Read the spec sheet on the part you are using for a driver. The size of the resistor on the paralleled driver side will be set by the maximum current of the devices.
Decide how conservative a design you want. Do you want it to drive into a shorted load and survive? [If each output is rated at 20 mA, in a 5 Volt system, then R=E/I, = 250 Ohms] Or do you only want it to work into a 50 Ohm load (to ground)? Or perhaps only a Thevenin load? 50 Ohm load, but 100 Ohms up to +V, and 100 Ohms to ground. Remember that 50 Ohms in a 5 Volt system will draw 100 mA on the 'High' which can be a lot of current for a modern IC. --- Graham == On Sat, Jun 17, 2017 at 4:01 AM, Hal Murray <[email protected]> wrote: > > [email protected] said: > > Q3: It's only a 1Hz frequency, but is low inductance a desired trait of > the > > chosen resistors? > > It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time. > If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1 > GHz. So, yes, you want low inductance. That includes the power to the > chip > as well as the resistors. Surface mount is your friend. So are > ground/power > planes. > > How good is your scope? > > > > Q: Why does everyone pick FIVE x 100 Ohm resistors? That's 20 Ohm out, > not > > counting the gate impedance on the hex inverter... > > The FIVE is 6-1. The one is for isolation. The 5 is the rest of the > package. You might as well use them all as drivers. You don't want to use > them for another signal (even if it is supposed to be identical) or you > will > get minor crosstalk when you do things like plug or unplug a cable. > > I don't know why you are saying "gate impedance". That's over on the input > side. I would have said "output impedance" or "driver impedance" of the > chip > or section. > > I'm not sure why they picked 100 ohms. Assume the net source impedance is > 25 > ohms. Suppose the far end is terminated with 50 ohms. There won't be any > reflections so the source value doesn't matter. 25 ohms will provide a > higher voltage at the far end than 50. If you have a CMOS driver and a > CMOS > receiver, 1/2 the voltage at the receiver is nasty. It might be OK if you > have HT type receivers. > > I would suggest a bit of lab work. What are you going to use on the far > end? > > Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance > takes > care of the reflections and leaves (almost) the full voltage at the > receiver. > > > [email protected] said: > > thus I can only include that I need to use something slightly more than > 250 > > Ohms on a 5 gate parallel setup) > > More than 250 divided by 5 and rounded up a bit for the output impedance of > the chip will be more than 50 ohms. I'd do some experiments. > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > 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.
