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ADS124S06: input impedance issue

naveen Namani
Prodigy 50 points

Part Number: ADS124S06
Other Parts Discussed in Thread: ADS131A04

Hey TI..,

We are Devoloped a PH Meter with the following circuit Refference images are (PH1.PNG &ADS124S06.PNG ). 

we have a working Reference Design circuit Refference Images are (PH.PNG & ADS131A04)

 

Issue here is with Refference Design Working properly with sensors having 230Mohm  & 990Mohm with no issue.

when we tested with ADS124S06 Low impendace Sensor 230Mohm Working Properly and with High Impedance sensor 990Mohm it is not working.

when i tried to analyse the Results with 230Mohm & 990Mohm i found results are

Z=230Mohms                                         Z=  990Mohms

pH buffer --->   result               pH Buffer  -----> Result

4.01pH ----> 173mV                4.01pH  ------> 140mV

10.01pH  ---->150mV              10.01mV ------> 105mV

 

Circuit Configuration we used are 

  1. Op-Amp we used as buffer.
  2. offset circuit with 10Kohms are added to measure evento measure negative values also with 2.5V referrence provided by internal ADC.
  3. ADC is to convert mV reading to Digital code.

 

kindly help us to work design with both 230Mohms and 990Mohms sensors.ADS124S06.PNGPH.PNGPH1.PNG

 

 

  • 0
    TI__Guru 59441 points

    Hi naveen Namani,

    Some questions for you:

    • Have you measured the voltage directly at the input pins (AIN4 and AIN5) to determine if the ADC is causing the issue or the external circuitry?
      • You can also measure across all of the resistances in the input path to determine if there is a meaningful voltage drop that could cause the offset you are seeing
    • What are the ADC register settings you are using?
    • What is the voltage at VREF_P?
    • You mentioned a working ref design with ADS131A04, but I didn't see any info about this in the images you sent. Do you have a schematic for that circuit as well?

    -Bryan

  • 0
    Prodigy 50 points

    This Circuit is Working Properly for Impedance 230M ohms. but the problem is with 990M ohms. results are mentioned above

    1. ADC Ref voltage is 2.5V iiternal.

    2ADC Configurations

           

      if (!WriteRegister(dev, 0x02, 0x54))

        return false; // I/p MUX: AIN5-AIN4 0x54

      if (!WriteRegister(dev, 0x03, 0x00))

        return false; // GAIN: PGA=1, 20SPS

      if (!WriteRegister(dev, 0x04, 0x14))

        return false; // DataRAte : 20SPS, osc-4.096MHz

      if (!WriteRegister(dev, 0x05, 0x3A)) //0x3A

        return false; // REF : internal 2.5V On always.

      if (!WriteRegister(dev, 0x06, 0x00))

        return false; // IDACMAG`

      if (!WriteRegister(dev, 0x07, 0xFF))

        return false; // IDACMUX

      if (ofcData != NULL) {

        if (!WriteRegister(dev, 0x0A, ofcData[0]))

          return false; // OFCAL0 (LSB)

        if (!WriteRegister(dev, 0x0B, ofcData[1]))

          return false; // OFCAL1

        if (!WriteRegister(dev, 0x0C, ofcData[2]))

          return false; // OFCAL2 (MSB)

  • 0 in reply to naveen Namani
    TI__Guru 50405 points

    Hi naveen Namani,

    It appears the positive reference buffer is currently disabled. To enable it, please write 0x1A to the Reference Control Register (address 0x05) instead of 0x3A.

    If the issue persists, please measure the voltage directly at the input pins (AIN4 and AIN5) following Bryan's suggestions, let us know the conversion code you read from the ADC when you measure the voltage. This will help us determine whether the root cause lies within the ADC or the external circuitry.

    Additionally, any further details regarding your reference design with the ADS131A04 would be very helpful for our investigation, as Bryan mentioned above.

    BR,

    Dale