74HC14 schmitt inverter issue - Page 1
Forum section: electronics / beginners

author: psubond
author_rank: Contributor
timestamp: January 24, 2020, 03:14:50 pm
content: I am trying to get more familiar with using a 74HC14, as an experiment i have an ldr set up in a voltage divider configuration and the center tap goes to the input of the 74HC14 input 1. I have all of the other inputs tied to ground. everything is soldered together on a perf board. I have a .1uF, .01uF, .001uF, .0001uF cap on the power pin (probably overkill). Input voltage is 5v. The voltage divider and the IC are both powered off the same railThe IC triggers as expected when light is shined on/removed from LDR. Below is the waveform I am getting, it is very consistent from test to test. What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0. The rising edge has an overshoot to about 6v but is smoother. Any information/guidance would be appreciated.https://drive.google.com/open?id=1oG6Eh2Gwj9bf4IXGBJ9i5LoJoLvwybN6

author: wraper
author_rank: Unknown
timestamp: January 24, 2020, 03:25:49 pm
content: 1. Measuring with long ground connection. For best results you should use short ground and signal connection directly from IC.2. Probe compensation probably is not properly adjusted.

author: psubond
author_rank: Contributor
timestamp: January 24, 2020, 03:38:00 pm
content: Thanks, will give that a try!

author: mvs
author_rank: Frequent Contributor
timestamp: January 24, 2020, 03:44:01 pm
content: Quote from: psubond on January 24, 2020, 03:14:50 pm74HC14 edge is typ. 7ns fast (140MHz), you need a quite fast scope to see real signal.

author: iMo
author_rank: Super Contributor
timestamp: January 24, 2020, 04:03:10 pm
content: QuoteThe recommended ratio for the decoupling capacitors is 1:100 (for demanding applications).Like100u || 1u || 10n10u || 100n || 1n1u || 10n || 100pIn your case 2 capacitors would be ok.

author: psubond
author_rank: Contributor
timestamp: January 24, 2020, 08:20:13 pm
content: Quote from: mvs on January 24, 2020, 03:44:01 pmmy scope is a 200Mhz scope, I don't have a function generator that can push it that fast to see if it can actually do it though.

author: rstofer
author_rank: Super Contributor
timestamp: January 24, 2020, 08:38:45 pm
content: To see a 7 ns rise time requires a scope with around 50 MHz of bandwidth.B = 0.35 / trise= 0.35 / 7E-9 = 50,000,000 Hz = 50 MHz Not a lot, actuallyhttps://interferencetechnology.com/rise-time-vs-bandwidth-and-applications/You can build a somewhat reasonable square wave generator from the spare inverters:http://www.talkingelectronics.com/pay/BEC-2/Page49.htmlA 555 timers is probably a better source for a square wave.

author: mvs
author_rank: Frequent Contributor
timestamp: January 25, 2020, 11:49:17 am
content: Quote from: psubond on January 24, 2020, 08:20:13 pmBandwidth is just a metter of signal attenuation in analog part of a DSO. Sampling rate is a bit more important here.Let assume your DSO has 1GSpS (1 sample per 1ns). You will get less then 10 samples to reconstruct a signal from. Depending on interpolation function your scope is using, you may get ringing on the screen which is actually not in real signal.Dave has a vid on this topic.