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AC Coupling Question

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Hello, If i am ac coupling a signal output from a clock gen ic say, do I need a 50ohm resistor right after the capacitor? My thought is that I need to present a 50ohm source impedance? Same goes for when ac coupling into an input.

Assuming the output impedance from the clock generator is low impedance (like less than a few ohms) then yes, you could place a 50 ohm resistor in series with any ac de-coupling capacitor to give you a 50 ohm output (source) impedance. The capacitor can basically be considered a short circuit at AC.

An average CMOS logic-level clock generator will have an output impedance comparable to 50 ohms. Don't expect precise source termination, even if you add extra resistance.  Better to use an attenuator (if you can tolerate the loss), or load termination (if the source can put out that much power without overheating).


Okay, that is what I was looking for. The output impedance is listed at 45ohms suggesting I need a 5ohm resistor.

--- Quote from: evb149 on August 06, 2016, 11:25:15 pm ---Of course if the line is less than a fraction of the transition time long then it isn't really a transmission line and you don't have to worry about impedance matching for SI so much.

--- End quote ---
As a rule of thumb, how do you determine when a trace will begin to act as a transmission line for both sin/square waves? I've seen .5/rise time as an approximation for knee frequency for square waves and can find wavelength from that, but I'm not sure how to relate the wavelength to where impedance matching becomes important.


--- Quote from: jmsigler on August 07, 2016, 03:27:15 am ---Okay, that is what I was looking for. The output impedance is listed at 45ohms suggesting I need a 5ohm resistor.
--- End quote ---

Correct. May not even be necessary at all depending on what tolerance you need.

What frequency is the clock generator producing? is the rise time very high? PCB track design only matters if you're into 20+ Mhz rather than Khz or less. (roughly). Does it matter in this application?


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