# Homework Help: TTL circuit voltage offset, why -2.5~2.5V and not 0~5V

1. Apr 1, 2016

### Jason Chuang

My teacher asked me to explain the reason why we need to tune the offset of the function generator of a ttl circuit to 2.5v(or -2.5v::didn't hear it clearly!) to create a shift from -2.5v to 2.5v instead of having it shift between 0v to 5v.

I've done some research regarding the high voltage and the low voltage of a ttl circuit, saying that most ttl circuit have a low voltage between 0~0.8v and a high circuit between 2.35v~2.5v, is that reason why we need to tone down(or bump up) the offset??

The question is in the following:

In TTL digital circuits, the “clock” signal like the left-handed picture below is frequently needed. However, the waveform output from F.G. is like the right-handed picture. How do we make the output waveform from F.G. be acceptable by TTL circuits?

can anybody help me explain the reason also tell me if the offset is to be bump up or tone down, in MHO I would say the offset needs to be tone down to -2.5v, but somehow I heard my teacher said 2.5v.

2. Apr 1, 2016

### Staff: Mentor

Hi Jason Chuang.

If the output from your function generator swings between -2.5v and +2.5v then it needs to be modified to operate TTL which needs signal levels between 0v and 5v. Can you explain now what he would have told you?

3. Apr 2, 2016

### Jason Chuang

I'm new to this forum, what exactly is the 3 header homework template??? Sorry if I sound ignorant.............

But last week we did some sort of experiment about a counter on a 7 segment display, and when the offset was 0v, I was unable to have it function properly as only 2~15 would show up not the 1, but after I adjusted the offset voltage to 2.5v, everything went well and I finished the experiment.

Why did this happened?? I'm sure setting the offset to 2.5v was the key point to have my 7 segment display to work.
I'm a EE student

4. Apr 2, 2016

### Staff: Mentor

Have a look at other threads in the Homework Help forums, you'll see the section headers in each opening post.

You gave the right explanation earlier. I thought I as good as handed you the wording you could use.

TTL won't work properly until you provide it with the signal levels that it is designed for.

5. Apr 3, 2016

### CWatters

With the offset set to zero the output swings from -2.5 to +2.5 eg it's centered on 0V. When you change the offset to +2.5 the whole waveform moves up 2.5v so it now swings from 0 to 5V.