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DAC8568 Cannot get any analog output

Dmitry Ponomarev
Prodigy 140 points
Other Parts Discussed in Thread: DAC8568

Why is a $30 chip not able to do the simple thing it's designed to do, when it has so many other useless functions?

The following are the codes I am sending to it via SPI (1 kHz) - directly from the datasheet

0x090A0000 - Internal Reference Always Powered On

0x000FFFF0 - write to A

0x004FFFF0 - write to B

0x006FFFF0 - write to G

0x027FFFF0 - write to H AND simultanously update all DACs

Nothing happens after the last command is set,

I make sure that the CS (SYNC) pin is low the entire time of the transfer

Can anybody help explain why this chip is not working?

Thanks, 

  • 0
    Prodigy 140 points

    I appologize, I meant the CS pin is low the entire time of the transfer of 32 bits, it does not rise back up after each byte to cause an interruption and the signal read to be aborted.

    THanks,

  • 0 in reply to Dmitry Ponomarev
    TI__Guru**** 181361 points

    Hi Dmitry!

    I'm sorry to hear that the DAC8568 is causing you problems!  Can you let us know how you have the /LDAC pin configured and possibly post a screen shot of your SPI interface to the DAC8568?

  • 0 in reply to Tom Hendrick
    Prodigy 140 points

    Hi there,

    Thank you for your prompt reply,

    The LDAC pin is tied to ground. There is a .1uF capacitor between the pin and +5V power that goes into AVdd

    The VrefIN/Out has a 150nF capacitor betweeen it and the ground. It's not connected to anything else.

    I send it 0x090A0000 to enable internal reference always

    Then I send 0x027FFFF0 to set output of DAC channel H to max. (what you see on the scope)

    Scope photo of CLK and Data lines (sending 0x027FFFF0)

    http://postimage.org/image/ni3bja89z/

    Scope photo of CLK and CS (SYNC) lines

    http://postimage.org/image/jtuw7mthl/

    Delay between sends is 100ms

    I understand that lines appear very noisy, this is entirely because I did not connect the ground of the oscilloscope to the PCB ground. I did not find it necessary as I could read the signals with the noise. The CS line is the one where I connected the scope ground to the ground and you can see the noise disappeared.

    EDIT: I read in the datasheet that if the Internal Reference is enabled, the VrefIn/Out pin is supposed to have 2.5 volts. I Measured it, and it shows 2.28 volts. I'm assuming this means I enabled Internal Vref correctly.

    Do I need to have the output pins tied to anything? Right now they are open, but will be connected to a buffer op amp once I get them functional/

    Thanks,

    -Dmitry

    Thanks

  • 0 in reply to Dmitry Ponomarev
    Prodigy 140 points

    Goo daftewrnoon,

    I found the answer to my issues, I set the SPI mode to clock data on a Falling rather than rising edge and everything worked well

    I just have a few follow up questions:

    1. There is no explanation in the datasheet for Flexible and Static Modes, what do these mean?
    2. I assume from the examples that out A corresponds to address 0b0000, output B to address 0b0001 .. etc but there is no definition of what address is for what pin in the datasheet.
    3. My output maxes out at 2.5 volts, I assume this is because of the internal reference, but my AVdd is 5 volts? If I wanted a 5 volt output, do I connect 5 volts to VrefIN? Will I need to make sure that I switch to external reference to be always on before i do this? Can I do this with internal reference?
    4. Can the DAC output a negative voltage with Internal/external reference?
    Thanks,
    -Dmitry
  • 0 in reply to Dmitry Ponomarev
    TI__Guru**** 181361 points

    Great News!

    For your follow on questions:

    1. The Flexible and Static modes of operation are described on pages 34 and 35 of the data sheet.
    2. Yes - for the operating examples on page 44, A corresponds to address 0000, B to 0001, G is 0110, and H is 0111.  The Pin Configuration diagram and Pin Description table on page 6 of the data sheet shows which physical pin is connected to DAC out A through H.
    3. If your output maxes out at 2.5V, you must have either the A or B grade device.  These parts have an internal 2.5V reference and would need an external 5V reference to reach 0-5V on their outputs.  The C and D grade devices have an internal 5V reference and can output up to 5V with no additional circuitry.
    4. Bipolar operation is possible using the circuits depicted on page 50 (Figures 127 and 128).  The DAC8568 though cannot output negative voltage on its own.
    Have a great weekend!