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DAC8568: DAC8568C - do not stable internal reference 2V5 voltage as required

Andrzej Szupiluk
Prodigy 230 points
Part Number: DAC8568
Other Parts Discussed in Thread: DAC80508

Hi

I use in my board DAC8568C converter.

It is configured to give 2V5 on reference output.

After 1 minute this volgtage is 2.50059V (measured with DMM 0.003%).

After 30 minutes this volgtage is 2.49951V (measured with DMM 0.003%).

The difference is about 1mV. I

t is tragedy and not super stable reference voltage.

I've checked a few chips. The results are generally the same.

Any idea.

Regards

Andrzej

  • 0
    TI__Mastermind 46290 points

    Andrzej,

    Can you please share a schematic for the design? Also, have you probed this with an oscilloscope to verify that the reference is stable?

  • 0 in reply to Kevin Duke
    Prodigy 230 points

    Kevin

    Oscliloscope do not show nothing bad, but only a little noise.

    I'll try to insert my schematic PDF file, but I do not know how to make it? Below is my trial by using option Insert File.

    dac1.pdf

  • 0 in reply to Andrzej Szupiluk
    TI__Guru 52663 points

    Hi Andrzej,

    Is the device experiencing a significant temperature change during this time? Is the load on the DAC output changing dynamically?

    There could also be some aspect of thermal settling on the device.  If you power the device on and leave it on for a few minutes, is the output stable at that point?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 230 points

    Hi Paul

    Thanks. Temperature is raising about 10 deg. I think, no more absolutely.

    Each output of DAC is loaded only by 20kOhm resistor to GND.

    Scheme of output connection of reference voltage you can see on the picture in the post above.

    Output of reference voltage stabilize minimum 30 min. After this time change of level of reference voltage is negligible.

    No sources of hotness are near DAC8568C chip.  

    Last time I've measured temperature drift of 4 DAC8568C chips 30 minutes after power on.

    1) First chip - after 30 min drift is  -1000µV (fatal).

    2) Second chip - after 30 min drift is -100µV (acceptable).

    3) Third chip - after 30 min drift is   -10µV (OK).

    4) Fourth chip - after 30 min drift is  -2100µV (fatal).

    Additional information:

    a) Power supply of this DAC-es is 5VDC.

    b) Power supply 5VDC has the drift -30mV after 30 minutes.

    c) Chips were soldered and tested in the same way and on the same type of PCB.

    You see from 4 DAC8568C chips , 2 pcs are out of specification.

    Any new idea?

    Regards

    Andrzej

  • 0 in reply to Andrzej Szupiluk
    TI__Guru 52663 points

    Hi Andrzej,

    I do not think that what you are seeing is real reference drift, but rather thermal settling, board/package stress, and burn-in drift.  Could you do an experiment: take one of the boards with significant drift (chip #4) and heat the device up with a hot air gun or oven (with the board powered off) to re-flow the solder.  Is the reference shift in the first 30 minutes the same?

    There can also be some shift after burn-in.  For example, you are currently measuring at t=0 minutes and t=30 minutes, can you measure again at t=2 hours and t=24 hours?

    Can you leave a board on for 24 hours, power cycle the device and repeat the t=0 minutes and t=30 minutes measurement?

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 230 points

    Hi Paul 

    Yersterday I've made 2 experiments/solutions already.

    1) I've removed stress from soldering using hot air gun. It helped a little but not completely.

    2) Significantly helped powering on the board and leaving it in this state for about 24 hours.

    Thanks.

    Best regards

    Andrzej

    PS. My conclusion: case of this DAC is very very too delicate to maintain chip parameters stable in many applications.

  • 0 in reply to Andrzej Szupiluk
    TI__Guru 52663 points

    Hi Andrew,

    I'm glad to hear that the two tests helped a bit.  Have you considered our new device, the DAC80508?  This device has an improved reference architecture and we implemented some new IC layout and fabrication techniques to greatly reduce the package stress effects on the device.

    If it is okay, can I email you at the address you registered your myTI account about getting a DAC80508EVM to you?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 230 points

    Hi Paul

    In my analog board I use 2 DAC-es form TI.

    The first is DAC8568C and second DAC7568C.

    I'm producer and designer of Data Acuisition Systems from 1990.

    It will be nice to get good replacement form these DAC chips.

    In my drawer I have got a few junk  DAC-es from the serie mentioned above with fatal parameters of reference output voltage.

    The answer for your quesion is OK.

    (please see also zaeata.webatu.com)

    Best regards

    Andrzej