Electronics > Beginners
Schmitt disappointment
PerranOak:
I have a Schmitt trigger (74HC14) with an input of: square wave, 1kHz, 5Vpp, 2.5V offset.
It triggers as expected but the overshoot is nearly 2V.
OK, I have it all in a breadboard with unshielded jumpers but 2V!
Is this normal?
langwadt:
--- Quote from: PerranOak on October 01, 2018, 04:57:24 pm ---I have a Schmitt trigger (74HC14) with an input of: square wave, 1kHz, 5Vpp, 2.5V offset.
It triggers as expected but the overshoot is nearly 2V.
OK, I have it all in a breadboard with unshielded jumpers but 2V!
Is this normal?
--- End quote ---
how are you measuring it?
schmitt trigger:
Without you providing any additional details, and without seeing your actual circuit or layout; I would say that the overshoot is caused by:
* poor grounding and supply routing
* lack of adequate capacitor decoupling
* attaching the scope probe to an incorrect ground location
I mean, it is after all, only 1Khz.
To a logic circuit, any logic type, this is glacial speed.
PerranOak:
Thank you both.
I am looking at the input and output traces on a scope.
It is the most simple layout. One trigger has the sig gen input (scope channel 1) with a 1K resistor to ground on the output (scope channel 2). All other inputs are to Vdd. 5V battery pack as power with 100uF cap across it. 100nF cap across Vdd to ground - can't get is very close as Vdd and ground are at opposite ends of the chip and I keep shorting things if I try to connect the cap close to them!
It is a rats' nest of jumpers and the (10x) scope leads and sig gen leads all terminate in jumpers into the breadboard.
No matter what frequency (1kHz to 10MHz) the overshoot is remarkably stable!
Cheers.
schmitt trigger:
Scope photos?
Breadboard photos?
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