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DAC8812 volume control

Cedric Bompart
Prodigy 40 points
Other Parts Discussed in Thread: DAC8812, OPA2132, TIPD137

Hi,

I'm planning to implement a volume control using a DAC8812. I'm just wondering which oamp should I use after the DAC8812?  ...I think the proper term for this application is "unipolar" if I'm not mistaken...

The idea is too control the DAC via an Arduino so ideally I would like to power everything from 0-5V.  I've seen a circuit which uses two opa2132 with a -15V/+15V power rails.  

Cheers,

  • 0
    TI__Mastermind 46290 points

    Hello Cedric,

    The DAC8812 is a multiplying DAC, or MDAC. Conventionally MDACs are designed in a two-quadrant or four-quadrant configuration. What you're describing would most specifically be a two-quadrant MDAC circuit where the MDAC is simply followed by a transimpedance stage that converts the output current into a voltage. The output of this circuit is indeed a unipolar voltage though, so it is correct to refer to this as a unipolar output circuit.

    For these conventional circuits we have created a TI Precision Design called TIPD137 (which is technically for a four-quadrant MDAC, but if you chop of the second stage you've got the two-quadrant design you're interested in). I encourage you to give that document a read - it will go through all of the MDAC specific considerations for choosing this amplifier then you can make addition considerations based on any other application specific requirements you may have (i.e. slew rate, bandwidth, noise, etc.).

    I should point out that the typical two-quadrant MDAC circuit (MDAC + transimpedance stage) outputs a voltage that is inverted with respect to the reference input voltage. If you had a +5V reference your output will swing from 0 to -5V. If you had a -5V reference your output will swing from 0 to +5V. This is inherent in the operation of an amplifier in the transimpedance configuration.

    If using bipolar supplies is a challenge for you there are alternatives, but I would encourage you to share the full details of your requirements of this DAC before we go down that path so we can choose the most appropriate solution.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Thanks for the useful reply.

    Everything in my pre-amp is powered with a +5V supply therefore I would like to avoid a bipolar supply which is going to be used only by the oamp.

    Here is a schematic which provides a volume control with a DAC8812/ AD8599. Please ignore the SPI isolator on the left on side and the gain on the oamp (the double R2 and R3).

     

    ...what about a rail-to-rail oamp which is suitable for the audio bandwidth with low noise/ distortion?

  • 0 in reply to Cedric Bompart
    TI__Mastermind 46290 points

    Cedric,

    Your schematic did not post correctly. Can you re-try sending the file as an attachment?

  • 0 in reply to Cedric Bompart
    TI__Mastermind 46290 points

    Cedric,

    That design still requires a negative supply rail for the output amplifiers or a negative voltage reference. The extra resistors around the transimpedance stage are just contributing some extra gain on the output.

    Are you looking to use an MDAC to achieve faster settling time with this system or are there other concerns? String and buffered R-2R structures are more likely to comply to your supply requirements.

  • 0 in reply to Kevin Duke
    TI__Mastermind 46290 points

    Cedric,

    Any updates or further information concerning this request?

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Kevin,

    No nothing else, thank you. I'll have a look if I can do something with this design. You can close this ticket/request.

    Regards,

    Ced.