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LMX2581EVM: LMX2581E schematic questions

Part Number: LMX2581EVM
Other Parts Discussed in Thread: LMX2581E

I am working on schematic capture for LMX2581E but has several questions. My reference schematic is figuure 8 at snau136c.pdf

1) RF output pull up: RF output As used 51ohm resistor between RFoutA+ and RFoutA- but RF output Bs used 18nH inductors between RFoutB+ and RFoutB-. Is there specific reason for this or either one of the configuration such as all 51 ohm or all 18nH inductors would be fine for them. FYI, I am going to use single ended +7dBm output. So, I will terminate complimentary ports with 50hm anyway.

2) I have seen two big decoupling cap networks fro "VccPlane", one is with C32, C33, C34 and the other is with C29,C30, C31. I think these decoupling networks has different VCc pins for decoupling. So, can you let me know what pins C29, C30 and C31 is taking care of and what the other C32, C33, C34 does? I think there are sensative areas to have separate decoupling rather than tie them up together.

3) VCCDIG (pin#28): This is digital Vcc pin. is it ok to tie this Vcc with ananlog one? Usually analog and digital VCCs are separate.

4) OSCin (pin #29): I am going to use external source of 10MHz refererence coming out of the other board. I see Figure 8 schematic has resistor network R14, R15 and R16. Do we have specific reason for this network or just power divider? Also, you have L3 ferrite between GND_OSC and GND but in my case, since I don't have on-board OSC, I think I don't need this L3 but R15 directly connects to GND. Please let me know if this can be an issue.

5) SPI: I saw series and shunt resistor for SPIs hook up such as R35 thrugh R41. I am ok with series 10kohm resistors but do we need shunt 12k ohm resistors? do we specific reason for this?

6) BUFEN: If this can be controlled by software, I am going to leave this pin open.

Please advice the questions above.


  • Youngho,

    1) the two outputs are the exact same for the device. The EVM has different pull-up components only for users to see the difference they get when using different types. If you see from datasheet with 50 ohms you get a more flat output across frequencies, whereas with inductor it is more tuned to high output power for specific frequencies.

    2) These are just split placement of decoupling caps balanced around the EVM, they are not necessary to be in the position they are. It is useful for user to experiment like that for the EVM.

    3) good point! it is generally good idea to isolate digital VCC and analog, often we use a ferrite bead in series between the digital supply and analog. But our devices have operated well with this configuration and performance has not degraded with this kind of connection. Most of the analog supplies have internal LDOs that protect these sensitive internal analog circuitry.

    4) This is also just EVM design for convenience of user to have a tunable network, in the case there are tweaks to be made or different terminating impedance for the type of clocking signal they are providing. The inductor between planes is also experimental to show if there is difference with different connection between those two planes on OSCin spurs.

    5) No, those are also placeholders, as long as you have above 1.8V SPI signal level it is ok.

    6) Yes you are right!

    Let me know if any other question. thanks!

    Brian Wang