[time-nuts] Isolated 1PPS-input for distribution-amp?

Attila Kinali attila at kinali.ch
Thu Aug 31 15:20:57 EDT 2017

Hoi Anders,

On Thu, 31 Aug 2017 15:12:00 +0300
Anders Wallin <anders.e.e.wallin at gmail.com> wrote:

> I'm looking for a solution for galvanic isolation of a 1PPS signal-input to
> a distribution amplifier.

What is the goal of the isolation? Breaking ground loops? Or safety?
How much isolation voltage do you need?
> We have an old box using HCPL-2411 at the input followed by 74LS04, but the
> degraded rise-time (from below 3ns out of a clock, to >12 ns out of the
> box) and increased jitter (from below TIC-resolution of 20ps out of the
> clock to 700ps std.dev at 100s out of the box) are now limiting our
> measurements.

Yeah.. These optocouplers are not ment for stuff like this.

> Does anyone have experience with a circuit with good rise-time and low
> jitter?
> Should I look for better optoisolators, or pulse-transformers, or something
> else?

My first pick would have been an ADuM, like the ADuM1280 or so.
They offer decent rise/fall times and I guess the delay variations
should not be too much. But they have a temperature coefficient for
the delay, so beware of that!

Another idea would be to use one of the LVDS isolators like
the ADN4650, which has a jitter spec of 4.8ps RMS max (random). 
You will need a TTL->LVDS and LVDS->TTL conversion, of course.
But the jitter added there should be small compared to the one
added by the isolator.
> PS. I started on publishing my distribution-amplifier (non isolated) on the
> ohwr.org site, see https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/wiki

Cool! Thanks a lot!

			Attila Kinali

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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