[time-nuts] Experience with THS788 from TI?

Attila Kinali attila at kinali.ch
Thu Mar 22 09:51:09 UTC 2012

On Wed, 21 Mar 2012 12:18:21 -0400
Ben Gamari <bgamari at physics.umass.edu> wrote:

> On Wed, 21 Mar 2012 09:44:14 -0500, David <davidwhess at gmail.com> wrote:
> > I am surprised it is not more accurate and precise.  Even old discrete
> > designs can get down to 10ps or better.  I wonder what market it is
> > for where space is at that much of a premium.
> > 
> Out of curiosity, would you happen to have an example of discrete TDC
> design? Recently I've been exploring the TDC design space as these
> devices are a critical part of our experiments (I do spectroscopy of
> biological molecules). I'm currently (slowly) working on a FPGA TDC
> design (based on the PandaDAQ[1] and CERN's Spartan 6 TDC design) but it
> seems it will be non-trivial to get down to the 12 ns the commercial
> offerings provide (although at great cost). What would a discrete TDC
> design look like?  Are there any designs in the open?

There are multiple designs out there, some of which have been documented
openly (in manuals, like the SR620 which is available from Didiers site)
others are only known from papers. In general, i'd say if you need more
documentation for a design done by a research group, you should be able
to get help from them directly. Most people i know that work in research
are more than happy to share their tools with others.

If you are really going to build your own design, then i suggest you
read these papers:

"Time Interval Measurement Literature Review" by... uh.. dont know
Gives you an easy overview of different methods of time interval measurement
and how they work.

"Review of methods for time interval measurements with picosecond resolution"
by Jozef Kalisz, 2003, http://ztc.wel.wat.edu.pl/kalisz/met4_1_004.pdf
A very detailed, but broad overview of the methods that are used in todays
TIM/TDC applications.

"Time-to-digital Converters" by Stephan Henzler, 2010
DOI: 10.1007/978-90-481-8628-0
A more theoretic overview of TDCs. Nice if you need more math. But if
your library doesn't have access to the books from Springer, i wouldn't buy
it (you probably do not need all that much of math).

"Error analysis and design of the Nutt time-interval digitiser with picosecond
resoulution", by Kalisz, Pawlovski, Pelka, 1987 give you a much better
treatment of how errors occur in TDCs and how to mathematically treat them.

"An FPGA Wave Union TDC for Time-of-Flight Applications", by Jinyuan Wu, 2009
"The 10ps Wavelet TDC: Improving FPGA TDC Resolution beyond its Cell Delay",
by Wu and Shi, http://www-ppd.fnal.gov/EEDOffice-W/Projects/ckm/comadc/WaveletTDC_abs08.pdf
"A 20ps Resolution Wave Union FPGA TDC with On-Chip Real Time Correction",
by Qi, Deng, Gong and Liu, 2010
The wave union TDC is a quite interesting design that allows to get a quite
high resolution with an FPGA only implementation. But this design depends
highly on good placement, stable enviorment conditions (temperature, supply
voltage) and permanent re-calibration (which in turn needs uncorrelated time

"Time-Interval Measurements Based on SAW Filter Excitation", by Petr Panek, 2007
"Time interval measurement device based on surface acoustic wave filter
excitation, providing 1ps precision and stability", by Panek and Prochazka, 2007
"Random Erros in Time Interval Measurement Based on SAW Filter Excitation",
by Petr Panek, 2008
A very nice idea on how to use a high frequency startable oscillator with
an ADC as phase detector. Panek claims to get below 1ps with a 200MHz clock
and a 525MHz filter/oscillator. His calculations indicate that the ultimate
limit of resolution is given by the sampling jitter of the ADC and the
frequency and bandwidth (ie the Q of the oscillator). There 

If you search the IEEE archvies, you will find many more examples on
how to build a TDC. Some more, some less documented. Papers by Kalisz and
Pelka are always a good read. A good, and easy to understand example of
an TDC with a Nutt interpolator is the PICTIC
which gives you already 680ps (RMS) with its very simple design.
I guess, you can get to <50ps without too much effort using a 200MHz
oscillator and ECL devices. But getting to below that will not be
easy. Mainly due to all those side effect, non-idealities and other
stuff you have to deal with. And be aware, that you are dealing with
an high frequncy/high speed circuit. Crudly said, you are in the ballpark
of a 1/10ps = 100GHz system. Everything has to be right to get you there.


			Attila Kinali

The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin

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