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Does impedance not given if not stated on electrical characteristics?

Latest Updated:07/01/2009

Question:

For most products, there is no mention of the input or output impedance in the electrical characteristics. Does this mean that the impedance is not prescribed?

Answer:

In semiconductors, the impedance differs depending on the voltage and other factors. For this reason, the impedance is generally not prescribed in the electrical characteristics. For the actual component, the impedance can be calculated by using Ohms law, under the special conditions indicated in the specifications. For example, consider the case of a CMOS input/output.

If the high-level input leakage current ILIH = +1 uA (max.) when VI = 5 V, the input is as follows:
RI (min.) = 5/(1 x 10-6) = 5 (MΩ)

The input becomes high impedance like this because the input gate is isolated by an oxide film. For this reason, the state of the MOS input is called high impedance (Hi-Z). Note that generally, the absolute value of the low-level input leakage current ILIL is the same as ILIH when VI = 0 V, so the impedance is the same even when the input level is low.



In the case of the output, if the high-level output voltage VOH = (VDD - 0.5 V) (min.) when the current IOH = 1 mA, the voltage drop from VDD will be 0.5 V or lower, so the P-channel (VDD side) on-resistance will be as follows:
ROH (max.) = 0.5/(1 x 10-3) = 500 (Ω)



Moreover, if the low-level output voltage VOL = 0.7 V (max.) when the current IOL = +5 mA, the N-channel (GND side) on-resistance will be as follows:
ROL (max.) = 0.7/(5 x 10-3) = 140 (Ω)

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