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Interfacing LMX2581 to ADC12D1800 ADC

Other Parts Discussed in Thread: LMX2581, ADC12D1800

Hi,

I am planning to use LMX2581 to provide clock to ADC12D1800,  How to interface this two devices(Clock signal alone) . Since both the device supports differential clock signaling.  What is the use of pullup resistors at the output pins ? whether is it must ? what will be the o/p power level if pullup resistors are not provided ?

Note: As per my application I want 3.1GHz sampling frequency & RFout channel B I planned to use it for another application with divide factor of two.

Regards,

Rajesh

  • Rajesh,

    The output of hte LMX2581 is open collector.  This means if there is no pull-up component, then no ouptut.  So these are a must.

    For the pull-up component, there are two options.  One option is to use a 51 ohm pull-up that makes the output impedance look closer to 50 ohms.  The other is to use an inductor (we use 18 nH on our eval board) that looks like high impedance.  The output impedance is high and therefore not matched, but you get more output power and you can fix the match with a resistive pad after.   The datasheet goes into much more detail on this in the applications section.

    For the output, you can do one divided and one straight through as you say.  Of course, as with any clock generator with multiple output frequencies, just be aware that the 1.55 GHz divided output frequency will create a spur on the 3.1 GHz output.

    Regares,

    Dean

  • 1. What will be the o/p power level of LMX2581 in case of 51 ohms pull up to 3.3V?

    2. Whether this o/p shall I interface directly to  ADC12D1800 ?

    3. Is there any series resistor is required ?

    3. How to control the O/P power level of LMX2581 ?

    Regards,

    Rajesh

  • Rajesh,

    1.  The output power is a function of output power setting, pull-up component, pull-up voltage, and frequency.    For a pull-up of 51 ohms, 2.7GHz output, 3.3V pull-up voltage, and output power of 15, the output power is about 6.5 dBm in accordance to figure 8 in the datasheet.  This also shows variation over frequency.   Table 7 says that you can decrease this by 9 dB or increase this by 2.4 dB with the OUT_PWR setting.

    2.   I am not an expert on the ADC12D1800, so I can't answer this.

    3.  As for the LMX2581, no series resistor is required.

    4.  The output power is controlled with teh OUTx_PWR word.   See table 7 in the datasheet.

    Regards,
    Dean

  • Hi Dean,

    Thanks for the suggestion, Still I have few clarification on this chip o/p power level:

    1. As per your previous post it was mentioned that the o/p buffer type is open collector it means my Pk-pk voltage is 3.3V correct me if I am wrong ?(note: My operating frequency is 3.1GHz & 1.55GHz)

    2. As per the table:7 the maximum o/p power level 2.4dBm  for resistor pullup but in the data sheet some places more than 6dBm is mentioned(ex:Figure 8) .

    3. As per the layout recommendation(page 49 in the datasheet), it seems that the o/p's are routed as single ended signals , not as a differential , whether it will create any issue if I routed as a 100ohm matched differential trace ?

    Regards,

    Rajesh