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DAC8775: Using a buck-boost to power external circuitry

user6004682
Prodigy 50 points
Part Number: DAC8775

I have an existing DAC/ADC solution with an external +/- 15V buck-boost providing the positive and negative analog supplies for both.

I'm hoping to switch to the DAC8775 for the additional channels. With the integrated buck-boost, it would be very nice if I could get rid of the external buck-boost as well. The one issue is the ADC.

I've combed through the manual and couldn't find any mention of using one of the channel's buck-boost to power external circuitry. Assuming issues such as power supply sequencing, noise, etc. for the ADC are not an issue, is there any problem using one of the channel's buck-boost to supply the ADC?

  • 0
    TI__Guru 54133 points
    My colleague will get back to you on Monday
    Thanks,
    Paul
  • 0
    TI__Expert 5820 points
    Hello,

    It is possible to use the integrated buck-boost to power other devices. You will have to make sure to clamp VPOS, as by default the voltage changes based on load in current output mode.

    What are the current and voltage requirements of the ADC and additional circuitry?

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Prodigy 50 points

    Great. I'm planning on clamping them to +/-15V (at least the buck-boost I use to power the ADC).

    The ADC needs to be able to read +/-10V with headroom of 4V on each end, so using a +/-15V buck-boost would work well.

    The maximum current draw would be approximately 10mA from either +/-15V rail during the acquisition period. The acquisition period is about 1/10th of the total sampling period for the ADC. The buck-boost wouldn't be used to power any other circuitry, although there would be a couple bypass capacitors on each rail for the ADC.

  • 0 in reply to user6004682
    TI__Expert 5820 points
    Hello,

    Are you using the current output? One issue I can see is the current output will exceed the clamp voltage in open circuit condition. The device has not been tested to power other devices from the internal buck-boost and 10mA is fairly high current draw on the supply.

    What current/voltage output ranges do you plan to use and what is the maximum load? You may need to restrict your load range to accommodate the additional power draw.

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Prodigy 50 points

    Hey,

    The outputs would be configured as current outputs or voltage outputs depending on the specific deployment. That being said, I was planning on restricting one of the outputs to a voltage output in order to clamp the buck-boost to +/-15V for the ADC.

    The current outputs would be 0-20mA. The voltage outputs would be +/-10V. When configured as a voltage output the load would generally be some sort of external analog input, typically a PLC's analog input.

  • 0 in reply to user6004682
    TI__Expert 5820 points
    Hi,

    What will be your load resistance range for the current (~1kohm max) and voltage outputs (~1kohm min). You may need to restrict the load resistance you can drive in current output mode.

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Prodigy 50 points

    Hi Garrett,

    Do you mean restricting the load resistance for all outputs or just the output I'm powering the ADC with? The output powering the ADC would be restricted to a voltage output. The existing version of the board has 1k on its voltage outputs to protect the outputs from a short circuit event so it's restricted to 1k by default.

    The existing version of the board also only supports a maximum of ~600R on its current outputs.

  • 0 in reply to user6004682
    TI__Expert 5820 points
    Hello,

    Just the one you will be drawing the extra current from. Since you are using the voltage output only on one channel it should be okay. The 1kohm will limit the current to 12mA so the DC-DC should have some headroom to provide 10mA to your ADC. That being said, the device has not been extensively tested to power other devices so I would suggest testing thoroughly in your application.

    Thanks,
    Garrett