[time-nuts] Experience with THS788 from TI?

[Tristan Steele]

Hi,

The NIM Negative pulse is indeed problematic when trying to interface to
FPGA logic!

The solution I have used in the past is a board based around a Maxim
MAX9601[1] acting as a comparator on the incoming NIIM pulse, and
configuring the output for connection to the FPGA.  Judging by the results
that I have seen from this chip, the timing uncertainty is certainly not
single figure picoseconds, but in the range of 30ps - not great, but still
better then nanoseconds.  If the input to this signal is a NIM timing
pulse, there are minimal issues with timing walk as the pulse height is
always the same.

It's certainly a lot easier then building a CFD!

Tristan

[1]http://www.maxim-ic.com/datasheet/index.mvp/id/3400

On 22 March 2012 08:58, Ben Gamari <bgamari at physics.umass.edu> wrote:

> Bob Camp <lists at rtty.us> writes:
>
> > Hi
> >
> > Ok, for a legit 12 ps with 0.1 ps drift and 200 mega samples per second -
> > not to many alternatives. The FPGA stuff will get you to 50 to 100 ps on
> the
> > same basis this gets you to 12 ps. They will get you to 20 to 40 ps on a
> > good day - sort of the way this chip gets 8 ps. The FPGA will do it at a
> > much lower data rate.
> >
> In our experiments, we are typically observing very low count rates
> (100kHz at absolute most). I've occassionally stumbled upon a paper
> which claims to get 10ps on a standard FPGA, but naturally they never
> show the code. Given that I'm a relative novice at high-speed
> electronics and FPGAing in general, I'll consider myself lucky if I get
> the 50ps advertised by the CERN core.
>
> In particular, one issue I've been struggling with is the
> discriminator. Our fast detectors produce a NIM negative-current pulse
> which will ultimately need to become suitable input for the FPGA. Of
> course, the most precise time measurement in the world is useless if the
> discriminator front-end has a nanosecond jitter. Unfortunately, I have
> yet to find any open, high precision discriminator designs. In principle
> a constant fraction discriminator doesn't seem to difficult to
> implement, but when it comes to preserving the high-speed signal
> integrity, it seems like it could get pretty hairy. Comments?
>
> > If you average over many samples, all of these will get you a better
> > estimate. How much better depends on a bunch of things. The TI part
> *could*
> > do very well if you have a 200 MHz signal to look at.
> >
> For time-correlated single photon counting (our primary use for
> precision timing), having high temporal resolution is quite
> important. That being said, all of those arrival times all get combined
> into a correlation function so shot-per-shot jitter will be in large
> part averaged out.
>
> Cheers,
>
> - Ben
>
>
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