[time-nuts] High rate, high precision/accuracy time interval counter methods

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed May 4 23:55:08 UTC 2016


On Wednesday, May 04, 2016 02:22:22 PM Gerhard Hoffmann wrote:
> Am 04.05.2016 um 10:46 schrieb Bruce Griffiths:
> > Integrating A Time interval to charge TAC at the front end of a capacitive
> > charge redistribution SAR ADC should allow a conversion time of 300ns or
> > so.. Using 16 such TDCs should permit 1ps resolution with a 50MHz
> > timestamp rate without too many cascaded gates in the selection logic for
> > the next available TAC. Bruce
> 
> One or two years ago I investigated a solution around a 16 Bit / 100
> MSPS ADC (LTC2165), a 2C64 Coolrunner,
> an Avago PHEMT as current switch and a little bit of analog voodoo. That
> would have fit on a 2"*2" board.
> Good enough for a 10 MHz event rate, with some easy pipelining for at
> least 20 MHz.
> That includes the coarse counter from the last 1pps.
> But we stayed with a classical time stretcher, and my private project
> pipeline is already full.
> 
> regards, Gerhard
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Yes, taking advantage of the fact that one merely uses the fine time interval 
measurement to measure the delay of a synchroniser clocked with the same clock 
as the ADC makes the design relatively simple. The output of the synchroniser 
samples a counter clocked with the same clock as the ADC to produce the fine 
count. The Time to amplitude converter output is merely held for 1 or 2 clock 
cycles so the ADC can sample the relevant part of the output. The TAC output 
is then reset to zero (or better the opposite limit of the ADC input). Either 
a buffer is used between the TAC and ADC or a direct connection should be 
feasible as long as the effect of sampling during ramping of the TaC output 
(TAC output capacitor is being charged) is corrected for. That is correcting 
for the charge transfer during this undesired sample. The sample taken when 
the TAC is in hold being corrected for the charge transfer incurred by the 
sample taken during ramping. Alternatively the TAC current could be used to 
drrive a network with a suitable impulse response so that no explicit reset is 
required. The output of this network being sampled by the ADC. The fine time 
interval can then be recovered by curve fitting to the samples taken by the 
ADC.

Bruce





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