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Voltage mode on DAC882x

christopher hudson
Prodigy 70 points
Other Parts Discussed in Thread: DAC8830, DAC8820, DAC8822, DAC8881, DAC9881, DAC7811

Can the DAC882X series of converters be used in voltage reference mode as described herein: http://www.ti.com/lit/ug/tiduaf0/tiduaf0.pdf 

thanks,chris

  • 0
    TI__Genius 10615 points
    Hi Chris,

    The DAC882x family has an internal diode between Iout and GND in order to protect the internal circuitry.

    If you can share of me the details of your application I can try to recommend a device that suits your needs.
    What specifications of the DAC882x are you most interested in?
    We have a few voltage output R2R DACs that might suit your needs. I think the closest to the DAC882x performance would be the DAC8830.
  • 0 in reply to Eugenio Mejia
    Prodigy 70 points
    Hi thanks Eugenio for the reply,sorry for the delay but I have been on travel for the past 2 weeks. my application is a high speed closed loop controller that is sensitive to phase.

    the things i am interested in for my application are the following:

    parallel bus data load (fast 2 clock cycle load)
    fast settle time (< or = 1uS)
    low glitch
    and the ability to power it off a single voltage rail due to the application is portable audio.

    the parallel bus not only reduced phase lag but it also allows me to slow down my processor to reduce power consumption.

    you help is appreciated.
  • 0 in reply to christopher hudson
    TI__Genius 10615 points

    Hi Chris,

    In this case, you might still be able to use the TI Design that can use the DAC8820, but it still requires an output amplifier.

    Single-Supply Unipolar MDAC

    In order to pick the most appropriate device for your application I have a few more questions:

    What output voltage range are you interested in generating?

    What resolution is required?

    What accuracy is required?

  • 0 in reply to Eugenio Mejia
    Prodigy 70 points
    I tried this: Single-Supply Unipolar MDAC. The IOUT2 is internally grounded in the DAC8822 so you can't bias it, the reference gets shorted.

    What output voltage range are you interested in generating? line level is fine ±1VRMS

    What resolution is required? 16bit... i need a noise floor less than -80dB

    What accuracy is required? the INL / DNL specifications should be within 1LSB.
  • 0 in reply to christopher hudson
    TI__Genius 11750 points

    Howdy Chris,


    I think what Eugenio's trying to point out is that with the DAC8822 device, you wouldn't need a negative supply, so there would be no need to connect it in the configuration that is displayed in the TI Design.  For instance you can use one supply to connect to both Vref and VDD of the DAC8822 device when using it in the below configuration.  Of course, you would need to pick a suitable operational amplifier that is capable of swinging from rail-to-rail.

    Best Regards,

    Matt

  • 0 in reply to Matthew Sauceda
    Prodigy 70 points

    According to the previously suggested application note:

    MDAC selection for this design is based on dc accuracy goals and the requirement of operating on a single-supply. Important parameters to consider for dc accuracy are gain error and linearity errors (INL and DNL). Since MDACs do not feature internal output amplifiers, they do not contribute offset error to the system output. In order to realize a design that operates on a single-supply the MDAC must provide external access to the IOUT2 node of the R-2R ladder.

    and the DAC8822 does not, its IOUT2 is grounded.  If i single-side the opamp then the feedback will try to drive it below 0V since everything is DC coupled.

    Can i assume that the opamp could be biased midrail on + input and also be powered from the same single voltage source as the DAC and have everything swing between 0V and bias?  see below?

  • 0 in reply to christopher hudson
    TI__Genius 11750 points

    Howdy Chris,

    You are correct, I overlooked the second stage of the circuit (negative rail is required by OPA to produce negative output). A possible solution are the DAC8881 or DAC9881 device. These devices provide a positive unipolar output with positive supplies. IOVDD/DVDD/AVDD can connect together, allowing a single supply. The reference can also connect to this supply or connect to a REF device, as shown in the TIPD. Generally, the analog supplies are separated from the digital supplies to help reduce any digital noise from appearing on the analog output, so in this case you should expect digital noise to couple to the analog output. The only downside is that these devices exhibit larger glitch energy than other DACs in the precision portfolio (37nVs). The glitch can be reduced with either a simple low-pass, which will affect the bandwidth of the system, or with a one shot solution. Both solutions are explained in the URLs below:

    e2e.ti.com/.../dac-essentials-glitch-be-gone
    http://www.ti.com/tool/TIPD142

    I've tried to find other MDAC devices similar to the DAC7811 (IOUT2) and with larger resolution, but haven't been able to filter any out. If this changes I will let you know and update this post.

    Best Regards,
    Matt