I am trying to get a 74HC14 hex inverting schmitt trigger to work. First thing I did was connect it to a power supply.

When I connect the Vin and Ground ports to my bench power supply (5V), some noise is added on the 5V signal. See the figures. The first figure just shows the 5V from the power supply without the schmitt trigger connected. The second pictures shows the situation after connecting.

I tried a few different ones, but they all give the same result

Am i doing something wrong here. I intend to use the trigger to clear up noise from a signal, not to add noise.

Any idea is appreciated.

Situation without connection enter image description here


You have to tie the inputs of unused MOS gates to ground or Vdd (recommended) or they will pickup random noise and amplify it. It the case of Schmitt triggers, this may cause them to flip the whole range with their full speed.

Also, you should add a 100nF ceramic decoupling cap right at the chip to care for that switching noise, which you cannot avoid during normal operation anyways.

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  • \$\begingroup\$ Thanks a lot! I connected all unused inputs and connected to Vdd using a 10k resistor. That seemed to do the trick. What do you mean with connecting the 100nF cap at the chip; you mean between the signal lines? \$\endgroup\$ – roadrunner38 May 17 '19 at 9:27
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    \$\begingroup\$ No, between Vdd and GND. Your power supply lines have an inductance. That means the power supply cannot deliver power as promptly as needed when an output flips. The cap fills that gap in supplied power. \$\endgroup\$ – Janka May 17 '19 at 9:54
  • \$\begingroup\$ You need to connect that 0.1uF within 2mm of the IC. If you want best results, do not use a proto-board nor a white-board. Instead build the circuit on a 2" by 2" square of copper ground plane, with twisted pairs bringing for the INPUT signal and OUTPUT signal. \$\endgroup\$ – analogsystemsrf May 17 '19 at 16:15
  • \$\begingroup\$ Thanks for the comments. Things work much better now \$\endgroup\$ – roadrunner38 May 19 '19 at 13:56

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