LMK1C1104: Output impedance on PCB

Prasoon Joshi

Part Number: LMK1C1104
Other Parts Discussed in Thread: LMK1C1102

Tool/software:

Hi,

I am designing a board for test and measurement purposes and I care about the signal integrity on the PCB traces as they will serve as my measurement test structures.

I wanted to cross-check my design for correct series resistance that should be built into the PCB at the output of the following devices. My PCB traces will be micro-strip and less than 15 cm.:

LMK1C1102/03/04: From the IBIS waveforms this looks to be 59 Ohm (rising) and 53 Ohm (falling). I am not planning to add any series impedance. The outputs are going into a bunch of SN74AUC devices, CLVC257AQPWRG4Q1 mux
SN74AXC1T45DRLR: I calculated the output impedance to be about 35Ohm. The output will go to SN74LV161APW. I plan to put 15Ohm impedance at the output of the transceiver.
SN74LV161APW: I could not estimate the output impedance as I could not find the IBIS model. The output will go to SN74AXC1T45DRLR. What series impedance at the output should I put here?
CLVC257AQPWRG4Q1: This is driving SMAs on the PCB. I could not find the IBIS file to calculate the output impedance. The SMA output will be connected to a scope, or, left unterminated.

Thanks,

Prasoon

4 months ago

0 vicente flores prado 4 months ago

TI__Mastermind 29362 points

Hello Prasoon,
Traditionally LVCMOS are for very low speed non-critical clocks where impedance matching isn't much of a concern.
Usually, LVCMOS only needs to be impedance matched when driving long trace lengths or higher frequencies.
The output impedance for LMK1C also varies depending on supply voltage.

Best regards,

Vicente

0 Prasoon Joshi 4 months ago in reply to vicente flores prado

Prodigy 210 points

Hi Vicente,

Thanks. My frequencies of interest are 30MHz and less for the LVCMOS.

What is "long trace lengths"? Longer than 15cm?

The supply voltage for LMK1C will be 1.8V.

Thanks.

0 Joshua Salinas 4 months ago in reply to Prasoon Joshi

TI__Mastermind 23600 points

Hello Prasoon,

As least for Logic devices (AXC1T45) the "long trace length" question can be answered in this FAQ:

[FAQ] What is the maximum trace length that a logic device can drive?

What is the expected output current for AXC1T45? Is the output voltage also 1.8V?

Regards,

Josh

0 vicente flores prado 4 months ago in reply to Joshua Salinas

TI__Mastermind 29362 points

Hi Prasoon,
If using less than 30MHz for LVCMOS I would consider that low speed.
That's almost 6" of trace - in that case I would suggest impedance matching.

Best regards,

Vicente

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LMK1C1104: Output impedance on PCB