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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