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DAC8803: DAC8803: Unipolar voltage output- regular vs reverse mode?

Donald Adede
Prodigy 90 points
Part Number: DAC8803
Other Parts Discussed in Thread: TIPD159, , , OPA192

Hello,

From my understanding, the DAC8803 is a multiplying DAC and needs an opamp to get the voltage output. After studying the DAC8803/14 EVM user guide and schematic, and the TIPD159 application note- "creates a unipolar voltage output from 0 V to 2.5 V. This design does not require dual supplies to realize a unipolar, positive output voltage as with typical MDAC circuits." I would like to know which application would be better for the settling time.

Also, from the DAC8803/14EVM user guide, which of the opamps(U3B/U3C) in this schematic is for a unipolar application and is the analog voltage directly at the output of the opamp?

And if i were to use the TIPD159  method, is the OPA4227UA opamp still a good choice for the application?

Many thanks!

  • 0
    TI__Mastermind 28738 points

    Hi Donald,

    The DAC8803EVM does not use the "reverse" method that is described in TIPD159. The unipolar output scale in this design will be the inverse of the voltage applied to the Vref pin. To get a positive output the reference input needs to be negative. U3B and U3C are both used for a bipolar output. U3 pin 8 is connected to R14/13 through the switch, and the bipolar output is seen on U3 pin 7. This configuration is easier to visualize when looking at the datasheet example circuit. 

    U3C is used by itself for a unipolar output.

    The benefit of the reverse configuration in TIPD159 is that you do not need dual supplies anywhere in the system. The settling time of the DAC will remain the same. The op amp dominates the settling time, and each unipolar application only uses one op amp stage so the settling times would ideally be the same.

    Extra care needs to be taken with the reverse configuration because the reference is being applied to the MDAC output which will have a load that varies with the position of the DAC switches. The settling time of the reference due to the load transient would increase the total output settling time. 

    TIPD159 mentions that this can be remedied by buffering the input or adding a charge well capacitor:

    The TIPD159 reference design was released more recently than the DAC8803EVM. I would recommend the OPA192 used in the reference design for either configuration. 

    Best,

    Katlynne Jones