Schmitt trigger question

[G.T. W.]

Hello

I'm trying to figure what I am doing wrong when comparing the
input/output of a schmitt trigger in an energized circuit.

This is a MC7414 chip on a pcb, in a centerfuge operating under its own
power supply. The circuit board is on a floating ground, I measured
5.1 Volts (RMS) between pins 14 and 7 with a Fluke meter.

Unfortuneately, the only o'scope available to compare input/output was
a digital unit, first one I've seen. Everything was set to 'auto',
there was no manual on how to confirm its settings.

I set the first probe to the 7414, pin 1(input). A positive pulse was
seen there. A second probe was put on the output (pin 2) to observe
the re-shaped output. There was no wave-shaping, no squared pulse.
Both traces appeared identical, a positive-going pulse, rounded at the
half-power points. Further, the output was not inverted.

Due to my ignorance with a digital o'scope, I can not say what the true
pulse amplitude was. Using pin 7 as a ground reference for the probe,
the digital readout indicated pin 1 was 174 volts (I looked
very hard for a decimal point, there wasn't one).

My questions is.

What condition could cause a schmitt trigger to pass a signal
unchanged, instead of re-shaping it? If the input pulse was, for
example, 1.74 volts, and the schmitt trigger won't go high until 1.8
volts, does it pass the signal unchanged? I had the same result with
each gate on this chip. Thank you.

 

[JeffM]

What condition could cause a schmitt trigger
to pass a signal unchanged, instead of re-shaping it?
G.T. W. rampart@easynews.com

Not possible (risetime, propagation delay, etc),
but the best approximation would be:
The input low level is the same as Vee;
The input high level is the same as the rail (minus Vsat);
and the edges of the input are very fast.

If the input pulse was... 1.74 volts,
and the schmitt trigger won't go high until 1.8 volts,
does it pass the signal unchanged?

If it doesn't reach the threshold, the output doesn't change at all.



2 WAYS OF SHOWING SCHMITT TRIGGER HYSTERESIS

HYSTERESIS LOOP
*************** Turn-off threshold reached (6)
|
|
v <--- Input voltage decreasing
(5)
------<-----======
/| | ---> Input voltage
/ | | continues to increase
/ | | with no change
Output drops low (7) | | in output (4)
| |
| | Output jumps high (3)
| | /
Input voltage continues to decrease <--- | | /
with no change in output (8) =======-------->---

0V 2V 4V 5V
Input voltage increasing (1) --------------------------->
^
|
Turn-on threshold reached (2)



WAVEFORMS
*********

Output
_________________ ........... 5V
| |
| |
| |
| |
_______| |________ ..... 0V
. .
. .
. .
. .
. .
. /\ .
. / \ . Turn-on threshold 4V
......../\..../....\.............
/ \ / \ .
/ \/ \ .
/ \ . Turn-off threshold 2V
..../..................\.........
/ \
/ \
/ \
__/ \____ ..... 0V

Input


After it trips, it doesn't matter that the input voltage continues to increase.
The output stays high until the input reaches the turn-off threshold.
The noise on the input (between the 2 thresholds) does not affect the output.