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The NRZ signal generated by DAC38J84 does not have a flat top/bottom?

Other Parts Discussed in Thread: DAC38J84

Hi everyone,

I'm using DAC38J84 to generate NRZ signal. However, the waveform I generated doesn't have a flat top/bottom.

Firstly I generated PRBS sequence whose length is 2^7-1, and the bit period as 27ns. I use this PRBS sequence to generate NRZ signal, and the NRZ signal has 10 samples in each bit period. However, the DAC output waveform does not have a flat top/bottom. 

I also tried to generate square wave, whose period is 27ns and duty cycle is 50%. But the generated square wave still doesn't have a flat top/bottom.

I used Integrated Logic Analyzer to monitor the data that has been sent into the JESD IP core, and verified that the data is correct. I also generated sinusoid wave and triangular wave, and the output are correct.

Can anyone tell me how to fix this?

Thank you!

Regards,

Tong

  • Tong,

    The DAC38J84 has a transformer output which has a 1dB passband from 50M-1G.  The fundamental frequency of the pulses you are trying to pass through this are below this frequency so the main frequency component of the pulse is being attenuated (37M).  This causes the pulses to droop.  The wider the pulses, the more the fundamental is attenuated and you see more pronounced droop.  The way to mitigate this is to use a faster pulse to get the fundamental above 50MHz, or use a similar transformer with lower frequency range.  Check out the ADT2-1T as a possible alternative transformer.

    Ken.

  • Hi Ken,

    Thank you very much. 

    The frequency of the signal we want to generate is lower than 50MHz. Could you tell me how can we replace the transformer? Do we need to remove the original transformer and solder a new one?

    Regards,

    Tong

  • Yes.  You will need to remove and replace.  The one I suggested is pin compatible.  You may use others as long as it has the same impedance ratio.

    Ken.

  • Hi Ken,

    I found another RF transformer (ADT1-1WT) whose frequency range is 0.4~800MHz. But its impedance ratio is 1, is is possible to use this one?

    Below is the link:
    www.minicircuits.com/.../ADT1-1WT.pdf

    If cannot, could you recommend another transformer which has similar bandwidth?

    Thank you very much.

    Regards,
    Tong
  • Tong,

    Please look at the original impedance at the DAC IOUT pin - I believe it is set at 25 ohms on each pin (50//50). If you use a 1:1 you will get 50//25 which will be less than 25 ohms. Your swing will be smaller by the same ratio.

    Ken.