Output impedance calculation

First, if you shorted the output to ground you may well get 16mA, not

3.6mA. Second, the output impedance is a measure of the change in output voltage for a small change in output current; it's not intended nor necessarily accurate as a measure of gross voltage and current changes. Third, 1.8V - (90 ohms)(3.6mA) is surprisingly close to your 1.44V output voltage at 3.6mA -- so maybe that quoted output impedance works for high currents after all.

If the microcontroller is sufficiently fast it should say. If it doesn't, and if it's CMOS (are there any these days that aren't?), then the output impedance can be roughly calculated from the voltage drop divided by the current at that drop. The data sheet -- unless it's truly horrid -- will quote current source/sink capabilities at certain voltages. Calculate the voltage drop, divide by current -- voila! Output impedance.

I'm used to matching impedance to traces with termination resistors -- this built-in matching in the chip is new to me.

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www.wescottdesign.com

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Thanks Tim. But, let's say, if the driver's output impedance is 50 ohm and I am sending my schematic for layout. For impedance matching, do I need to add the 50 ohm series resistors in the schematic and send it for layout without bothering pcb trace impeance matching while fabricating or impedance match the pcb traces while fabrication and not make any provision for series termination resistors during schematic and layout preparation? I basically do not understand when to use which option.

-B

If the output impedance is 50 ohm you certainly don't want to add another 50 ohms in series *and* have 50 ohm traces. If the nets are point-to-point (no taps) just match the trace impedance to the device output impedance. If the net is multi-drop it's going to get ugly anyway and a small series resistor somewhere in the net may be necessary. It may not go where you think it should, though. This can be a real PITA.