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LAUNCHXL-F28379D: Accuracy of DAC CMPSS

vincent liebart
Intellectual 600 points

Part Number: LAUNCHXL-F28379D

Tool/software:

Hello,

In my applocation I use a CMPSS module to make a PWM module to enter in a trip zone

In a first step I programmed the LAUNCHXL. The VDDA reference seems to set at 3V3.

I use this reference for the DAC of the CMPSS.

I set the DAC values at:

- 3000 for the High Comparator --> I expect to enter in the trip zone at 2.41V

- 1000 for the Low Comparator (output inverted) --> I expect to enter in the trip zone at 0.805V

Intead, I enter in the trip zones  at 2.32V and 0.830V.

So, it seems that I have:

- an offset error about 10%

- a gain error about 8%

I looked for explanations in the datasheet, and found this:

Can anyone can tell what is the link between the table and the graphs?

- the table deals with an error of less than 1% on the offset and 2% on the gain

- I can see an error of 400LSB on graphs (about 10%, which is in accordance with my tests)

Did I something wrong using the CMPSS ?

Thank you.

Vince.

  • 0
    TI__Mastermind 36526 points

    Hi Vincent,

    Today I have a straightforward answer for you. 

    vincent liebart said:
    Can anyone can tell what is the link between the table and the graphs?

    The graphs are not drawn to scale, they are simply meant to demonstrate the concepts of gain and offset.

    As for the gain and offset errors that you have observed, it is difficult to determine what portion of you error is gain vs offset. 

    To account for the worst case scenario in the datasheet, the error could be as much as 25mV from offset + 66mV from gain (2% of full scale range). Since your error is less than this, your device looks like it is complying with datasheet. 

    It does look like a rather large offset error, but still within spec, so you will have to design with these limitations in mind.

    Also, please measure your VDDA voltage, as a small inaccuracy in VDDA could easily affect CMPSS DAC voltage by several mV. 

    Best Regards,

    Ben Collier