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

Gerhard Hoffmann dk4xp at arcor.de
Tue May 10 20:18:58 EDT 2016


Am 09.05.2016 um 10:08 schrieb Magnus Danielson:
> Hi,
>
> On 05/08/2016 09:53 PM, Attila Kinali wrote:
>>
>> True and not true. Yes, there are many ADCs that do high conversion
>> rates, but these are optimized for piplined applications where 
>> conversion
>> happens at a constant rate. Ie they expect a constant conversion clock
>> with a constant rate. If you want to trigger conversion at an 
>> arbitrary time,
>> you either have to build your own sampler or need to  use one of the
>> non-pipelined ADCs whic are much slower (IIRC they stop around 5-10Msps
>> aka >100ns conversion time). Flash ADCs with direct access to the 
>> sampling
>> circuitry are basically extinct.
>
> You can let the ADC convert as a continuous process as long as you 
> filter out the samples you are interested in.
>

And, at least for the LTC2165, you could really use the encode clock to 
read the result
of the capacitor immediately. All one would have to do is to flush the 
pipeline with
5 or 6 clocks after the encode, with a cycle time not much faster than 
10 ns and
not slower than 1 usec.
That seems not too hard to do but costs a differential mux in the encode 
lines.


We then would not need to switch the current source, it could degenerate 
to a
20V source with a resistor & safety clamps; that would be as linear as 
it gets.
The reset level could be clamped by a Sky or Avago phemt, they can 
swallow the
current easily, are blinding fast and do not drop a voltage for the base 
current
over Re (from Gummel-Poon model). And an inverse transistor may have less
Re, but also less beta and therefore more base current.


I have already built a time stretcher and was restricted to spaceworthy 
components;
those Intersil transistor arrays had the only acceptable PNPs. These 
flatpacks are
huge, and the legs are endless; anything more complicated than a simple
current mirror would oscillate or would at least lurch to its 
destination, and you
cannot simply damp it to death. Not funny.


Minimizing the E-E-inductance: The next best thing to an onchip-connection
might be abusing 2 Infineon SiGETs on the 2 sides of a thin board; they 
are optimized
for low emitter inductance and have 2 emitter legs each. And their Early 
voltage
is huge, so they are not impressed by the VCE changes of the current source.
(SiGe BFP650, 750 etc, available in a tiny package)

regards, Gerhard




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