How much is the input impedance of the 74 TTL family

[mr_anderson]

Hi
how much is the input impedance of the TTL logic devices, for example if you have a transistor feeds a signal to a 7404 (for example) how you will deal with the input impedance of the 7404 when you make your calculations?
we hear that the TTL family has high input impedance in order not to sink too much current but the question is how much that high impedance is it 1 M or 2 M or what?
Any Ideas, reviews, comments?
Regards.
Anderson

 

[FvM]

we hear that the TTL family has high input impedance

It's completely wrong. Standard 74 TTL has a specified maximum input current of 1.6 mA in low state.

 

[dick_freebird]

I seem to recall more like 400uA IIL, but I was working with
LS series "back in the day". At any rate the databook is your
friend.

 

[FvM]

LS reduced the max. input current by a factor of four, increasing the standard logic circuit fanout from 10 to 40.
Only a few types of standard 74 are still in production, while 74LS is available in a large type variety. Because the
price is a multiple of 74HC types now, they are rarely used for new designs.

74HC have actually "high input impedance".