# 74HCT14 schmitt trigger doesn't improve rise/fall times

I have a square wave signal whose rise and fall times are too big to feed into another 74xx-series logic chip, such as the 74HC175 flip-flop, which requires rise and fall times in the nanosecond range.

I've tried feeding the square wave signal through a 74HCT14 schmitt trigger inverter, like this:

I've built the circuit on a bread board with components that were just lying around - so it's a bit messy:

Now, rise and fall times doesn't seem to improve when i feed the signal through the schmitt trigger (yellow: input, blue: output):

As can be seen, rise and fall times are roughly the same. Also, I find it a bit strange that in the two last pictures the output signal begins transitioning before the input signal has passed Vt+.

The data sheet can be found here: http://www.ti.com/lit/ds/symlink/sn74hc14.pdf

• Add 0.1uF ceramic capacitor as close as possible to the IC. Solderless breadboard prototypes suffer high inductance. For any fast switching part, anything more than about 5-10mm away "doesn't exist" as far as high frequency current is concerned. – MarkU May 27 '14 at 1:49
• Why do you have a pulldown on the input? – Ignacio Vazquez-Abrams May 27 '14 at 1:49
• Also, 1000uF is not a decoupling capacitor. – Ignacio Vazquez-Abrams May 27 '14 at 1:52
• Sorry, I forgot to mention: the input signal is connected to Vcc by a mechanical switch periodically. When it is not connected to Vcc, it must be 0V, hence the pull-down. – Troels Folke May 27 '14 at 1:56
• What is wrong with using a 1000uF capacitor as decoupling? I know big electrolytic caps have more stray inductance compared to ceramic ones, but I don't seem to have a lot of high-frequency noise on Vcc - especially not after connecting all the other inverter inputs to GND. – Troels Folke May 27 '14 at 2:02

It looks like you've got the oscilloscope displaying some uber-expanded portion of a capture- it's only changing slope every two horizontal divisions .. so anything faster than that all looks the same.

The first photo shows it changing slope every division, rather than two divisions in the bottom two photos, but in each case it's 20$\mu s$ per division, which implies your effective sample rate is only 50ksps, as @RJR comments.

If you want to see 74HC rise and fall times, 1Gsps would be more appropriate.

• Good point. It seems it only samples once every division - 20us which translates to 50KHz??? – RJR May 27 '14 at 3:01
• Thanks, I was not aware of my scope's sampling rate. – Troels Folke May 28 '14 at 13:56
• Your scope isn't 50KHz, it is 50MHz and should work fine for viewing this. Are you using triggering, or are you manually stopping it and zooming in? Zooming in doesn't work, you don't get as much resolution. Setup a trigger. – THEMuffinMan7 May 30 '14 at 5:37

I've used the same chip before with no issue. It definitely transitioned in the ns range, maybe about 10ns or less. Mine was wired up like this, but was sort of used as a pulse generator with the cap charging/discharging. The input goes into U8 there and the output is inverting like yours. Hope this helps.

Pretty sure we have the same scope

Edit: Use triggering on you're scope if your not! Don't just stop the waveform and zoom in - it doesn't work! I did that, took me days to figure it out.

• This was surface mounted btw – THEMuffinMan7 May 27 '14 at 2:27