Hi Well, based on about a hundred breadboards, it always comes out this way regardless of who does the layout or what caps are used for a buffer amp. Consider that this is *not* an oscillator, so the only cap that is sure to be there is a blocking cap.
Bob > On Apr 26, 2016, at 5:22 AM, Attila Kinali <[email protected]> wrote: > > On Mon, 25 Apr 2016 20:45:53 -0400 > Bob Camp <[email protected]> wrote: > >>> To get to CMOS levels, you usually add a simple inverter, with an >>> capacitor in front and a 1M resistor across the inverter (from input >>> to output). >> >> If you set up a modern (74AC or faster) inverter with a resistor from input >> to >> output, you are very likely to get it running as an oscillator all on it’s >> own. That >> oscillation may or may not add to the desired output. >> >> Stick with a two resistor bias on the input … it’s a lot safer. > > Not really. In order to prevent oscillation you have to ensure > that the lowpass filter formed by the input capacitor and the > bridging resistor has a low corner frequency. Putting the corner > frequency in the range of 10kHz-1MHz is usually good enough, the lower > the better. Though there are of course limits to this: > > * from a certain point on, the surface resistance of the PCB > dominates the resistor. A normal FR4+solder stop board has a > surface resistivity of 1M-10G depending on the actual solder stop, > humidity and dirt/grease on the PCB and the geometry of the wires. > (I usually go with 10M for close wires when i'm too lazy to calculate) > > * The larger the capacitor the lower its self-resonance frequency > becomes. Ie from that poin on the capcitor behaves more like an > inductor than a capacitor. Rule of thumb: a 4.7uF 0603 X5R has a > self resonance frequency around 1-3MHz > > For my squaring gates, I usually use 100nF+1M (~100kHz) on a > single gate inverter (usually LVC, but not always) and have > not seen any oscillation yet. > > > BTW: Does someone know about actual measurements of the small > signal transfer characteristics of a single CMOS gate? > It would be nice to put the above rule of thumb onto a more > solid foundation. > > Attila Kinali > > -- > It is upon moral qualities that a society is ultimately founded. All > the prosperity and technological sophistication in the world is of no > use without that foundation. > -- Miss Matheson, The Diamond Age, Neil Stephenson > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
