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ADS1248: Unable to get proper ADC count, however getting variations according to change in resistance.

Abhishek Dhumal
Prodigy 30 points
Part Number: ADS1248

I am using ADS1248. I have connected a potentiometer across AIN2 and AIN3, with internal excitation current 500μA.

I am getting variation in the count by changing the potentiometer resistance but it's not in a full range. Not sure for that, ADC count is correct or not.

I am following the Pseudo Code example.

Register settings:

MUX0 - 0x13
VBIAS - 0x0c
MUX1 - 0x30
SYS0 - 0x02
IDAC0 - 0x04
IDAC1 - 0x8c
SPI connection with controller is correct, as I am able to read back register values.
Support is appreciated!!!!
Abhishek
  • 0
    TI__Guru** 112705 points

    Hi Abhishek,

    Welcome to the E2E forum!  It would be helpful to see your schematic and the value of the pot you are using and how you have it connected to the analog inputs.  It is not clear as to why you are using VBIAS at AIN2 and AIN3 if you are trying to measure a voltage across these same inputs.  What you are doing with VBIAS is setting the AIN2 and AIN3 inputs at (AVDD-AVSS)/2 which is effectively shorting the inputs together. 

    Also, you must make sure that the total pot resistance times 500uA maintains the compliance voltage.  You must also make sure that the path to analog ground exists for the current to travel from the IEXC1 output to analog ground.  And lastly, you must make sure that the measurement you are making is within the correct analog input range for the ADS1248 PGA.  The ADS1248 has a common-mode input restriction which prevents a single-ended measurement if you are using a unipolar analog supply with one of the inputs tied to analog ground.

    A pot is usually used as test, so what type of sensor are you intending to use?  Will this be an RTD?  If so I would suggest taking a look at A Basic Guide to RTD Measurements:

    http://www.ti.com/lit/an/sbaa275/sbaa275.pdf

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Hi Bob,

    I have attached schematic for hardware connections. Moreover, POT range is 0 - 200ohm.

    we are going to connect RTD across inputs.

    Updated register settings:

    /*MUX0*/ = 0x13 

    /*VBIAS*/ = 0x00 

    /*MUX1*/ = 0x30

    /*SYS0*/ = 0x02 

    /*IDAC0*/ = 0x04
    /*IDAC1*/ = 0x8c
    I have disabled VBIAS from channels.
    after changing VBIAS values we are not getting any changes in ADC count1882.ADC.pdf
  • 0 in reply to Abhishek Dhumal
    TI__Guru** 112705 points

    Hi Abhishek,

    Mostly the schematic appears to be ok.  However I think the biggest issue is with the reference resistor.  Both R637 and R736 must connect to AGND.  As it is now the IDAC has an open circuit path.

    The second issue with the reference resistor is the value.  Ultimately you want the RTD measurement to be ratiometric where the excitation current excites both the RTD and the reference.  In this way the code returned (relative to total number of +full-scale codes) represents the ratio of the RTD to the reference resistance.  If you are using a 500uA current along with 820 Ohm resistance, the voltage drop is 0.41V which is less than the minimum of 0.5V required for the reference input voltage.  See the table in Section 7.3 of the ADS1248 datasheet.  So you will either need to increase the current or change the resistor to a value that is larger than 1k.  Although 1k should be sufficient, most likely due to tolerance and drift the voltage could drop below 0.5V.

    It appears that the circuit is designed for two, 4-wire RTDs in series.  When conducting the tests you must make sure that when using a fixed resistor, or a pot, that the resistance is installed in such a way that the current path flows through all connections.  For example, the IEXC1 current enters J45 pin 1.  If the fixed resistor is connected to J45 pins 2 and 3, the current needs to flow through the resistor and out J45 pin 4.  This means that J45 pins 1 and 2 need to be jumpered together as do pins 3 and 4 to complete the current path through J45.  The same is true for J33 as the current must flow into J33 pin 1, then through the resistor and out J33 pin 4.  To complete the current path, the current flows through R637 and to AGND (when R637 is connected properly).

    Right now the reference you have selected is the internal 2.5V reference which means you should be able to make a valid measurement.  However, as the measurement is not ratiometric, you will be subject to the affects of accuracy and drift of the excitation source.  For now in your early testing, it is ok to use the internal reference.  Eventually you will want to make the measurement ratiometric  by changing the value of the reference resistor and selecting the appropriate reference for the RTD measurement you wish to take.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points
    Hi Bob,
    As per your suggestion, we have connected  R637 and R736 with ground and set excitation current is 1mA.
    Below is the register settings:
    MUX0 - 0x13
    VBIAS - 0x00
    MUX1 - 0x30
    SYS0 - 0x02
    IDAC0 - 0x06
    IDAC1 - 0x8c
    this issue is fixed now.
    Thnak you for your support.