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likephysics
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How do you determine the output impedance of cmos gates?
likephysics said:How do you determine the output impedance of cmos gates?
likephysics said: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.
likephysics said: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)!
Determining the output impedance of CMOS gates is important for understanding the electrical characteristics of the gate. It helps in designing and optimizing circuits, as well as predicting the behavior of the gate in different operating conditions.
The output impedance of CMOS gates can be calculated using the following formula: Zout = (Ron||Rop)/(1 + gmp/gmn). Ron and Rop are the resistances of the pull-up and pull-down transistors, while gmp and gmn are the transconductances of the pMOS and nMOS transistors, respectively.
The output impedance of CMOS gates is affected by the size and type of transistors used, the supply voltage, and the operating frequency. In general, larger transistors and higher supply voltages result in lower output impedance, while higher frequencies can increase the output impedance.
The output impedance of CMOS gates is typically low because CMOS technology uses complementary pairs of transistors, which provide a low impedance path to both the supply and ground. Additionally, the use of large transistors and low supply voltages also contribute to the low output impedance.
Yes, the output impedance of CMOS gates can be adjusted by changing the sizes of the transistors or the supply voltage. However, this can also affect other electrical characteristics of the gate, so it is important to carefully consider the trade-offs when making adjustments.