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Schematic for LMX2541

Francisco Martins
Prodigy 160 points
Other Parts Discussed in Thread: LMX2541

Hello, I'm using lmx2541sq2060e, only on full chip mode. Before i order the PCBs,i would like to know if my schematic is good.

Please give me your opinions.

Regards, 

Francisco2577.LMX2541.pdf

  • 0
    Mastermind 7395 points

    Hi,

    I think that it needs a careful revision, consider making a design review for it with your peers, and/or your local TI representative.

    Many signals are floating (CE, LE, CLK, DATA)

    Many voltage pins don't have decoupling capacitors.

    It is recommended to check your 3.3V power feed, at least add decoupling close to the J7 connector. I would recommend adding an LDO for better results.

    Check the EVM for further details

  • 0 in reply to Avi Chami1
    Prodigy 160 points

    Hello Albert, thanks for your comment.

    CE, LE, CLK and DATA seems to be floating, but they are connected to their respective pins, the red label is the connection.

    I didn't put decoupling capacitors, to have the minimum space in the PCB, economical reasons. This schematic is based on the evaluation model schematic. I will use: 
    -Full Chip mode; OSCin Single ended.

    In the future i will use an LDO. Thanks again.

    Regards,

    Francisco

  • 0
    TI__Mastermind 28001 points

    Francisco,

    You might use the schematic in our evaluation board instructions at:  http://www.ti.com/lit/ug/snau067/snau067.pdf

    As for the components we use, don't go by the schematic in this document, but the bill of materials, which is exactly what we send to the boardhouse and use on our evaluation boards.

    Also, the datasheet gives some recommendations.

    In your schematic, it looks like you are doing similar things to the evaluation board.   Note that if you do not need the RFoutEN pin feature, you can also do this in the software and simplify your layout a little. 

    I know that we have all the ferrite beads on the board and you might want to try 0 ohms and see if it makes any difference.   I didn't make much on our boards.

    For the VccFRAC pin, this is the supply to the MASH engine, so this is the one pin that can generate fractional spurs energy, so the bead might be useful for this.

    On the OSCin pin, you have shunt 51 ohms.  However, many CMOS TCXOs do not have a 50 ohm output impedance and you need some series resistance.  If I was to do it and you have the space, consider a T pad.  You can put 2 series 0 ohm resistors and a shunt 51 ohm resistor, or you could use a series for a CMOS input and the shunt 51 ohms, or you could do a 3 dB pad with 18-68-18 ohms.  You might find that if there is very poor matching at this pin that the fractional spurs might be impacted.

    Regards,
    Dean