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Original question:

DAC1282: Problem understanding DAC1282 DC Mode

DAC1282: DC mode

ken cui
Prodigy 20 points
Part Number: DAC1282

Tool/software:

Hello,

This is Ken Cui(ken.cui@nxp.com) from NXP semiconductors, and I have a related question about DAC1282 in DC mode.

Given that: AVDD=2.5V, AVSS=-2.5V VREF=5V

by sending a command with 0X24027FFFFF Total of 40 bits data frame, do I get VOUTP = 1.25V, VOUTPN=-1.25V or VOUTP =1.15V and VOUTN=-1.35V refer to AGND?

 

Another words, is Vcom(-0.1V) applied when in DC mode.

 

Thanks

Ken

  • 0
    TI__Mastermind 46760 points

    Hello Ken,

    I assume you are writing to the DCG registers to generate an output voltage of +2.5*gain.  In this case, the 40b command should be:

    0x4402FFFF7Fh

    Also, you need to set the analog gain to 1 and configure DC mode by writing to the GANMOD register:

    0x400001h

    Yes, in DC mode, with the above configuration, you should get a differential output voltage of +2.5V with a common mode of -0.1V, VOUTP=+1.15V and VOUTN=-1.35V with respect to GND.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hello Keith,

    Thank you very much for your quick response. This is exactly what I was looking for.

    I will design this DAC into my evaluation circuit.

    Best Regards,

    ken 

  • 0 in reply to ken cui
    Prodigy 20 points

    Hello Keith,

     

    Thank you for your answer on last email.

    I have a further questions in DC mode DAC update.

     

    In DC mode:

     

    1. After program DCG and gain registers, should I apply a pulse width (2 clk pulse width, high pulse) on SYNC pin to update the DAC output. If not, how do I update the DAC output? I need to program DAC output in a fix period time (such as in 39.0625uS, 19.53125uS).
    2. What is DAC settling time in DC mode? It’s 25uS Typical from spec, but what is max spec?
    3. If there a timing diagram for DC mode including SYNC pin, can you please send it to me?

     

    Thank you in advance for your help.

     

    Best Regards,

    Ken

  • 0 in reply to ken cui
    TI__Mastermind 46760 points

    Hello Ken,

    1.  No, you do not need to apply any signal to the SYNC pin.  This is primarily used to synchronize multiple DACs so that you can get a known phase for the AC sinewave output, or to control the output value in PULSE mode.

    2.  For DC mode, the settling time is actually 100us.  This is typical because it is dependent on the exact clock frequency.  The exact number of clock cycles is around 410 cycles.  The maximum settling time will be the maximum clock period times 410.  If using a 4.096MHz typical clock with 100ppm accuracy, the maximum clock period would be 0.244us, resulting in a maximum settling time of 410*0.244us=100.11us.

    3.  The only timing requirements will be the register write command for the DCG registers.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hello Keith,

    Appreciate your help once again.

    ken

  • 0 in reply to ken cui
    Prodigy 20 points

    Hi Keith,

    Does TI have a DAC with Dual channels (or more), 24 bits resolution, which I can control the sample rate with similar pin like load DAC, the sample rates could be 25600Hz or faster. Thank you. 

  • 0 in reply to ken cui
    TI__Mastermind 46760 points

    Hi Ken,

    Not to my knowledge.  Any DAC greater than 20b is typically a delta-sigma topology that uses an interpolating filter, which requires additional settling time.

    The only multi-channel DACs that are +24b are audio codecs, but these parts operate on a continuous bitstream.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Thank you very much, Keith

  • 0 in reply to ken cui
    Prodigy 20 points

    Hi Keith,

     

    Thank you for all your answers. Based on all your inputs thar far, it seems to be very difficult to implement what I want to do with the DAC1282. But I haven’t given up on DAC1282 yet.

     

    I am thinking of 2 scenarios with DAC In DC mode as attached diagram of Case1 and Case2 in DC mode.

     

    If DAC1282 works as Case1, I should be able to use it. If it works like Case2, I don’t have a shot.

     

    Just in case it works like case1, what is min T would be?

     

    Thank you very much for your help.

     

    Ken

     

  • 0 in reply to ken cui
    TI__Mastermind 46760 points

    Hi Ken,

    I will get back to you on Friday of this week.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    TI__Mastermind 46760 points

    Hi Ken,

    I took a closer look at your diagram.  Do you need two independent outputs?  If so, then you would need to use two DAC1282s.  The DAC1282 has a switch on the output, but you can only provide a single voltage at a time.

    You can use two DAC1282s and write to both at the same time, using the same /CS and SCLK signals, but different DIN data.  The two DAC's will then update their outputs at approximately the same time (withing a few ns of each other).

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hello Keith,

    Thank you again for your support. 

    What I really need is to use DAC1282 to generate 50Hz or 60Hz sinewave with better than 125dB THD. Figure 24 in DAC1282 datasheet indicates that DAC1282 is capable of doing better than -118dB THD, which is very good but worse than our expectation. To solve this problem, there are 2 different ways I can think of:

    1. Find a way to purchase DAC1282s with better than -125dB THD in sine mode. I don't know if you guys can cherry pick them and sell them to us.

    2. Generate a sinewave in DC mode with faster than 12.8Khz (12.8khz, 25.6khz, or 51.2khz) sample rates. I can find a way to improve THD up to -130dB with some algorithm. Based on your description you provided, DAC1282 can only work at fixed sample rate at approximately 9.6Khz (410 system clock cycles + SPI frame of 4uS with 8.333Mhz SPI clock). If there is a FIFO on DAC1282, we may be able to complete this task. Is there FIFO in DC mode

    Your thought?

    Best Regards,

    Ken

  • 0 in reply to ken cui
    Prodigy 20 points

    Hi Keith,

    Following up on previous question. 

    Thank you very much for your help,

    ken

  • 0 in reply to ken cui
    TI__Mastermind 46760 points

    Hello Ken,

    We are not able to screen DACs for a specific THD.  There is no internal FIFO, and even if there was, the DC mode would not produce as good a sine wave as the sine function built into the device.

    You can get typical distortion of -125dB THD as long as you limit the gains between 1 and 1/8.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Thank you very much for the answer.

    Ken