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DAC8803: Vref and output current

Ya Li
Prodigy 20 points
Part Number: DAC8803
Other Parts Discussed in Thread: DAC7811

I am confused about the polarity of Vref and output current. If I want to have both positive and negative current output, does that means I need to switch between positive and negative Vref?

Thanks.

  • 0
    TI__Expert 5820 points
    Hi Ya Li,
    Welcome to the e2e forums!

    This MDAC device is typically used with an external op amp as a transimpedance amplifier, converting the output current to voltage. There is an internal feedback resistor as a feedback path to support this configuration. That being said, if you want positive and negative current out of the device, the reference voltage will have to change between positive and negative as well. Can you share additional details of your application and how you plan to use this current output? There may be a device better suited for your needs.

    Regards,
    Garrett
  • 0 in reply to Garrett Satterfield
    Prodigy 20 points

    Hi Garrett,

    Thanks for the quick reply!

    I am building a stimulation circuit for stimulating multi electrode array. The circuit needs to pass a stimulation current into the electrode. Because I have a size restriction on board size, I need a current output DAC that has high channel counts. The output current is 100uA, and can be both positive and negative.

    I have looked into several TI DACs and 8803 seems to be my best choice. Do you have any other suggestions?

    Best,

    Veronica

  • 0 in reply to Ya Li
    TI__Expert 5820 points

    Hi Veronica,

    The current output of this device shouldn't be used to directly drive a load. It is meant to be buffered with a transimpedance amplifier. I suggest you look into designing a current source/sink with an external amplifier. Since you need to both source and sink, you might want to investigate the Howland current pump (shown below) which has this capability. Note how U2 is converting the output current of the DAC to voltage. For source and sink capability the voltage going into current pump will need to be bipolar. There will likely be some design challenges for accurate operation at such low current. A high precision op amp and precision resistors will be required. Refer to the DAC7811 datasheet and the linked application report.

    Taken from the DAC7811 Datasheet (8.1.5 Programmable Current Source Circuit):

    DAC7811 Datasheet:

    http://www.ti.com/lit/ds/sbas337d/sbas337d.pdf

    Comprehensive Study of the Howland Current Pump:

    http://www.ti.com/lit/an/snoa474a/snoa474a.pdf

    Regards,

    Garrett