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DAC084S085 output not working

wireHound
Intellectual 430 points
Other Parts Discussed in Thread: DAC084S085

I am not sure if this is the right forum for this question but I figured i would put it out there just in case.  I am working with a DAC084S085 digital to analog converter.  It is a 4 channel DAC, but I am using only 3 channels, (A, B, and C).  I am interfacing with the IC using SPI.  I have successful comms because I can change the output of channels A and C successfully.  But for some reason if I set channel B to anything other than zero it affects the outputs of channels A and C attenuating them significantly.  Is this normal operation?  What is causing channels A and C to be affected when i try to set them to anything other than 0?  Can the DAC be configured to use 3 of the 4 channels or if a channel is not used does it need to be physically tied to a load and driven so the op amp functions normally?

  • 0
    TI__Genius 16890 points

    Hello,

    The channels are independent. They should not affect each other under normal operation. Unused channels can be left unconnected.

    What is the B channel driving? Is the B channel circuit different than the A and C? Is the CHB output value correct while the other channels are attenuated? Is the reference voltage and Va correct during this time?

    What are the commands you are using to program the part to switch between the channels?

    Mike

  • 0 in reply to Michael Stout
    Intellectual 430 points

    Mike,

    Thanks for your reply.

    - Channel B is not driving any load.

    - The circuits for channels A, B, and C are all the same.

    - No, (CHB output is never correct).

    - No.  vref and va dip anytime i try to set channel B to anything other than 0.  If I set channel B output to any set value then vref dips to about half.  If I configure channel B to be continually adjusted so it should be a saw tooth output then vref will attenuate to about half and hold for about 2 seconds, then it spikes and plateaus at vref for about 400 mSec then drops again.  This continues to be repeated while trying to generate saw tooth wave.

    - have a 16bit register that i OR my set point target value, channel ID, and write mode, (see constant defines from my code below).  set point is shifted four places to left so it lines up with DAC spec on page 15.  I can successfully set channels A and C to a constant output or a saw tooth wav using this method, but if B is set to anything other than 0 vref dips affecting my channels A and B outputs.

    /* Defines

    -------------------------*/
    /* Address Bit Defines */
    #define DAC_CHANNEL_A           0x0000u
    #define DAC_CHANNEL_B           0x4000u
    #define DAC_CHANNEL_C           0x8000u
    #define DAC_CHANNEL_D           0xC000u

    /* Operation Mode Defines */
    #define WRITE_BUT_NO_UPDATE_TO_OUTPUT   0x0000u
    #define WRITE_AND_UPDATE_OUTPUT         0x1000u
    #define WRITE_TO_ALL_AND_UPDATE_OUTPUTS 0x2000u
    #define POWER_DOWN_OUTPUTS              0x3000u

    Thanks...

  • 0 in reply to wireHound
    TI__Genius 16890 points
    Hello,

    It seems like it is acting like the channel b output is shorted to ground somehow, maybe through a small resistance. This could be through a solder short or it may be internal to the part, if the part has been damaged somehow.

    Can you remove the part and check the channel b trace for a small or no resistance short? You can also try another part and see if it works better.

    Mike
  • 0 in reply to Michael Stout
    Intellectual 430 points

    Mike,

    I have checked channel B for being shorted to ground but there is no resistance between channel B and ground.  I will look to see if I have an extra part to replace the current DAC with.  Thanks for your feedback.