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ADS124S08: Trouble Reading Conversion Data with Internal or External Reference

Part Number: ADS124S08

Hello, 

I am trying to read conversion data from the ADS but am seeing some strange behavior. I have been trying to run using the internal and external reference seeing different behavior.

  • I am using a differential input with one Ain pin tied to ground as the negative input and one tied to a variable power supply as the positive input
  • I have tried with the PGA enabled with a gain of 1 and with it disabled
  • I slowed the data rate down to the minimum 2.5 SPS
  • I have confirmed all register settings by reading them back and have successfully controlled a GPIO input / output to prove functionality

Using internal reference:

I first set the device to use the internal 2.5V reference with the buffers disabled. This register set up is shown below:

  • Reference Register: 0x3A

This setup gave me conversion data of all [0x7F, 0xFF, 0xFF] on every conversion regardless of the input voltage (.5V - 2V)

Using external reference

I then tried to set the device to use an external reference on the REFP0 and REFN1 pins of 2.5V. I set the register configuration to use external reference (REFP0 / REFN0) and enabled the buffers. This new register setup is shown below:

  • Reference Register: 0x30

This setup gave me conversion data in a closely repeating sequence regardless of the input voltage (.5V - 2V). This sequence looks close to the following data:

[0x7F, 0xFF, 0xFF], [0x7F, 0xFF, 0xFF], [0x7F, 0xFF, 0xFF], [0x17, 0x5E, 0xDD], [0x7A, 0x78, 0x91], [0x28, 0xF8, 0xFE], [0x6A, 0x17, 0x78], [0x01, 0x8A, 0x5E], [0x7F, 0xFF, 0xFF]

I saw this same pattern of data even if I connected the external reference set the reference register to use the internal reference. It seems as though the internal reference is not working at all and I don't really have any ideas as to what would be causing the strange pattern of conversion data? 

Thanks for any help!

  • Hi Braden,

    Can you share a schematic and all of the register settings you are using?  One item to note is that if you are using unipolar supplies (AVSS connected to ground) then you must disable the PGA if you have one of the inputs connected to ground.

    You can verify if the internal reference is powering up by measuring REFOUT.  REFCOM should be connected to ground and there should be a minimum of 1uF cap between REFOUT and REFCOM.

    Did you have a typo when you described the external reference connections?  REFP0 should connect to the 2.5V external voltage and REFN0 should connect to ground.

    Are you running in continuous mode or single-shot mode?  Are you using the Start command or are you setting the START pin high?  How are you determining when to read the conversion data?  Are you monitoring the DRDY pin?

    Are you reading the conversion data directly or are you using the RDATA command?

    Best regards,

    Bob B

  • Hi Bob, 

    Below is a snip of the schematic. Note I made one edit to tie AINCOM to ground. To answer your questions:

    1. AVSS is connected to ground and have tried enabling PGA and disabling PGA with same results
    2. I will measure REFOUT and verify internal 2.5V. Have 1uF cap between REFOUT and  REFCOM
    3. Yes I did have a typo, REFP0 had 2.5V connected and REFN0 had ground connection
    4. Running in continuous mode and polling the DRDY pin. Using the Start command and have the START pin held low / RESET pin held high.
    5. I am reading the conversion data directly after I see the DRDY pin go low

    Thanks!

    Braden

  • Hi Braden,

    Thanks for the information.  AGND and GND3V3 should be at the same potential (connected together).  The PGA should be disabled and bypassed when measuring ground referenced inputs.

    Just to rule out any issues relative to conversion timing I would suggest to try using the RDATA command instead of reading data direct.  What SCLK frequency are you using?

    Best regards,

    Bob B

  • Hi Bob, 

    AGND and GND3V3 are tied together but I forgot to include that in the screen shot. I have tried with the PGA disabled and enabled with the same results but I will go ahead and try testing the conversions using the RDATA command. 

    We are using 1Mhz for SCLK. I'll let you know how the testing goes. 

    Thanks for your help, 

    Braden

  • Hi Bob,

    Updating the code to send the RDATA command instead of poll the DRDY signal and using the internal reference (and disabling PGA), I now see the conversion data read back as 0x80, 0x7F, 0xFF repeating.

    Using a DMM I measured REFOUT (C116 in schematic) and I see 0V. I grabbed another board and also saw 0V on it so don't think this is a hardware issue? I am writing 0x3A to the reference control register so believe that should be enabling the internal reference to be always on?

    Should the negative/positive buffer bypass bits be set to enable or disable? 

    Thanks,

    Braden

  • Hi Braden,

    You should see 2.5V at REFOUT if the internal reference is actually turned on.  Have you tried reading back the registers after the register write?  Do you have confirmation of the communication by either oscilloscope shots or logic analyzer that you can share with me?

    Best regards,

    Bob B

  • Hi Bob, 

    I can get you some traces tomorrow. But I am very confident that the registers are being programmed correctly as I can properly set and read back GPIO on the device as well as see the data rates successfully change when programming the rate register. 

    Thanks,

    Braden

  • Hi Braden,

    All I can say is if the REF register is written as 0x3A the reference output should have 2.5V and not 0V.  The most common cases where the reference doesn't turn on is with either improper communication or inadequate supply for the analog.  If the supply droops when turning on the internal reference, there could be insufficient grounding or inductance in the AVDD supply restricting current.  This may actually cause the ADS124S08 to reset to default values when turning on the internal reference.

    Best regards,

    Bob B