potential difference between 1&0

Discussion in 'General Electronics Chat' started by jayanthyk192, Apr 21, 2010.

1. jayanthyk192 Thread Starter Member

Jan 23, 2010
80
0
hi,

in any digital circuit when a voltmeter is used to measure the potential difference between logic 1 and ground the reading would be 5v. but,what would the reading be if it is measured between 1&0?

for instance,if i'm using a flip flop,and i measure the potential difference between Q & Q' ,what would the reading be?

another question:in any digital circuit does '0' mean ground or something like "nothing"?is it something like connecting one end of voltmeter to a hig potential and leaving the other end just without connecting?

thank you

2. Papabravo Expert

Feb 24, 2006
11,781
2,496
This example is close but not quite factually correct.

For TTL and LSTTL:

VIL is 0.8 Volts and anything less than that is supposed to be a zero
VIH is 2.0 Volts and anything greater than that is supposed to be a one
This means there is a no mans land between 0.8 V and 2.0 V where the logic level is indeterminate. Transitions through this region are allowed, but lingering and loitering are not.

For CMOS:
The basic threshold for a CMOS gate is at Vcc/2 If you look carefully at the datasheets you often see output specifications like 0.7Vcc and 0.3Vcc as the minimum value of a one and the maximum value of a zero. You need to be careful when looking at these specifications to determine if they are input or output specifications.

I've never heard of a smith trigger. I think you mean a Schmitt Trigger
http://en.wikipedia.org/wiki/Schmitt_trigger
This gate has two thresholds; one for a positive going transition and one for a negative going transition. The thresholds may be set in a variety of ways, but the individual datasheets for each logic family will tell the story.

3. t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
A voltmeter will always the difference between the inputs. And since this a DC circuit this will be related to the circuit ground reference. so let us say the Q' output is 0.1 volt in respect to ground, and the Q output is 4.78 volt in respect to ground. I you place the com input on Q' the voltmeter will read (4.78-0.1) volt

4. bertus Administrator

Apr 5, 2008
18,898
3,716
Hello,

There are several logic families with different logic levels.
See these links for more info:
http://www.interfacebus.com/voltage_threshold.html
http://www.interfacebus.com/Design_Translation.html

These are link from the EDUCYPEDIA from this page:
http://www.educypedia.be/electronics/digitaltechnology.htm

There are more pages like that on digital electronics:

Digital electronics:
Arithmetic circuits D/A-A/D converters Number systems Codes and decoders Flip flops Technology Counters and registers General overview Technology-CMOS Digital logic Memories Technology-TTL Timers and oscillators

Greetings,
Bertus

5. jayanthyk192 Thread Starter Member

Jan 23, 2010
80
0
now i got some basic idea about it,but can anyone answer my second question?

"in any digital circuit does '0' mean ground or something like "nothing"?is it something like connecting one end of voltmeter to a hig potential and leaving the other end just without connecting?"

i was using an l293d motor controller and i had 3 outputs coming from 6,11,14.at a time i was giving 5v to one input of the ic to get output at 6,11 or 14. i.e the output would be the voltage at pin8(i had given 8v). the potential difference between the high and ground was 8v and that between low and ground was 0v.but the potential diff between high and low it was 0v.why was ot that way?and i cannot figure out what kind of IC it is(cmos,ttl etc.).

6. t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
Picture you a 5 volt battery. In logical 1 you have the voltmeter connected to each pole. In logical 0 you have both voltmeter input connected yo the negative pole. This is somewhat simplified. But you should never treat logical level 0 as disconnected from the circuit. Also if you leave an input unconnected or floating. Then the state of this input will be undefined. It will most probably behave as the input is 1, but it is NO guaranty for this. So you shall always tie unused inputs to either a logical 0, or logical 1 level. Depending on the gate function