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ADS1246 power issues

cody lohse
Intellectual 650 points
Other Parts Discussed in Thread: ADS1246, MSP430F2274

 Support,


I am working with the ADS1246 and I am experiencing some difficulties with the chip. I have a strain gage connected to the ADS, and the ADS connected digitally to a MSP430F2274. In order to reduce the number of data lines running between the MCU and ADS. I have tied CS to GND, START to VCC (I do not have a pull up resistor in place, problem?), and I am using the DRDY line to signal new data to the MCU. I am unable to determine if my problem is power related or if it is one of the digital lines. Basically when I connect power through the battery and try to capture data from the ADS the DRDY never sets off the interrupt on the MCU, but when connecting to a DC power supply the ADS works just fine. I have confirmed that everything code wise is functioning correctly.


I am using a 3.8V Lithium Ion battery and a 3.3V voltage regulator capable of providing 50mA output and I am no where near that (11mA).  All of the components(MCU, ADS, Volt. Reg) are all mounted on custome PCB's and filtered according to the applicaiton notes. As stated above I don't have a weak pull up between the START pin and the VCC pin on the ADS, is this part of the problem?


Any help would be greatly appreciated.


Cody

  • 0
    TI__Guru** 113765 points

    Cody,

    If the ADS1246 works with the DC supply and not with the battery, then usually there is a connection problem.  I would suspect there is a ground problem.  Do you have a schematic that you can share?  Is the supply common to both the micro and the ADS1246?

    Connecting START direct high and CS direct to ground should not be causing a problem.  The DRDY signal should be active if the START pin is high and the device is not in power down (either by missing a supply connection or by SPI command.)  Double check all your connections and verify with a meter that you the proper voltages.  You may also want to use an ohm meter to make sure the grounds of the device are connected to the battery.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 650 points

    Bob,

    Thanks for the fast response.

    I have attached a schematic of the ADS1246 in my system.

    The supply is common to both the MCU and ADS. I have checked all ground connections between the battery and the ADS and everything checks out fine. I have also removed the trace on the PCB between START and VCC and ran to MCU, no change. I will put another board together and see if that resolves the problem. Let me know if you have any other solutions.

    Thanks,

    Cody

     

    0131.protoV1.2.2_ant.pdf

  • 0 in reply to cody lohse
    TI__Guru** 113765 points

    Cody,

    I see that from the schematic that you are not monitoring the DRDY pin, but rather DOUT/DRDY.  As the device defaults to the DOUT/DRDY pin as DOUT mode, perhaps there is a communication problem.

    If you have a scope available, check the DRDY pin itself to see if it is toggling.  If it is, then you know for sure that the device is converting.  Perhaps there is a problem with communication changing the mode of the DOUT/DRDY pin.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 650 points

    Bob,

    Sorry I updated the schematic to show that CS is tied to GND, and DRDY line is sent out to the MCU.

  • 0 in reply to cody lohse
    TI__Guru** 113765 points

    Cody,

    I'm guessing that you have a couple of different boards wired together as your prototype.  I would focus on what is different between going from the DC supply to the battery.  For example, are you using the 3.3V regulator in both cases?  Do you have a clip lead in one setup but not in another? 

    Is DRDY always in a single state, like always high or always low? Or does it ever transition?  Is there any noise in the supply lines that could drive the system to some unknown state?  Have you tried a manual reset to make sure the device starts from a known state?  For example, clipping a battery on versus the DC supply and using a switch can make a difference on power on reset of the device.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 650 points

    Bob,

    I just noticed that the voltage reference input max is (AVDD-AVSS)-1; I am feeding the the ref+ at 3.3V(AVDD) and ref- at GNDA(AVSS). It appears that I am violating the specs, this looks like the problem that I am having.

    I have verified this to be true. I dropped the voltage across the ref inputs down to 2.3V and the ADS seems to be working correctly now.

    Thanks for the support,

    Cody