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MSP430F5359: Inaccurate internal DAC12

Robert Helvestine
Intellectual 275 points
Part Number: MSP430F5359

Hello. I'm using the internal 12 bit internal DAC of the MSP430F5359. It's powered by 3.3V, and we're using the internal 2.5V reference as the top end of the DAC. I can verify this 2.5V is indeed 2.500V per a DMM on the VREF+ pin.

I've noticed the DAC output itself has some gain error. Specifically, from 0x000 counts to 0xFFF counts, I'm seeing a gain error of about -1.22% across the entire span. Therefore, actual output voltage is about 1.22% lower than ideal output voltage. I've tested this on 11 of my devices, and they all seem to fall in line with this statement (1.22% is an average over the entire span, actual error varies from 1.13% to 1.33%).

My question is, is this expected, or is there a design error in my circuitry? The output of the DAC goes to a voltage divider with a total resistance of ~140k ohms (top and bottom resistors combined). The midpoint of the voltage divider is the + terminal of an op-amp. Is there any way to increase the accuracy of the DAC (other than just adding this error value to every DAC count)?

Thanks.

Rob

  • 0
    TI__Mastermind 26420 points

    Hey Rob,

    This seems to be within the DAC Gain Error Specs, defined in Table 5.46 of the Device Datasheet.   

    Have you run the calibration on the DAC?  If not, I would recommend it. 

    The DAC offset can be adjusted via calibration to try and help reduce this error.  Section 31.2.6 of the Family User's Guide describes this process.  http://www.ti.com/lit/ug/slau208q/slau208q.pdf

    Thanks,

    JD

  • 0 in reply to JD Crutchfield
    Intellectual 275 points

    Thanks JD. 

    That is what I figured. Just wondering if perhaps there was a way my circuitry could be improved, as I expected the error to be more evenly distributed in the +/- 1% range, instead of all being -1%. 

    We are doing calibration. I'll make sure it is working correctly. 

    Can you pass along the document showing Table 5.46? I have the datasheet and user manual, and neither have that Table. 

    Thanks,

    Rob

  • 0 in reply to Robert Helvestine
    TI__Mastermind 26420 points

    Hello Robert,

    Apologies, it is actually Section 5.46 of the User's Guide titled "12-bit DAC, Linearity Specifications".  

    As for the error always seeming to be -1%, this seems to be something that calibration should be able to reduce by adjusting the offset voltage of the DAC as seen in Section 31.2.6 of the Family User's Guide.  Copied below for quick reference too:

    Thanks

    JD

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