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DAC8820: Cross reference specifics?

Part Number: DAC8820
Other Parts Discussed in Thread: DAC80004

Hello, the datasheet contains reference to LTC1597,  however, makes no mention of what degree these parts supposed to be equivalent. 

Specifically, one of the features of the Linear part is code uniform glitch, this is quite important in my application. 

Is DAC8820 "covered" in this respect ? Eg. code independent glitch ?

  • Hi,

    Welcome to E2E and thank you for your query. Regarding the LTC1597, I don't think they have mentioned anywhere the glitch is code dependent or not in the datasheet. Did you find the information from some other source? Regarding the DAC8820, I am checking this with the internal team - will get back ASAP.

    Could you please let us know about your application, in the meanwhile so that we may have a better understanding of the requirement?

    Regards,
    Uttam Sahu
    Applications engineer, Precision DACs
  • Hello,

    My two cents on the matter -

    Both devices are unbuffered MDACs, therefore the glitch specification will have dependency on code where major carry transitions are the worst cases with maximum charge injection from all of the switches in the DAC switching, similar to an unbuffered R-2R DAC though maybe a bit lower in general. The LTC1597 specifies mid-scale glitch energy with a footnote explaining "Note 12: Mid-scale transition code: 01111111111111 to 10000000000000 for the LTC1591/LTC1591-1 and 0111111111111111 to 1000000000000000 for the LTC1597/LTC1597-1." which is the major carry transition. A very similar specification is provided on page 4 of the DAC8820 datasheet called "DAC Glitch Impulse" with the same conditions. Both devices specify 2nV-s. I don't see any indication on the LTC device datasheet to suggest uniformity of glitch impulses.

    Certainly Uttam can follow up with any information maybe specific to the DAC8820. It's possible architectural generalities were subverted with unique design techniques in these devices.
  • Feb 1999 16-Bit Parallel DAC Has 1LSB Linearity, Ultralow Glitch and Accurate 4-Quadrant Resistors

    "DESIGN FEATURES 16-Bit Parallel DAC Has 1LSB Linearity, Ultralow Glitch and Accurate 4-Quadrant Resistors by Patrick Copley"

    The LTC1597’s new proprietary deglitcher reduces the output glitch impulse to 1nV-s, which is at least ten times lower than any of the competition’s 16-bit voltage output DACs. In addition, the deglitcher makes the glitch impulse uniform for any code. 

  • That is interesting to claim 1nV-s when the datasheet shows 2nV-s typical and with a stated code condition :) From a guarantee perspective nothing is promised in the datasheet.

    Still working to verify my assumptions on the previous post.
  • Hi,

    Let me add my two cents on this..

    For using the DAC in AC applications with high THD requirement, the code-to-code glitch energy becomes a constraint, especially the code-dependant glitch; because it appears as a hamonic at a frequency (fs - 2fo), where fs is the sampling frequency and fo is the fundamental. Depending on your external filter, this harmonic may or may not get filtered and will appear in THD. The impact of this harmonic will be minimized if the overall glitch energy is small. The other way is to use a larger sampling rate so that the harmonic falls outside the filter passband. Whether the DAC has an internal deglitcher or some other design mechanism to minimize glitch, the overall glitch amplitude will get impacted and not the code-dependancy.

    As far as the competition is concerned, DAC8820 has same glitch number i.e. 2nV-s typical. Further if you look at the THD numbers, the competition just mentions it at one frequency while we show the graphs across a frequency range and for different filter bandwidths.

    Just iterating again, if you provide us with more details on the actual use case, we will be able to suggest specific solutions.

    Regards,
    Uttam
  • About 1nV vs. 2 ,

    "Understanding Glitch in a High Speed D/A Converter" by Intersil 

    mentions it's common practice among manufacturers to specify "peak glitch" and average glitch seperately.

    I am contemplating if the DAC output should be grounded for a duration during settling. As per patent US5646620 by ‎Christopher G. Regier.  Looks like the best practice still for AC waveform generation.

  • Hello,

    That is a novel idea from the folks at Intersil, and that may be true for "high speed" DACs which are more commonly considering things like THD and SNR, however the devices in the discussion to this point have been more commonly considered precision DACs. Having supported precision data converters for nearly a decade I would say that it is not common practice for us, or our competitors, to provide both specifications as these precision products are usually focused on DC specifications. Generally you will find a typical specification, which may be an average specification for typologies like the string DAC, but in a majority of R-2R or MDAC datasheets you will find a worst-case specification at a major carry transition. These typical figures are based on statistics, you may also find something more "average" in the typical characteristics curves.

    I'm not familiar with Mr. Regier's patent. We have published some approaches to "de-glitch" DACs with external circuits and you'll find more and more products going forward like the DAC80004 which also implement glitch reduction sample and hold circuits internally - this just wasn't a priority in the 90's and early 2000's.

    Maybe one loose end on this thread - I did confirm that there is not extra implementation in DAC8820 for "uniform" glitch. It's not as bad as an R-2R DAC but it will vary from code to code, what is in the datasheet is representative of what you could expect to be the worst case at a major carry transition.