[time-nuts] Single ended or differential input to TDC chip

Attila Kinali attila at kinali.ch
Tue Mar 28 10:16:44 EDT 2017


I do understand the advantages of having differential input.
But my problem is that we are really sensitive to delay differences
between channels. And I mean _really_ sensitive.

The TDCs will have a total usefull range of about 100-200ps.
The input pulses will arrive within this time frame. There is
some provision in there to calibrate delay differnces between
different paths, but it's not much. If there is an electronic
component in the path, that has a delay variation between
different devices of 1ns (like the ADN4661 that Bruce recommended),
this would mean we would need somehow extend our measurement range
to at least 2ns to be able to compensate for this delay variations.
This >10 fold increase in measurement range means that we need more
circuitry after the TDC, ie the digital circuitry would increase
by more than a factor of 40 (grows with approximately n+n*log(n))
and that will cause us all kinds of other headaches.

This sensitivity to delay variations also means that we have to
minimize the number of components in the signal path. Hence things
like having the option of selecting single-ended or differential is
would degrade our signal path too much. The multiplexer alone would
add too much delay variation.

As the signal is distributed to multiple boards and the number
of connections we need for that is already large enough, i want
to keep the signal going between the boards single-ended. This
also makes it easier for us to find cables that are withing
a few ps (a non-representative sampling of cables of same type
i had at hand gave me <30ps variation between cables)

On Tue, 28 Mar 2017 02:00:11 +0200
Gerhard Hoffmann <dk4xp at arcor.de> wrote:

> Somewhere you'll have to produce sharp differential edges and this
> semi-analogue stuff is probably hard on a chip shared with thousands of
> other gates and a digital process. Going differentially into the chip will
> reduce the effects of ground bounce etc a lot.

It's a full custom ASIC, with only the circuitry to support the TDC
around it. The digital logic after the TDC are only there to
convert the thermometer code we get from the TDC into binary.
There is also a bit of arming logic and various other small bits, but
nothing substantial. We are also going to split the power supplies
on the chip for the TDC and the digital part, to isolate them even
further from eachother.

> This is what you can expect from an ADCMP580 on a homebrew
> board and soldered-in semi rigid cable:

I was actually thinking about the ADCMP572. The datasheet unfortunately
does not specify a variation of delay between devices, not even a maximum
propagation delay. Guessing from the typical propagation delay of 150ps,
the 35ps rise/fall time and the 10ps deterministic jitter, it is probably
safe to assume that the device to device variation is in the order of 20
to 100ps. This means, in best case this device will work, in worst case
we would need to enlarge our measurement range to at least 400ps.

I am now trying to figure out whether we have any other posibilites
to get path delay variations down.

> Some will cry: eeek - a comparator, and an ECL or CML one at that, but 
> in the end
> it is just a differential amplifier like that Wenzel design, made by 
> people who
> know their SiGe process.

I've seen worse :-)
We are currently designing CML in a pure digital, Si 65nm process
(nothing fancy, not even SOI). So, having a CML chip as input source
isn't going to kill us, quite to the contrary.

> If you have a slowly rising source, you'll need multi-stage slope 
> amplification
> and low pass filtering in Collins style anyway which will be safer to do 
> off-chip.

Slope is reasonably fast. I was thinking about using an LMK00101 into
an 50R load (~10cm of coax). This should provide us with <500ps of
rise/fall time.
> If you can specify the interface, go for something speedy, then
> the guys who do the the signal source have the hard time and
> can be pointed at.  :-))

The "guys who do the signal source", that's me :-)
As I am the only one in the group with significant experience
in both analog and digital electronics, I have to not only
come up with a feasible overall design, but also figure out
solution for all these pesky details.

			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

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