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ADS124S06: Some questions about IDAC of ADS124S06

Lin_Charles
Expert 4260 points
Part Number: ADS124S06

Hi all,

I have used ADS124S06 to design RTD temperature measurement. Below is my schematic: When the temp=850C，the resistor of RTD is about 400R. IDAC1 comes out from AIN5, IDAC2 comes out from AIN3, AIN1 and AIN2 are the input. And IDAC from ADS124S06 is 1mA, I have calculated the compliance voltage of IDAC, 0.8(R22)+0.4(RTD)+1.72(R2)+0.4(L1)=3.12 < AVDD-0.6=4.4. But when I use this circuit to measure 400R resistor, the voltage of R2 will be unstable. And the voltage wave of RTD is seen fig.2, it was also unstable. If I remove L2, means I did not use IDAC2, the wave will be normal again.  What is more, if I reduce the current of IDAC to 0.5 mA, the wave will be normal again. I cannot find the reason, can anyone give me some advices on it? Thanks a lot.

Fig.1

Fig.2

  • 0
    TI__Guru** 112895 points

    Hi Charles,

    I don't think that the analog is the big problem but rather the digital.  Are your connections to the micro really 5.2V or should this be 3.3V?  Also, DRDY is an active output that you have a 33 ohm pullup resistor to 5.2V.  When DRDY goes low, this will sink a lot of current that could damage the device.  Double check your digital and verify that the supply needs to be 5.2V and make sure you remove the R4 resistor from DRDY.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 4260 points
    Hi Bob,

    Thanks for your continuous support. Your advice helps me a lot. After I remove the 33ohm from my circuit, the wave is normal. But I am curious about why this pull resistor will have a influence on the IDAC? Or why this digital signal affect the analog circuit part? Thanks for your support again.
  • 0 in reply to Lin_Charles
    TI__Guru** 112895 points

    Hi Charles,

    DRDY is an active driving output regardless of CS.  When DRDY transitions low, DRDY will sink a lot of current when pulled up to 5.2V and the pullup is 33 ohms.  This current is (5.2V/33 ohms) is about  157mA.  This is way more than the normal driving current for the device which is closer to 30mA.  This excessive current may be sagging the supply or may be causing an internal issue within the ADS124S06. 

    When using a pullup on a driving circuit, the minimum resistance should be 10k ohm.  But there is no reason to use the pullup at all as it is an output and can be left to float.  Only digital inputs should not be floating.

    Best regards,

    Bob B