

    It appears that you want to make a current excited ratiometric measurement.  To use the IDACs, the internal reference must be enabled even if the internal reference is not used for the conversion reference.  The IDACs use the internal reference to establish the IDAC output current value.  As you have no reference voltage (because there is not current), the output will always be full-scale.

    The second consideration is that the reference resistor used on the EVM is 1k Ohm (R68) and is designed for using PT100 RTDs.  The IDAC output current chosen is 100uA establishing a reference voltage of 0.1V.  The minimum reference voltage required by the ADS124S08 is 0.5V.  So you will either need to replace R68 with a higher value resistor, or use a different reference voltage.  I would suggest that initially you turn on the internal reference and select it as the conversion reference for your initial checkout of the circuit.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 170 points

    Hi Bob,

    Thanks for your reply.

    Is the REFCON bits controls internal references? I tried leave it on (REF register :0x1A). But I don't see the reading changes.

    For your second point, what if I select 1mA IDAC current? It will be 1V cross R68.

    But with both changes, I still didn't get good readings.

    Regards,

    Jeff

  • 0 in reply to Bob Benjamin
    Prodigy 170 points

    I think my setup may have problem. The thermistor is 10kohm at 25C. As you mentioned, the R68 is too low for it. if I increase the IDAC, the voltage cross the thermistor will over the ADC input range.  The reference resistance should be large than Thermistor resistance.

    Now I am thinking to use figure 19 circuit. in this circuit, the REFP0/REFN0 is connected to J8 pin1 and 4. Are they used in the measurement?

    Thanks,

    Jeff

  • 0 in reply to Jeff Zhang22
    TI__Guru** 112635 points

    Hi Jeff,

    You are correct that the REFCON bits select the configuration of the internal reference.  When the REF register is 0x1A, the internal reference is used and powered on.  In your original configuration 1mA will be an issue with the 10k thermistor as the largest analog voltage that can be applied is 5V.  There is also a compliance voltage for the IDAC where the voltage drop across the entire current path must be less than AVDD - 0.4V for currents less than 1mA and AVDD - 0.6V for currents 1mA or greater or the current source cannot maintain constant current.

    Figure 19 from the EVM user's guide is a voltage excited thermistor circuit example.  In this example, the thermistor value is calculated from the known resistance of a voltage divider.  You don't have to use REF0 inputs as the ADC reference, but it is probably easiest to do so with the EVM.

    Best regards,

    Bob B