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Total noob here.

The tech sheet can be found here: http://www.ti.com/lit/ds/symlink/sn74as138.pdf

In the VOH parameter for the SN74AS138 on page 5, it states that it's MIN is VCC-2.

What does that mean? What is referring to? Is it referring to the actual VCC minus 2 volts? So 2.5V for VCC = 4.5V?

Here's the snippet I'm talking about:

Snippet from datasheet

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It is a simple sum.

It refers to the output voltage when an output is HIGH (\$V_{OH}\$) as a minimum (it's in the MIN column) of 2 volts below \$V_{CC}\$, so if \$V_{CC} = 5V\$ then \$V_{OH}\$ will be at least 3V.

For CMOS you will often see something like \$0.6V_{CC}\$, where the value is 60% of the \$V_{CC}\$ value.

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It refers to the pin diagram on the first page, where Vcc is pin 16.

On page 4 of the datasheet, you see:

enter image description here

So it's the supply voltage of the IC.

Also see this: http://en.wikipedia.org/wiki/IC_power-supply_pin - excerpt:

Typical supply pin labeling:

VCC / VDD / V+ / VS+ : Positive supply voltage
VEE / VSS / V / VS− : Negative supply voltage

And exactly, VCC - 2 refers to 'the supply voltage minus two volts'.

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