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DAC8750: Use of external Iset-R with the DAC8750 & DAC8760

Thomas Lucas
Prodigy 110 points
Part Number: DAC8750
Other Parts Discussed in Thread: DAC8760

I am looking to use both the DAC8750 & DAC8760 to replace AD5420 & AD5422 in 4-20mA applications. In the Analog Devices parts the TUE specification for the external Iset-R is better than the internal Iset-R, and the resistor precision has to be accounted for. For the TI parts the external Iset-R specifications are worse than using the internal Iset-R, and the external Iset-R is glossed over. Is there an assumed precision in your specifications or do I need to account for the external Iset-R which would further reduce the precision of the TI Parts. I currently use a .01% 15K ohm 5 PPM resistor. It appears there is no advantage to using an external Iset-R with the TI parts if I am not using a Hart Net Configuration. Is this correct?

  • 0
    TI__Expert 5820 points

    Hi Thomas,

    Both AD5420 and AD5422 datasheets provide the error assuming an ideal 15kOhm resistor. This is stated in the right column of the specifications table. The specifications for DAC8750 and DAC8760 use a 0.005% 15kOhm 2 PPM resistor. So the error from this resistor is included in the specification. Error wise there is no advantage to using the external resistor but it can also be used to tune the gain value in some cases. In most cases the internal gain setting resistor is used. Is there a reason you have for using the external resistor?

    Thanks,

    Garrett

  • 0 in reply to Garrett Satterfield
    Prodigy 110 points

    No, we have no other purpose for the external Iset-R other than getting better numbers with the Analog devices ICs. Than means we could drop them from the BOM when we switch to TI ...nice.

    Thanks for your time,

  • 0 in reply to Thomas Lucas
    Prodigy 170 points

    Good Day,

    I am curious about your transition from AD5422 to DAC8760. What if any changes have you had to make hardware or firmware to get the DAC8760 up and running in your design that started with the AD5422?

  • 0 in reply to cy drollinger
    Prodigy 110 points

    I have not made that transition to the DAC8760 yet. I am waiting on samples.  I have swapped out the DAC8750 and it was a drop in replacement for the AD5420. I ran it with an external rset and without. I did alter my startup code so I would not activate the external rset in the end. It works great, and I am anticipating that the same will hold true with the DAC8760. I will let you know what I find when the samples arrive.   

  • 0 in reply to Thomas Lucas
    TI__Mastermind 46290 points
    Hello Thomas and Cy,

    Having compared the devices in the past in every test that I have run the DAC8760 features include a super-set of functionality compared to the AD5422 (same comments throughout the two families concerning w/ or w/o VOUT and resolution / package permutations) so if you are only using the features included int he AD5420 you should not have any issues. An example of specifically what I am referring to is with respect to the 8-bit CRC on the DAC8760 interface - if you were using this in DAC8760 sockets you could not drop in an AD5422 as that device does not implement a CRC.

    Hope this helps, and Thomas please feel free to update the community with the results on your board.
  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Having replaced the AD5422 with the DAC8760 in a current hardware design, the DAC8760 preforms as expected without changes in either hardware or firmware.  

  • 0 in reply to cy drollinger
    TI__Mastermind 46290 points
    Thanks Cy for the update!