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DAC7678: DAC7678SRGET

Moshe Levy
Prodigy 30 points
Part Number: DAC7678


Tool/software:

Hello.

I have a problem in my design (or i don't and just don't understand how it works).

1. I designed with the DAC7678SRGET and per the datasheet page 23 Vo should be Vo=(DIN/409)*2*Vrefout

- my VREF is internal so it is 2.5V

if I feed 0xFFF in DIN (in decimal equivalent) I get Vo measured 3.3V...I was expecting:

Vo=(409/409)*2*2.5 = 5V

what am I missing?

BR
ML

  • 0
    TI__Mastermind 42273 points

    Hi Moshe, 

    Your AVDD is 3.3V so the output is saturating at AVDD. Your understanding of the transfer function is correct, but you'd need to increase AVDD to at least 5V to see a 5V output. 

    Best,

    Katlynne Jones

  • 0
    Prodigy 30 points

    Dear Katlynne, 

    Thank you an noted. figured out this could be it from the datasheet. 

    but one thing i still don't understand. per the transfer function, for internal VREF of 2.5V the Vout(max) should be 2xVREF (i.e: 2*3.3V) and only if I

    feed external ref it is Vo(max)=Vref

    what is the explanation for this?

  • +1 in reply to Moshe Levy
    TI__Mastermind 42273 points

    Hi Moshe, 

    With internal 2.5V reference, the Vout max is still 2*2.5V = 5V. But the output will saturate at VDD if VDD < 5V. The transfer function does not change. 

    The DAC has a different transfer function when using the external reference. There is no 2x gain factor. This was just a design choice by the team to not include the gain factor with an external reference. The external reference cannot be greater than VDD, so because there is no gain, you won't run into the potential saturation issue because VOUT max will always be less than or equal to VDD.

    Best,

    Katlynne Jones