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DAC8811: I want to understand the speed

Part Number: DAC8811
Other Parts Discussed in Thread: DAC7811

Hi,

I went through all the questions asked regarding DAC8811. But I still unable to understand.

I saw that serial interface maximum speed is 50Mhz. but the settling time is 0.5us which means 2Mhz.

I have the input which has the max sped of 2MSPS. As the settling time is also same.

I want to understand that, whether I can use the full code or not.

In datasheet there is a figure 9 which has some explanation, but I am unable to understand that.

I just want to understand that will this DAC work fine for 2MSPS speed?

Regards,

Rohit Arora

  • Hi,

    Are you trying to generate a sine wave or some sort of arbitrary wave from generation?

    Since the settling time is 500ns, it will limit the frequency of the waveform. What this specification means is that if you were trying to generate a full-scale square-wave, you would likely not be able to go beyond 2MHz.

    For DAC8811, the update rate will be function of Settling time  +  16bit data transfer time +  set up time and digital delays as per the timing specified in the datasheet.

    Assuming SCLK is at 50MHz which is 20ns, and 16bit data transfer takes 16*20ns, DAC update rate will be approximately 500nS + 320nS = 820nS = 1.2Msps.

    We have TI precision video which explains this bit more in details and can be found in below link.

    TI Precision Labs - DACs: Settling Time and Update Rate | TI.com Video

    Regards,

    AK

  • Hi Akhilesh,

    Thank you for providing the DAC timing.

    My input is random changing signal, it is for sure not a sine wave.

    So , as you explained 1.2MSPS can be converted properly using this DAC.

    If I use any signal above 1.2MSPS the output will be distorted.

    Regards,

    Rohit Arora

  • Hi,

    Yes, your understanding is correct about the update rate.

    Are you feeding an arbitrary waveform to ref pin? What do you mean by " This can be converted by DAC " ?

    Regards,

    AK

  • Hi,

    I saw that TI video. As per that video settling time is greater than update time. So, settling time will be taken to calculate the speed that comes to 2MSPS.

    Ref pin signal is a constant 5V but the digital data coming is updating with 2MSPS.

    So tell me if my digital data will update with 2MSPS then can I get the full scale output on DAC Iout pin?

    Regards,

    Rohit Arora

  • Hi,

    As I said earlier, update rate includes settling time  +  Interface transaction time.

    In DAC8811's case, max update rate will be 1.2Msps if you consider full code settling. In theory, since your code changes are not from zero code to full code ( in the case of square wave), you should be able to update the DAC with 2MSPS update rate.

    From zero to full code, how many code transitions are you planning?

    Regards,

    AK

  • Hi,

    Update rate is not the sum of settling and interface transaction time.

    As per the video shared by you the Update rate will be considered as highest of these two time. So the update rate will be 2MSPS.

    if my code changes from zero to full scale what will happen or If I use 2MSPS speed then how much code change I can read without any error.

    Regards,

    Rohit

  • Hi Rohit,

    Most of the time, DAC settling time will be order of magnitude higher than the interface timing, that's the reason in the video, it is shown like that.

    Some unbuffered DAC's will have very low settling time and in that case if you want to get a realistic update rate of the DAC, you need to include the interface timing as well. For example, Consider 12 bit DAC like DAC7811 with settling time of 200nS. With 50MHz SCLK, 12bits will take 240nS, if you consider only settling time or the highest in this case the interface time, you will not get the correct update rate. 

    If your code change is zero to full scale, for DAC8811, update rate will be 1.2MSPS.

    Hope this is clear now.

    Regards,

    AK