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IO pad drive strength selection

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zhangljz

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Hello,

I have a IO pad library with different drive strengths, say 4mA, 6mA, 8mA, 16mA, 24mA, etc. For signals below 100MHz, which drive strength is suitable?


And how to simulate the IO output impedance? I want to use a IO with output impedance of ~50 ohm to match the PCB impedance.

I did the simulation like this :
1.for a 8mA IO pad, I set input signal to 1.2V, and connect the output to a 'idc' source with dc current 8mA, ac magnitude 1A
2. do ac simulation, and plot the output voltage. The voltage is about 25V which means 25 ohm output impedance.

Is this setup correct?

Anybody can help?


Thank you
 

zhangljz said:
... how to simulate the IO output impedance? I want to use a IO with output impedance of ~50 ohm to match the PCB impedance.

I did the simulation like this :
1.for a 8mA IO pad, I set input signal to 1.2V, and connect the output to a 'idc' source with dc current 8mA, ac magnitude 1A
2. do ac simulation, and plot the output voltage. The voltage is about 25V which means 25 ohm output impedance.

Is this setup correct?
Click to expand...

I understand you want to measure the ac output impedance?

If the measured voltage of 25V is the ac part only, this would be correct. If you aren't sure, separate the DC and ac sources by a (huge) capacitor, and measure the separated ac part at the ac isource side.
 

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