i want to capture an exact point of the output waveform of a fast Track-and-Hold stage, which hast to happen within a few nanoseconds. Skew alignment because of different aperture delays between T&H/ADC is done via delay lines and should be no problem at all.
The instant of time when the sample has to be taken is not corresponding to a clock or something else predictable. It is generated by a Trigger circuit and can come at any time.
My conclusion is: I need an ADC which starts conversion immediately after a clock edge is presented - at an arbitrary time point.
- ~12 bit resolution
- fast acquisition time < ~2ns, this corresponds to a high input bandwidth resp. low aperture jitter
- preferably a low aperture delay
- Flash converters are not available with the needed resolution
- SAR ADCs are way too slow
There are pipelined ADCs that satisfy the requirements - except of the clocking aspect. Clock has to run all the time(?).
The AD9238, for example, has a latency of 7 cycles, a minimum clocking frequency of 1 MHz, requires a duty cycle of +-50% and duty cycle stabilizer can be deactivated.
In this case i would deactivate the duty cycle stabilizer and leave the clock low until the trigger fires, so that the "Single Shot" is taken. After 7 cycles with proper frequency and duty cycle, the correct digital value should appear at the output.
The datasheet tells nothing about such an application, it tells nothing about the state in which the ADC is with clock turned off.
Information about single shot in connection with pipelined ADCs is rare; AD's "The Data Conversion Handbook", viewable in Google Books, says (p.399, bottom): "Although most pipelined ADCs cannot be directly operated in the single-shot or burst mode, they can be operated with a continuous sampling clock, ..."
I talked with different people and no definite answer could be found.
Is there a simpler approach i did not see?
Under which circumstances can a pipelined ADC be operated in the desired Single-Shot mode?
Does the switched-capacitor input or the pipeline do something undesirable when clock is turned off?
What happens when i operate AD9238 like i described before?
I hope i could show and explain the relevant facts of this problem.