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DAC8871 Carry glitch question

1351769
Intellectual 275 points

Other Parts Discussed in Thread: DAC8871

Hello,

I have a customer trying to minimize the Major carry glitch of the DAC8871.  Is there a way to do this?  The operating voltage is +/-15V for VCC/VSS andVdd is +3.0V.

Ali,

As discussed on the phone earlier, I need some support from the TI AFE on the DAC8871 Major Carry glitch characteristics.

       On page 13 of the datasheet it specifies typical glitches of ~450-500mV every time the DAC is updated. 

 

Is there any way to minimize these glitches digitally? if so can you provide any suggestions on how to accomplish this?

The dac SCKL is running at 31.25MHz, but the DAC is only updated at a 244.14KHz rate (31.25MHz/128), The update frequency is a requirement and cannot change, however the SCKL frequency can change if it will help with the glitches.

Thanks,

Ali Kozak

Avnet FAE

  • 0
    TI__Mastermind 43915 points

    Hi Ali,

    The DAC8871 is an unbuffered DAC allowing you to customize the output stage. So, if you want to drive any kind of load with the DAC, you are going to have include an op amp at the output of the DAC in order to drive any kind of current. Depending on which op amp you choose, you may see a decrease in the code to code glitches on the output. Additionally, you can add a capacitor at the op amp output (if the op amp you choose can drive a capacitive load), or add a RC combination filter at the op amp and that will help with the glitches.

    My recommendation is to look to see what op amp the customer chooses and try to add a capacitor or a RC network to the output. That is probably the easiest way to decrease the glitches.

    Changing the SCLK frequency is not going to remove/help with the glitches. The glitch energy is there and will be shown on the output. If the amplitude of the glitch is the issue, a capacitor will help smooth that out at the cost of increased settling time.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 270 points

     

    Hi Tony

    Im not sure if i should post here on a verified answer post.  However, im intrigued about one thing about your answer:  you say there will be a decrease in the code to code glitches on the DAC8871 depending on the amplifier you choose... can you please be so kind and explain that to me?

    I know some opamps will degrade the performance of a 16 bits DAC (via Vos, Vn, Ibias and large resistors, etc) but i dont know how can an amplifier help to improve the characteristics (unless you mean an amplifier used as an output filter)

    Thank you very much and best regards

  • 0 in reply to JACR
    TI__Mastermind 43915 points

    Hi JACR,

    If you take a look at the period of a major carry glitch, you will see that it is on the ns or us scale. Depending on the speed of your op amp, it may be that it is not fast enough to ''catch'' the entire glitch. Another option would be to use an output amplifier and limit its bandwidth to help with ensuring it does not pass through the glitch. Just keep in mind, doing something like this may result in increased settling time.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 270 points

     

    Ty very much Tony, i won't keep asking here because i already opened a new thread, but your answer about this topic was pretty clear.

    Best regards