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How do you determine the output impedance of cmos gates?
A reasonable approximation is to look at the Voh/Ioh and Vol/Iol numbers.How do you determine the output impedance of cmos gates?
The Voh number is for when the output stage is pulling up, so the voltage drop is across the upper CMOS transistor. Its output Z is (Vcc-Voh)/Ioh, since that transistor is sourcing the Ioh current.Exactly! I am trying to calculate Zout to fix the series termination resistor value.
Are you sure about (Vcc-Voh)/Ioh.
Voh/Ioh gives the correct value.
How are you doing the simulations? Are you getting the models from the manufacturers' websites? Can you provide links to the models?Guys, I tried to calculate the Zo of some ICs.
For a clock buffer - CDCVF25081
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cdcvf25081&fileType=pdf
Zo = (Vdd-VOH) / IOH. This gave me (datasheet pg.5) Zo = 0.9/12mA = 75 ohms
From the IOH row, Z0 = Vo/IOH = 1.65/30mA = 55 Ohms
VOL/IOL = 0.8/12mA = 66.67Ohms
Simulation gave me close to 50 Ohms. (55 rising edge and 51 falling edge).
Now For another part CY25811
http://www.datasheetcatalog.org/datasheet2/e/0lajeyeqd5a8q0ytgz70kd4gfcky.pdf
Zo = 2.97-2.4/4mA = 142.5 Ohms
Zo = VOL/IOL = 0.4/4mA = 100 ohms
Now for one more - P2811B (onsemi)
http://www.datasheetcatalog.org/datasheet2/e/0lajeyeqd5a8q0ytgz70kd4gfcky.pdf
Zo = 3.3-2.5 /15mA = 53 Ohms
Zo = VOL/IOL = 0.4/15mA = 26 Ohms (This is given as output imp in datasheet)
Last one
Cyclone FPGA
http://www.altera.com/literature/hb/cyc/cyc_c51004.pdf
Pg 2 LVTTL
Zo = (3-2.4)/24mA = 25 Ohms
Zo = VOL/IOL = 0.45/24mA = 18.75 Ohms
The Zo from simulation is much less (7 ohms)!