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Using a HCF4049 to invert a signal, I simply fed the input to one of its not-gates and got its output. Here is the result:

Oscilloscope screen shot

The highlighted red lines is the output of the not-gate. (Yep! my analogue oscilloscope can display colors, it knows how to use my laptop ;D )

The output signal has a smaller peak-to-peak value! Both channels are set to 2 volt/div. So the output's peak-to-peak voltage is about 0.4 to 0.6 volts less than the input. Why?

I switched the probes and also the channels to check if there is any problem with the oscilloscope calibration. But got the same result.

If it is important, here is the original circuit (the link to schematics is in the middle of the page.)

This is my modified version. I have added A4, and the CH1 and CH2 are where the probes are.

My modified circuit

To check the oscilloscope accuracy, here the CH1 is connected to 5V and the CH2 is the output as before:

VCC on oscilloscope

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    \$\begingroup\$ What's connected to the output of A4 (READ)? If (a decent amount of) current is sunk, than that's the explanation. If not, note the datasheet (Electrical characteristics) only defines the minimum output \$V_{OH}\$, no typical value. So, deviations on typical values may be the explanation. Try the 2 other NOT gates and check if you see the same behaviour. \$\endgroup\$
    – Huisman
    Commented Apr 6, 2019 at 11:57
  • \$\begingroup\$ Try adding a 0.1 uF capacitor from Vcc to ground. \$\endgroup\$
    – Blair Fonville
    Commented Apr 6, 2019 at 13:01
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    \$\begingroup\$ A 741 op amp powered with +5V only? Unbelievable! That schematic appears to have multiple errors. What is the actual op amp part number? Can you show us the waveform at its output? \$\endgroup\$
    – Bruce Abbott
    Commented Apr 7, 2019 at 0:24
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    \$\begingroup\$ @Sohail The LEDs wont light up when they are conneted as shown in the schematics above. Did you swap them to the correct polarity and do you see LED2 burning sometimes? Did you add a resistor in series with LED2? If your answers are resp. yes, yes and no, then driving LED2 is likely the cause of the problem. \$\endgroup\$
    – Huisman
    Commented Apr 7, 2019 at 12:37
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    \$\begingroup\$ @Huisman Great! You are right! the LEDs didn't light up when I first assembled the circuit on bread-board. I simply thought it was my mistake and swapped them to match the GND and VCC, and they work. I didn't guess that maybe the schematic were wrong! I didn't add a resistor, and none of the LEDs have been burnt out yet. \$\endgroup\$
    – Sohail
    Commented Apr 9, 2019 at 10:15

1 Answer 1

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Yes, there's a tiny difference, but the point is, it's a difference that doesn't (or at least, shouldn't) matter in any way to whatever is receiving the signal. That's the whole point of digital signals — only two states matter, and as long as you can cleanly distinguish the two states, you're good to go.

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  • \$\begingroup\$ I think OP’s question is why does it deviate from the datasheet’s specs. \$\endgroup\$
    – Blair Fonville
    Commented Apr 6, 2019 at 13:04
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    \$\begingroup\$ @BlairFonville: But the point is, it doesn't! The datasheet only specifies a minimum level for \$V_{OH}\$ and a maximum level for \$V_{OL}\$, which are clearly being met. (Curiously, the TI datasheet doesn't specify the load current for these values.) \$\endgroup\$
    – Dave Tweed
    Commented Apr 6, 2019 at 13:41
  • \$\begingroup\$ But doesn’t the minimum level (4.95) say that the output should be that, or higher? Ex. this shows min. 4.95 V and typically 5 V. ti.com/lit/ds/symlink/cd4050b.pdf \$\endgroup\$
    – Blair Fonville
    Commented Apr 6, 2019 at 13:53
  • \$\begingroup\$ @DaveTweed Blue line peak-to-peak is 2.2div, so 4.4V. So, I wonder if the level for \$V_{OH}\$ is clearly met. \$\endgroup\$
    – Huisman
    Commented Apr 6, 2019 at 18:57
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    \$\begingroup\$ @DaveTweed No. I'm not. But would neither draw the conclusion (yet) the datasheet is met. \$\endgroup\$
    – Huisman
    Commented Apr 6, 2019 at 19:04

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