Reading datasheet of an Inverter swith Schmitt Trigger Inputs

[luckyvictor]

Hi All

I have a question regarding reading a datasheet of a 54LS14/DM74LS14 Hex Inverter swith Schmitt Trigger Inputs. www.ti.com/lit/gpn/54ls14

In the table of recommeded operating conditions, it says

VT+ Positive going input threshold voltage min = 1.5V nom = 1.6V Max = 2.0V
and
VT- Negative going input threshold voltage min = 0.6V nom = 0.8 Max = 1.1V

My question is, how do I understand these information? Does it means an input voltage above 1.5V is considered as a HIGH and will output LOW to me? a voltage below 1.1V is considered as a LOW and will output HIGH to me? what about an input voltage between 1.1V and 1.5V? what do I get in theory?

Also, I actually power up the chip, and the 'floating input' seems to be 1.1V, why is it please?

Many thanks

 

[betwixt]

There is a 'no mans land' region between the lower and upper thresholds in a Schmitt device. The threshold is adjusted according to the present logic output state so you need to go below the lower threshold to make the output high and it will stay that way until you go above the upper threshold. The same applies the other way around, you have to go beyond the other threshold for the output to change.

A non-Schmitt inverter would have an abrupt state change at a single threshold, above it produces low output and below it produces a high output, the threshold satays constant though.

This is exactly why Schmitt inputs are used, for example, if the threshold was 2V and the input was 1.99999V the output would theoretically be high and at 2.00001V it would be low but in real life the electrical noise would randomly make it change state. By having two thresholds, the noise immunity is greatly improved.

The 'floating input' voltage on a logic gate means nothing, it is just leakage through the internal circuits. Because it is an input, you have to actively provide it with a voltage or current source from outside.


Brian.

 

[luckyvictor]

There is a 'no mans land' region between the lower and upper thresholds in a Schmitt device. The threshold is adjusted according to the present logic output state so you need to go below the lower threshold to make the output high and it will stay that way until you go above the upper threshold. The same applies the other way around, you have to go beyond the other threshold for the output to change.

A non-Schmitt inverter would have an abrupt state change at a single threshold, above it produces low output and below it produces a high output, the threshold satays constant though.

This is exactly why Schmitt inputs are used, for example, if the threshold was 2V and the input was 1.99999V the output would theoretically be high and at 2.00001V it would be low but in real life the electrical noise would randomly make it change state. By having two thresholds, the noise immunity is greatly improved.

The 'floating input' voltage on a logic gate means nothing, it is just leakage through the internal circuits. Because it is an input, you have to actively provide it with a voltage or current source from outside.


Brian.

Many thanks.

So from what you say, I understand that the Schmitt input has two input thresholds, this can help to increase reliability because you don't want a signal that fluctuates below and above a single threshold and keeps changing the output.

Then, the question still holds, what do I expect to see at the output when the input voltage falls between these two threshold please?

And you mention that "The threshold is adjusted according to the present logic output state", do I can define the input threshold? how do I do it please?

Lastly, the reason I ask about the floating input voltage is because it seems to define that voltage node when its input is not made, e.g the switch is not closed to give a logic level, and this 1.1V seems to be able to cause another part of the circuit operates.

 

[Audioguru]

If the voltage of an input is between the Schmitt threshold levels then the output stays at the high or low it last was at.
74xxx and 74LSxxx are old fashioned TTL logic that have inputs with a bias current that pulls them high if they are floating. They require a fairly high current to ground to drive them low.

 

[betwixt]

And you mention that "The threshold is adjusted according to the present logic output state", do I can define the input threshold? how do I do it please?

In a Schmitt input device the thresholds are decided in manufacture. If you want to be able to control it, which is a common requirement, the solution is to use a comparator instead. A comparator compares two voltages (hence the name!) but by feeding some of the output voltage back to the reference input, the voltages it compares can be made different according to the present output state. By varying the amount of feedback, the 'hysteresis' (difference between thresholds) can be adjusted from nothing to any reasonable voltage level.

Lastly, the reason I ask about the floating input voltage is because it seems to define that voltage node when its input is not made, e.g the switch is not closed to give a logic level, and this 1.1V seems to be able to cause another part of the circuit operates.

TTL gates normally assume a high logic state if the input pin is disconnected but that comes from internal design, not because the pin actually has a measurably high voltage on it. You can't assume anything from an outside measurment and in the case of CMOS devices the measuring process alone may be enough to change the logic state. This is why it is considered bad practice to leave the input of any logic gate floating, they should always be tied high or low if they are not actively driven from a voltage source.

Brian.