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DAC3484EVM: OSTR connection

Expert 1700 points
Part Number: DAC3484EVM
Other Parts Discussed in Thread: DAC3484, CDCE62005, CDCP1803, THS3217, TRF3705

Hello,

   1)  In the EVM OSTR is generated from the CDCE62005 which is capable of generating LVPECL signals. However OSTR generated from CDCE is not connected to the DAC3484 directly but instead through a 1:3 LVPECL buffer CDCP1803. What is the reason for doing it this way? 

2) How can I modify the transformer circuit (DAC output) to take out single ended DC coupled output (to be fed to an external IQ modulator that takes in single ended inputs).

Thanks for your help,

  • Hi SM,

    1. CDCP1803 buffer is used to divide OSTR output frequency from CDCE62005 by 2. This is needed when interpolation factor 16 is used.

    OSTR input of DAC3484 needs to f_DAC_CLK / (8 * interpolation factor* n). For DAC interpolation 16, OSTR output should be f_DAC_CLK/ (8*16*n) = f_DAC_CLK/(128*n). But CDCE62005 output divider maximum division ratio is 80. So, for this case, we can use output clock divide factor of 64 in CDCE62005 and as CDCP1803 provides a didvide-by-2, OSTR input will be f_DAC_CLK/128. 

    If interpolation factor 16 is not used, CDCP1803 buffer is not needed. CDCE62005 can be directly connected to OSTR input of DAC3484.

    2. An IQ modulator like TRF3705 with differential inputs is ideal as there are advantages in using fully differential circuits. But if a single-ended output is needed, differential to single-ended buffers like THS3217 can be used with proper signal conditioning. 

    Regards,

    Vijay

  • Hello Vijay,

              Thank you for the answers.

    We need to test an IQ modulator with the existing circuit and so I am afraid that the THS3217 can be used when we make our custom board only. 

    Please suggest a modification to the existing circuit in order to complete the evaluation (can we just terminate the unused output with a 50 Ohms resistor?)

    Thanks,

    SM

  • SM:

    It is a little unclear what configuration is needed/wanted for the final circuit.  For quick evaluation, you can take the single-ended output from the transformer of the DAC348x EVM and feed it to your modulator of choice or to the TRF3705.  If you use the TRF3705, you can terminate the unused ports.  Note, driving the TRF3705 SE will introduce an automatic 6 dB drop in gain.  Further note, using the transformer precludes operating at or near DC on the baseband signal.

    If you want to use a TRF3705 modulator, the TSW3084 is configured with the DAC + TRF3705.  It is a convenient platform for evaluating both devices together.

    If you want to configure the DAC output for SE without the transformer, some modifications are needed to maintain proper common mode voltage and load resistance to maintain compliance voltage.  The Datasheet provides some basic guidance on the interface.  Essentially a proper pull-up/pull-down resistor is needed to set the proper load resistance and common mode.  The values are dependent on the voltage available.  If both differential ports are properly terminated then you can leave one unconnected.  You will also take an automatic 6 dB voltage level hit, but you can interface with a (high impedance) SE load.

    --RJH