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problem in reading system monitor ADS1148

Manoj Prabhakar
Prodigy 35 points

Hi, 

while reading the system monitor of ADS 1148, i am unable to read the correct default values of the device, instead i get a wrong default values, i am using the following electrical parameters 

AVDD = +2.5v

AVSS = -2.5v

DVDD = 3.3v

DGND = GND

can any one help, 

Regards

Manoj

  • 0
    TI__Guru** 112895 points

    Hi Manoj,

    There are no 'default' values for the system monitor.  Have you turned on the internal reference and is it selected? Some of the monitor functions require the internal reference.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 35 points

    Thanks Bob, sorry for the delay....

    yes, iam enabling the internal reference, but still it reads the following reading 

    AVDD : 8.16v

    DVDD: 8.16v

    Temperature: 456.58

    Gain : 0.996

    but the expected value is the following

    DVDD: 3.2v

    AVDD: 4.8v

    Temperature: 26.5

    Gain: 1

    but why such thing is happening, can you just help me.

    Thanks in advance.

  • 0 in reply to Manoj Prabhakar
    TI__Guru** 112895 points

    Hi Manoj,

    Are you sure you are using the correct conversion formulas?  Can you send me the formulas you are using to convert the codes to volts/temperature?  Can you also send me the raw digital codes for each of the measurements?  The actual returned code will be related to the reference and about 1/4th of the real voltage for the voltage measurements.  An internal voltage divider is required to drop the measurement within the correct common mode input range and also the correct full-scale range.

    For the temperature measurement, you need to convert the codes to a voltage and then determine the relative difference based on 25 deg C. Lets say you measure 120mV.  25 deg C is 118mV.  This is a 2mV difference.  The temperature coefficient is 405uV/deg C, so 2mV/405uV is about 4.94 degrees above 25, or 29.94 deg C.

    Can you send me your schematic?  You need to have at least a 1uF cap between VREFOUT and VREFCOM, and VREFCOM should be tied to AVSS.

    Thanks,

    Bob B