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DAC11001A: AGND-OUT

Part Number: DAC11001A

The datasheet says the output voltage is referenced to the AGND-OUT (datasheet page 4). But according the output voltage formula (datasheet page 23), the voltage is referenced to VREFN. How is the internal connection of the AGND-OUT in the DAC11001A? Can you provide a simplified schematic?

Is it possible to drive the AGND-OUT with an opamp?

I have a multichannel system with one reference (VREFP=7V, VREFN=0V). According the datasheet the output voltage is referenced to AGND-OUT, but AGND-OUT should be the same potential as VREFN.

The idea is to buffer the VREFN voltage with an opamp and drive the AGND-OUT on each DAC to prevent crosstalks.

Thanks for your help.

  • Hi,

    Output of DAC11001A is referred to AGND_OUT only. From the output equation, how you arrived at DAC output is referred to VREF_N ?

    Lets say VREF_P = 5V and VREF_N = -5V, DAC output will span from +5 to -5V, thats the interpretation.

    You can buffer AGND_OUT with an op amp and connects to reference ground circuit for each channel to minimize cross talk between channels.

    Please see the figure below for multi channel grounding scheme.

    Regards,

    AK

  • Hi,

    Ok, I understand.

    But I have a different configuration in my application. I have only one reference and I have two DACs which build a differential pair. The relevant output voltage is DACOut1 - DACOut0.

    Since the DAC is based on an R2R network, I do not understand how the AGND-Out is connected? Can you clarify this? The R2R network is only between VREFP and VPREFN.

    What is your advise to minimize crosstalk?

    Best regards,
    Lukas

  • Hi,

    I cannot share the details of the internal architecture of the device.

    This R2R DAC is not straight forward R2R architecture. We use segmentation and other techniques to improve our parameters, so going into detail for the architecture is difficult and I am bound not share those details. 

    Now coming back to the question of avoiding cross talk, for your system, You can have single reference IC and do not short AGND_OUTs.

    Also make sure that there is only single point short between your load GND and AGND.

    Regards,

    AK

  • Hi,

    Thank you for your answer. What do you mean exactly with "do not short AGND_OUTs"? Should i leave it floating?
    I thought the AGND_OUTs should be somewhere connected to AGND? And If I do this, I will short the AGND_OUTs.

    One additional question: Does current flowing thru AGND_OUT? If yes, how much?

    Best regards,
    Lukas

  • Hi,

    What I meant by saying  "do no short AGND_OUTs "  is that between channels. You need to short AGND_OUT with AGND  for individual channels and star connect it on the board. No static current flows through AGND_OUT. We connect AGND_OUT internally to  the capacitor of the track and hold of the de-glitcher circuit. 

    Layout wise it will be shorted since AGND is common for both channels. How is your Load side circuit ? is it referred to AGND?

    Can you share a block diagram of your circuit with load so that we can come up with optimum grounding scheme?

    Regards,

    AK

  • Hi,

    Please see attached a simplified diagram:

    I have one differential output channel, but several DAC channels. I use multiple DACs to lower the noise and to increase the resolution. The load is not referenced to ground. As seen in the picture, the load is between the two output buffers.
    I have two DAC groups. The first group is DAC0 and DAC1. The second group is DAC2 and DAC3. With a 5Vreference, the first group produces lets say 3.5V, whereas the second group produces 1.5V (2.5V common mode).

    Best regards,
    Lukas

  • Hi,

    Since your outputs are differential in nature and load is not referenced to GND, I suggest the following ground scheme.

    1. Connect REFGND, AGND  and AGND_OUT at singe point preferably a star connection on the board. 

    2. Have separate DGND plane and short with the above ground at one point.

    3. Make sure all the digital signals are referred to DGND plane and no crossing of analog and digital signals between the planes.

    4. Have a single reference IC referred to REF_GND and fan out to all the DAC's.

    5. If you use different reference IC for DAC pair, this recommendation will not hold.

    Regards,

    AK

  • Hi,

    Thanks a lot. I followed your suggestion except the DGND plane. Due to some restrictions, I cannot have separate DGND and AGND plane.
    Instead I use one common continious GND plane, but keep the analog and digital signals separated.

    Best regards,
    Lukas

  • Hi,

    This should work fine.

    Can you send me the layout, so that I can have a look at it? If you are not ok with sharing in e2e, we can take this to email conversation.

    send me email at a.k@ti.com

    Regards,

    AK