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DAC80502: DAC ouptput during power-on with midscale ( RSTSEL = VDD)

ttd
Mastermind 8580 points
Part Number: DAC80502

Tool/software:

Dear Technical Support Team,

I'd like to confirm DAC ouptut(Vout) with   midscale ( RSTSEL = VDD) .

According to the datasheet, Vout is defined by the following formula and default settings are DIV=1 and GAIN=2, internal refefence=2.5V.

I believe that this default settings means 5V fulll scale with internal reference.

Q1:

The default setting with midscale code(=8000h), what timing does Vout show 2.5V? 

For example, VDD POR circuit is following, Vout shows 2.5V after green zone?

   

Q2:

When using external reference, I need to change from internal ref=2.5V to  external ref=5V and REFDIV=1 (reference divided by 2) and GAIN=1 (gain at both the DAC outputs is 2) following steps.

At this time, will Q1's midcode (Vout=2.5V) be maintained until write new DAC code?　

Best Regards,

ttd

  • 0
    TI__Genius 10220 points

    Hi ttd,

    1) From my own testing, the output goes to 2.5V (midscale) when VDD = 3.7V. When under 3.7V, the output is 0V. This is in line with the image you sent in your second question, Line 2: "When using 3.3V VDD and the internal reference of 2.5V, the DAC outputs are disabled unless the reference is divided by 2." 

    2) The DAC buffer will hold the midscale code, but the time between disabling the internal reference and applying an external reference will cause the DAC output to drop to 0V. One you apply the external reference the DAC output will return to normal. The DAC output will not be stable without a reference voltage.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Mastermind 8580 points

    Hi Erin,

    Thank you for your reply.

    1) Internal Reference case

    Erin said: 

    From my own testing, the output goes to 2.5V (midscale) when VDD = 3.7V. When under 3.7V, the output is 0V.

    →Is VDD=5V  better for Vout=2.5V(midsale) with default settting? 

       

        When using VDD= 3.3V and change from DIV=1 to DIV=2, I think that configuration is 2.5V fullscale and 1.25V midscale(DIV=2,GAIN=2,Int REF=2.5V).

       It means VDD=3.3V doesn't work for 5V fullscale with internal reference=2.5V.  Is it correct?

    2)

    I got your comment, thanky you.

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Genius 10220 points

    Hi tdd,

    Yes, if you are using 3.3V VDD you will not be able to do 5V full scale. 2.5V full scale would be the max. You'll want at least 3.8V to do 5V full scale. Note the limit is based on VDD. So if you used 4V VDD, the DAC would be configured for 0-5V, but will not be able to go over 4V. If you want to use 5V full scale, you'll ideally want 5V VDD.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Mastermind 8580 points

    Hi Erin,

    Thank you for your reply.

    I will use VDD=5V for 5V full scale with internal reference.

    I have additional Question with 

    (3) When using the internal reference, the maximum output is 5V(2 × VREFIO), but is it possible to exceed VDD?
    VOUTx to AGND(Absolute max) shows VDD+0.3V(max).
    I'd like to confirm whether it is possible to output 5V even when using a VDD=5V ±5%(5V±0.25V) power supply.

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Genius 10220 points

    Hi tdd,

    The output range is based on the VREF. If you are using the internal VREF = 2.5V, the max output is 5V. If you are using an external reference that is higher than 2.5V, the output range will scale higher, but it will be limited by the VDD voltage. You will not see a situation where the output will be higher than VDD. 

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Mastermind 8580 points

    Hi Erin,

    Thank you for your reply.

    >You will not see a situation where the output will be higher than VDD.

    →Is this comment for the internal VREF = 2.5V, the max output is 5V?

       VDD=5.25V(5V ＋5%), I believe that the max output is 5V(no saturation).

       How about VDD=4.75V(5V -5%) situation? Does Vout satuate 4.75V(max) due to VDD=4.75V?

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Genius 10220 points

    Hi ttd,

    The VOUT is limited by the VDD voltage, as this voltage is what powers the output buffer. If your VDD is 4.75V, that will be the max output voltage. 

    The Absolute Max Ratings table is more of a warning for applying an external voltage to VOUT. As one might figure, applying a voltage over VDD to VOUT will break the device. 

    Thanks,
    Erin