Hey guys first time poster. Was just hoping you could shed some light on converting analogue to digital signals using a comparator and schmitt trigger. I understand how the schmitt trigger works but not to sure about how it converst A/D.

Thanks

 

See the the section of the AAC eBook here...

http://www.allaboutcircuits.com/vol_4/chpt_3/10.html

If you have any questions we'll try to tackle them after that. A really good example of a Schmitt trigger is the 555 wired in this configuration. Its threshold values are 1/3 and 2/3 of the power supply voltage. Here is an illustration how a Schmitt Trigger works.

Circuits like this are also refered to as signal conditioners, as they remove the noise and create a signal that a digital gate can use. Examples of this include debounce circuits for push buttons, which can make/break several times off on button press, a counter will interpret each make/break as an input, and count them accordingly.

 

ok, so the schmitt trigger is a comparator, it compares a signal voltage with a reference voltage. But the output of the schmitt trigger is still an analogue signal or is it now a digital signal?? We have a quad 2-input nand schmitt trigger to use.

Thanks

 

The difference between a comparitor and schmitt trigger is the comparitor uses 1/2 the voltage, or whatever reference you set, while a schmitt trigger uses 2 separate threshold points. The signal has to cross the threshold on both to switch. Both output a digital signal of a sort, but the schmitt trigger cleans up a signal a lot better.

 

A comparitor is an op amp that is not used in a linear mode. It is digitial only in that its output is on one side of the power supply or the other. A schmitt trigger is made from a comparitor, but is not a comparitor. The comparitor is a component of the schmitt trigger. I don't have time to diagram it entirely, but look at the three signals below, one is a digital signal after going through a transmission line, and has lost a lot of digital characteristics, the second is an interfering signal, and the third is 2 combined. Think how a schmitt trigger would handle this signal vs. a comparitor.

 

OK, I've finished a drawing of the differences in how the two convert, and why it's important. Note that 1/3 and 2/3 levels of the Schmitt trigger and 1/2 level of the comparator are arbitrary, different chips/designs will have different values. The gap between the 1/3 and 2/3 levels on the Schmitt is called hysteresis, and is very important, fundamental to all Schmitt triggers.

Hope this helps.