[time-nuts] Hints on PPS Buffer design...
Graham / KE9H
ke9h.graham at gmail.com
Sat Jun 17 08:43:26 EDT 2017
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 <hmurray at megapathdsl.net> wrote:
>
> cautery at montac.com 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.
>
>
> cautery at montac.com 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.
>
>
>
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