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ADS124S08EVM: About thermistor measurement

Weiming Han
Prodigy 60 points
Part Number: ADS124S08EVM
Other Parts Discussed in Thread: ADS124S08, ADS124S06

Tool/software:

Hi Support Team,

I am using ADS124S08EVM to measure Thermistor and connected J8-4 to GND.  Please see the setting below.  The voltage between AIN8 and AIN9 is 3.356V.   I am wondering why the results could be negative in the data analysis.  thanks,

  • +1
    TI__Guru 52146 points

    Hi Weiming Han,

    You have set the OFCAL registers to 0x7FFFFF, which is the maximum positive offset. This gets subtracted from each measured ADC value before being output on the SPI bus, so it makes sense that you are seeing large negative values.

    Please reset this value to the default and try again

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Thanks Bryan, it solves my problem. I have one more question about it. Do you have to add a follower as buffer to AIN8 for thermistor input? thanks, 

  • +1 in reply to Weiming Han
    TI__Guru 52146 points

    Hi Weiming Han,

    That depends on the performance level you need. The ADS124S08 PGA must be disabled to measure ground-referenced inputs, so if the leakage current is too high under these conditions you should consider adding a buffer

    You could also add another dummy resistance between AIN9 and ground, so the thermistor is no longer a single-ended measurement. This would allow you to enable the PGA even with a gain of 1, and the leakage (input) current decreases significantly with the PGA enabled

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    thanks Bryan, what value of dummy resistance between AIN9 and Gnd you suggest to be added is?

  • +1 in reply to Bryan Lizon86
    TI__Guru 52146 points

    Hi Weiming Han,

    Usually the thermistor bias resistor is selected to be the same as the thermistor nominal resistance. So if the NTC_nom = 10k, then you would select 10k. If you use two resistors (one on top and one on bottom), you would probably select each to be 5k in the previous example.

    Or you can remove the top resistance in the image below and move it to the bottom, so now your measurement is differential and you can use the PGA 

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    thanks Bryan,  Below are the two circuits. Could you please let me know what their pros and cons are? And one more questions: Because ADS124S06's datasheet says that AVDD needs a LDO. Do you have any suggestion on its LDO?   - Weiming

  • +1 in reply to Weiming Han
    TI__Guru 52146 points

    Hi Weiming Han,

    I believe we already discussed the differences and therefore pros/cons of each circuit (differential vs single-ended measurement, PGA enabled vs bypassed, etc.).

    You can use the ADS124S08EVM as a reference for you second question. This design uses the TPS7A4700

    -Bryan