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

Bob kb8tq kb8tq at n1k.org
Mon Mar 27 21:48:51 EDT 2017


Eventually you will want multiple inputs to any chip like this. The more I/o the more crosstalk.
Properly done, differential inputs will reduce your crosstalk quite a bit. You might as well debug that part of it now.


> On Mar 27, 2017, at 9:05 AM, Attila Kinali <attila at kinali.ch> wrote:
> Hi,
> We (the group I am with and a group at TU Vienna) are currently designing
> an ASIC (digital 65nm process) that will contain a TDC part. The TDC will
> be a simple delay line TDC design using differential buffers, which we
> expect to give us something in the order of 20ps of resolution (hopefully
> better, but we will not know until we get post-layout simulation data).
> We are loosely following the design CERN came up with for their new TDC chip[1].
> Now, the TDC expects a differential input, but the system gets single-ended
> pulses as input (50R coax input, level likely to be CMOS 3.3V, but level not
> fixed yet, ie can be freely choosen). I can either convert these single-ended
> signals into differential off-chip or on-chip. Unfortunately, I lack knowledge
> and experience to judge either approach. The issues I see are:
> * Single-ended input in a chip might lead to shifting ground potential
>  on the chip and thus to measurment jitter.
> * There are different architectures to preform the single-ended to differential
>  conversion on-chip, but I have no clue which one to choose or even how
>  to judge them without extensive simulations for which we do not have the
>  time, know-how and probably not even the tools.
> * Conversion to differential off-chip means another component off-chip
>  that might introduce additional delay uncertainty (our application is
>  very sensitive to that) and an unknown amount of jitter.
> My google foo has been so far not strong enough to find answers to these
> questions.
> I would appreciate if someone could give me some hints in this matter
> or tell me where I could find appropriate literature and maybe even
> tell me whether I am missing anything.
> Thanks in advance
> 				Attila Kinali
> [1] https://indico.cern.ch/event/228972/contributions/1539621/attachments/378552/526492/TDC_TWEPP_2013.pdf 
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