| Electronics > Beginners |
| Schmitt disappointment |
| << < (4/5) > >> |
| Zero999:
It's the physical layout of the circuit which is the problem. The long leads have inductance, which resonates with the parasitic capacitances, generating relatively high voltages. Build the circuit on a strip-board with as shorter connections as possible, the supply decoupling capacitor right next to the supply pins and use the proper probing technique described above and it will be much better. |
| sdouble:
yep, a breadboard like taht one is not suitable for relatively high frequency circuitry. I can't see low ESR/ESL caps close to the chip. Are they any ? |
| abraxa:
--- Quote from: PerranOak on October 03, 2018, 05:22:00 pm ---So, this is a common problem with fast switching things is it? Is it only an artifact of the scope rather than a "problem" with the circuit? --- End quote --- Yes, all switching things - be it due to fast switching or a high signal frequency (which naturally requires short fall/rise times, i.e. fast switching). The ringing is a very real property of your circuit, the scope's inadequate probing merely exaggerates the effect. |
| Zero999:
A simple fix would be to swap the 74HC14 with the HEF/CD40106B, which is much slower and won't produce as much ringing on the output. The most optimal solution is to use as smaller package as possible, so the 74HC1G14 if only one Schmitt inverter is required. Build it on a small PCB with a ground plane and a small packaged capacitor near the power supply pins. I was going to suggest a low-Z probe, but that would load the circuit down too much, as the 74HC14 can't drive a 500R load. You could make your own probe with a 5k input impedance, but it will be a x100, rather than 10 probe i.e. the output voltage will be 1/100 of the input voltage, but modern 'scopes can be set to compensate for that. |
| schmitt trigger:
--- Quote from: abraxa on October 01, 2018, 06:57:15 pm --- --- Quote from: schmitt trigger on October 01, 2018, 05:18:43 pm ---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. --- End quote --- While I agree with your assessment of the situation at hand, I'd just like to point out that a squave wave frequency of 1 kHz can still have tons of high-frequency components if the rise and fall times of the edges are short. I'd say that this is the case here. --- End quote --- Thanks for pointing this out. :-+ The repetition rate has nothing to do with the rise/fall times. But my basic suggestion still apply. |
| Navigation |
| Message Index |
| Next page |
| Previous page |