• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 134 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1118: About Resistance Sampling

TI-CSC
Genius 14760 points
Part Number: ADS1118


Hello E2E Experts,

Good day.

I found that when using the thermocouple measurement circuit provided by the ADS1118 datasheet to measure a 2MOhms resistance , it takes about 5-10 seconds to read stable data, and after removing the resistance, it also takes about 5-10 seconds to stabilize. I want to quickly read the correct reading. Pictures are the circuit diagram provided in the  Datasheet, my design, and oscilloscope input and output waveforms.Additionally, my voltage channel acquisition is fine, which I believe can prove that the chip itself has no issues.

Design Files.zip 

Regards,

TICSC

 

  • 0
    TI__Intellectual 1360 points

    Hello Tang, 

    The thermocouple measurement circuit provided in the datasheet is meant for a thermocouple measurement which is ideally lower than 100Ohms. The high resistance added to your system increases the time constant for the RC filter settling time which would explain the delay for the data to stabilize.

    If you aren't measuring a thermocouple this circuit is probably not very helpful.

    From my understanding you are most likely measuring a voltage divider, in which case the pull-up and pull-down resistors could be removed. For the RC filter, this could also be changed. The first order anti-aliasing  RC filter can be changed depending on the cut off frequency you need for your measurement. 

    If you can share a little more about the system, such as expected input voltage range and what is being measured across the 2Mohm resistor I will be able to give more tailored advice. If this is sensitive information communication can continue via email.

    Regards, 

    Andrew 

  • 0 in reply to Andrew Kalthoff
    Prodigy 10 points

    Hello Andrew,

    I'm glad you replied to my question.

    My purpose is to use ADS1118DSGR to measure resistance within the range of 1 MΩ to 5 MΩ.Previously, I used the recommended circuit diagram for measuring thermocouples on the datasheet, but the measurement speed was not ideal.Can you provide me with design suggestions of measuring resistance within the range of 1 MΩ to 5 MΩ?

  • 0 in reply to tang minghui
    TI__Intellectual 1360 points

    Hi Tang, 

    Welcome to the E2E forum, thank you for your interest in our ADCs. 

    Using the 16-bit ADC you will need ~11.1τ for the result to settle within 1 LSB. Where τ = R*C.

    Since you are measuring between 1MΩ and 5MΩ I would consider removing the pull down resistor on Res-. You can create a voltage divider with a precise pull-up resistor on Res+ to calculate the resistance change across your measurement. Additionally, to help the settling time the differential and common mode capacitors could be removed or significantly reduced to make the time constant (τ) smaller.

    To settle within 1 LSB use the equation above settling time within 1 LSB = 11.1 * R * C

    Depending on how fast you would like to sample you can use the equation to find the values of passives that work best for your system. 

    Regards, 

    Andrew