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I have a PCB with voltage regulator (5V), op-amps, 2 multiplexers, 1 counter, several pots, caps, and resistors.

I supply 9V. The voltage regulator converts it to 5V. Without connecting any TTL signal through the function generator, I probe the input and output pins of the counter and I see a jump from 0V to about 2V in ~4ms. Then when I turn the power supply off, I see another jump from 0V to about 1.44V in the 10ms range.

I notice this behavior when I turn on and off the power supply. I removed the counter and op-amps, and I still see the spike in voltage but in the ps range and only when I turn the power supply off.

There is an overshoot from the power supply where it jumps to about 10V then back to 9V. There is also a tiny transient voltage but it is very small and I have the necessary passive components at the voltage regulator to control this.

I do not know what is causing this issue. Tried different things, I breadboarded the voltage regulator by itself and I do not see any issues at the 5V output, I added just the counter, and I saw this issue, it seems like whenever an active component or IC is added this issue occurs. Can this be a PCB manufacturing issue? Any suggestions?

I see, I was not clear enough.Lets forget what I said in the block above. I made something to make this clear, see below. I also wrote my questions there.

I kept the recommended design set up for the regulator and I'm only powering the counter as seen below. I'm not providing any input signal or anything else. All of this is done on a breadboard. I have a more complex circuit on a PCB but I still see similar issues. That jump at the output pins is an issue for me.

Data input is grounded on the counter.

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  • \$\begingroup\$ Does the datasheet for the regulator indicate that this is unusual behavior? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 21, 2018 at 22:49
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    \$\begingroup\$ thanks for the scope capture. Maybe a quick schematic of your circuit would make it easier/faster to understand your question. Actually I read through your question 3 times and I am not sure I can give you a clear answer. One thought: if multiple, not directly connected signals change simultaneously, it might be an issue with your ground. \$\endgroup\$
    – MAB
    Jun 21, 2018 at 22:57
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    \$\begingroup\$ Can you show a photo of layout with schematic and PSU & Regulator links to datasheet? Is this just a lab experiment? \$\endgroup\$
    – Tony Stewart EE75
    Jun 21, 2018 at 23:08
  • \$\begingroup\$ @Ignacio Vazquez-Abrams they suggest to use caps to smooth signal, but nothing out of the ordinary. \$\endgroup\$
    – Fugy
    Jun 22, 2018 at 16:39
  • \$\begingroup\$ @MAB I have the schematic, any help is appreciated. \$\endgroup\$
    – Fugy
    Jun 22, 2018 at 16:40

1 Answer 1

Reset to default
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Try tying the MR input high. If the effect goes away, attach a decent reset chip to the MR input.

For example, an ADM803 (of appropriate trip point for your actual supply voltage). It is guaranteed to assert reset down to 1V or so.

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    \$\begingroup\$ Thanks for the help. Setting the MR input high works. This actually helped me figure out a design flaw in the overall PCB I was working with. A circuit was suppose to set the MR high for some miliseconds to avoid the jump and therefore avoid a false positive signal. The problem is that a MOSFET was missing. So the MR was always low. I did not have to use ADM803, I just needed to latch the MR high enough for sometime. \$\endgroup\$
    – Fugy
    Jun 22, 2018 at 21:46

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