G
G.T. W.
Guest
Hello,
I was attempting to compare input / output of an MC7414 chip on a pcb.
The board was in the device (centerfuge) operating under its own power
supply. The circuit board is on a floating ground, but 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 model I've never seen before, there was no instructions on
how to confirm its settings. Everything was set to 'auto'.
The first probe was set on 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 pulse. 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 scope insisted the input pulse to pin 1 was 174 volts (I looked
very hard for a decimal point, there wasn't one).
My two questions are:
1. What was I doing wrong?
2. What condition could cause a schmitt trigger to pass a signal
unchanged, instead of re-shaping it?
PS. I had the same result with every gate on the 7414. Thank you.
I was attempting to compare input / output of an MC7414 chip on a pcb.
The board was in the device (centerfuge) operating under its own power
supply. The circuit board is on a floating ground, but 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 model I've never seen before, there was no instructions on
how to confirm its settings. Everything was set to 'auto'.
The first probe was set on 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 pulse. 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 scope insisted the input pulse to pin 1 was 174 volts (I looked
very hard for a decimal point, there wasn't one).
My two questions are:
1. What was I doing wrong?
2. What condition could cause a schmitt trigger to pass a signal
unchanged, instead of re-shaping it?
PS. I had the same result with every gate on the 7414. Thank you.