Digital families and analgue devices

Discuss the implications of interfacing TTL and CMOS chips. Extend this discussion to the issues of interfacing these digital families to analogue devices.

Originally posted by indie_matt@Jan 4 2005, 08:19 AM
Discuss the implications of interfacing TTL and CMOS chips. Extend this discussion to the issues of interfacing these digital families to analogue devices.

[post=4412]Quoted post[/post]​

Click to expand...

TTL has a problem known as fanout. CMOS does not.

Fanout is when your logic circuits begin to load down your inital logic gate. Think of an opamp. As best they can supply 50mA. You put a load that requires more, and you load the opamp down. You can do the exact same thing with TTL logic.

As you keep adding more and more transitors/gates to the original gate, you may reach a point where there is not enough current to turn on all the connected gate.
CMOS circuits do not run into this problem. CMOS do not require a base current operate, rather they require only a voltage to be present.

Go for the rest.

Hi,

If you can still find them, the CD4049 and 4050 will interface between CMOS and TTL, if you can still find TTL chips. You'd need logic-level fets to go from TTL to CMOS. TTL is current operated, CMOS is voltage operated.

There are hundreds of digital-to-analog converters available to interface logic to analog circuits.

A tip:

Sometimes its easier to use "HCT" versions of a chip. Thats High speed Cmos TTL compatable as in - 74HCT04. These chips can be used interchangably with CMOS/TTL.

n9xv

Hi,

Yeah, I just got an education about CMOS. My old favorite 4000 series has just about gone the way of straight TTL. I have some stuff from a couple of years past that is getting redone in 74HCxx because it's available.