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ADS1248: ask a question about burnout ,confused!!!

dongsheng yi
Prodigy 20 points
Part Number: ADS1248

hi 

i use ads1248 to measure 3 wire PT1000, i can measure a correct value when connecting a high precision resistance.

but it has a problem when i detect if a sensor exist or not, using burnout current. sometimes it give me a wrong information .

below is register i set:

MUX0 = 0xC8  (burnout is 10uA, AIN1 is positive input, AIN0 is negative input)

VBIAS = 0x00

MUX1 = 0x30 (internal reference is on)

SYS0 = 0x22

IDAC0 = 0x90 (turn off)

IDAC1 = 0xFF(turn off)

as datasheet said:"In case of an open circuit in the sensor, these burn-out current sources pull the positive input towards AVDD and the negative input towards AVSS, "

does that mean , positive input will have a voltage ,equal to AVDD? negative input have a voltage , equal to AVSS.

in this design  , AVDD = 3.3V ,AVSS = GND.

after setup register , i halt the software,  using a multimeter to measure voltage on AIN1 and AIN0 , no voltage on that, 

confused!!!

  • 0
    TI__Mastermind 47625 points

    Hi Dongsheng,

    Thanks for your post and welcome to the forum!

    Instead of using a multi-meter, what does the device read as an output code?

    I would expect that the burnout current source would pull the positive pin close to AVDD, and the negative pin close to AVSS such that the reading is full-scale. 

    When measuring this voltage, are one of the RTD leads disconnected resulting in an open circuit? 

  • 0 in reply to Alexander Smith
    Prodigy 20 points

    hi smith

    thank you for replying.

    output code is 0x7FFFFF.when in the open circuit. but almost zero when using mulit-meter to measure this voltage.

    it is supposed to be AVDD when burnout current source pull the positive pin to AVDD.

    output code varies a lot with a fixed resistance.

    so i am very confused about this.

    and  afraid of  that it will have a problem when mass production. 

  • 0 in reply to dongsheng yi
    TI__Mastermind 47625 points
    Hi Dongsheng,

    How much variation do you see with a fixed resistance? Can you provide data from a known DC input signal that shows this variation?