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AD9951 Amplitude

Dear Technical Support Team,

Hi,

My circuit shows 290mVpp on 50Ω pullup and 550mVpp Single-End through transformer.

Settings of AD9951 are 10mA(FullScale with ASF=0x3FFF) and 12.88MHz 

Is it expected amplitude?

Here is schematic and waveform.

PDF

It seems that there is a resistance of about 50Ω in parallel on the secondary side of the transformer.
When the impedance on the secondary side reaches about 38Ω,

Primary side: 275 mVpp
Secondary side: 550 mVpp

It will be about.
If there is about 50Ω in parallel on the secondary side

50Ω // (51Ω + 49.9Ω + α) = 38Ω
α is the impedance of the LC filter (at 12.88MHz)

Best Regards,

ttd

Parents Reply
  • Hi Jules,

    Thank you for your bench test.

    It seems like my results.

    I assumed 50Ωx10mA = 500mVpp, but the level is actually low. I feel that it is less than the original 10mA full scale depending on the load, please tell me the reason. If the amplitude is not exactly determined, it is very difficult to estimate the amplitude before board development.

    Best Regards,

    ttd

Children
  • Hi  ,

    Just want to ask the value of your Rset?

    Regards,

    Jules

  • Hi Jules,

    DAC Rset=3.92kΩ for 10mA.

    Best Regards,

    ttd

  • Hi  ,

    With 10mA into 50Ω yields 500mVpp, as expected. The problem is that the 50Ω load connected to the DUT has other circuitry connected (namely, the transformer). If you disconnect the transformer you should see 500mVpp across the DUT load resistor. However, 500mV is on the cusp of violating the DAC compliance voltage, so you may not see a full 500mVpp output (should be close, though). With transformer connected, things get complicated in a hurry.

    Regards,

    Jules

  • Hi Jules,

    Thank you for your reply.
    Is the following understanding correct?

    --------------
    Since the compliance voltage (= 0.5V) is barely enough, it is NG if various load circuits are attached.
    The solution is to set the compliance voltage to 0.25V (= 25Ω).
    --------------

    Also, is it better to use a buffer amplifier instead of a transformer?
    Since the maximum frequency this time is about 13MHz, I think that an operational amplifier is okay.

    Best Regards,

    ttd

  • Hi  ,

    The maximum full-scale output current of the combined DAC outputs is 15 mA, but limiting the output to 10 mA provides the best spurious-free dynamic range (SFDR) performance. Given that 0.5V is on the cusp of violating the DAC compliance voltage, using a lower load resistor will help.

    A transformer by itself can change the voltage of a signal but does not change the amount of power. For a given increase in voltage, there is an equivalent decrease in the current.  The power remains constant.   In fact due to resistive losses the amount of power is actually a bit less.  An amplifier, on the other hand, can increase the power of a signal by using the input voltage to control a more powerful source. In this specific application, I suggest using the transformer since it is used for impedance matching.

    All the best,

    Jules