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ADC sample and hold
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JosiahCochran:
It is to my understanding that most ADC's have some sort of sample and hold circuit on the input. I know the basic theory of how these work, but how do they get them to work on the order of GSa/s? As far as discrete sample and hold circuits, most of the things I see online are on the order of microseconds. I tried making a circuit in LTspice and even the GHz op amps have too high of an input capacitance to work at these speeds. Is it possible to make a sample and hold circuit that works on the order of picoseconds with discrete components? how do they do it in high bandwidth ADC's?
David Hess:

--- Quote from: JosiahCochran on August 17, 2019, 05:32:36 pm ---Is it possible to make a sample and hold circuit that works on the order of picoseconds with discrete components? how do they do it in high bandwidth ADC's?
--- End quote ---

That was possible to do decades ago using various high bandwidth sampling configurations.  Tektronix wrote a book on the subject and details are available in their comprehensive service manuals up until about 1990.

As a practical matter where Tsampling > Tsettling, a 25 ohm source from a terminated 50 ohm transmission line into 2.5 picofarads (double the shunt capacitance of a 4 diode sampling bridge) gets to 2.5 GHz.  Beyond that, sampling at lower than 100% efficiency is used as described by Tektronix above.
JosiahCochran:
So what I gather is that you can't really do it with op amps, you have to build a high band width transistor amplifier to accomplish these time scales?
David Hess:

--- Quote from: JosiahCochran on August 24, 2019, 02:09:10 am ---So what I gather is that you can't really do it with op amps, you have to build a high band width transistor amplifier to accomplish these time scales?
--- End quote ---

Do you mean use operational amplifiers as part of a high bandwidth sample and hold?  No, and in general feedback amplifiers are not suitable.  Transistor choppers are also too slow and suffer from excessive charge injection.

Fast discrete samplers actually operate at the transmission line level.  I am not sure how the integrated ones built into modern ADCs work.
JosiahCochran:
could you elaborate on what you mean by transmission line level? I am not very familiar with RF circuits. How can a transmission line be gated to sample at a certain frequency? By transistor choppers you mean using an FET as a switch?
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