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| pcprogrammer:
But how? Like David wrote the charging of the capacitor might be to slow when it comes to GHz signals. Looking at analog to digital conversion it depends on the used technology if there needs to be a sample and hold circuit before the analog to digital converter. With successive-approximation the input needs to be stable during the conversion. There are several types and the question is which does not need a sample and hold circuit. https://dewesoft.com/daq/types-of-adc-converters |
| tautech:
--- Quote from: pcprogrammer on October 25, 2022, 08:42:32 am ---But how? --- End quote --- Do you think the sampling dots in a DSO in Dot mode are just invented ? |
| pcprogrammer:
--- Quote from: tautech on October 25, 2022, 08:48:53 am --- --- Quote from: pcprogrammer on October 25, 2022, 08:42:32 am ---But how? --- End quote --- Do you think the sampling dots in a DSO in Dot mode are just invented ? --- End quote --- That is not the point. Yes a DSO uses digital values that represent the signal. And we call these samples. But what part of semantics don't you get? David states that sampling is the act of capturing an analog value in a capacitor and not the converting it into a digital value. And what he, and me too, wonders about how this works for a high speed ADC. Does it use a sample and hold circuit or not. |
| gf:
--- Quote from: pcprogrammer on October 25, 2022, 08:56:27 am ---And what he, and me too, wonders about how this works for a high speed ADC. Does it use a sample and hold circuit or not. --- End quote --- Virtualy any ADC does. Continuous-time sigma-delta ADCs are the only exception i'm aware of. |
| tggzzz:
--- Quote from: David Hess on October 25, 2022, 07:20:09 am --- --- Quote from: rob77 on October 24, 2022, 06:35:04 pm ---synchronous sampling ... isn't that called logic analyzer ? ;) --- End quote --- Logic analyzers can operate synchronously or asynchronously. --- Quote from: tggzzz on October 24, 2022, 10:53:46 pm --- --- Quote from: Mechatrommer on October 24, 2022, 09:13:42 pm ---because generally when a person want to build a diy dso implicitly means the real-time one. a person will come with building a "sampling" scope in the topic when he knows what he is talking about. and nobody will recommend newbies asking about buying a new scope to buy a "sampling" scope. --- End quote --- Er, newbies are routinely recommended to buy a sampling scope. You, at least I think it was you, previously ignored the point that all modern scopes (e.g. the Rigol DS1054) tale samples of the input signal. You can even look and see the individual samples on the display, presuming you can navigate the menu system to find the right config parameter. --- End quote --- DSO stands for digital storage oscilloscope distinguishing it from digital sampling oscilloscope. The later term was almost never used, but does refer to a specific type of instrument which is not a DSO. Nothing requires a digital storage oscilloscope, or any ADC or digitizer, to use sampling, and when they do use sampling, it is *not* the same type of sampling that a sampling oscilloscope uses. Datasheets tend to no longer distinguish them, but ADCs are available in sampling and non-sampling varieties. Going back to the beginning of DSO evolution, the Tektronix 468, 2230, and 2232 all use non-sampling ADCs, however the 2230 included a discrete sampling unit before its 20 MHz flash ADC, and the 2232 included integrated samplers before each of its 100 MS/s flash ADCs. None of these worked the same way as the sampler does inside of a sampling oscilloscope. These were digital storage oscilloscopes. The sampler in a sampling oscilloscope operates with a strobe which is *shorter*, often a lot shorter, than the acquisition time needed to charge the storage element. This means that either the storage element must be reset between samples or the output is based on both the old and new value, which is where sampling gain come into play. By requiring only a fraction of the charge to be transferred, the "sampler" has gain which can be applied to increase its bandwidth. (1) (2) The distinguishing feature of a sampling oscilloscope is that sampling occurs first in the signal chain. Gain is applied after sampling in the analog domain. This means that bandwidth is determined entirely by the the sampling strobe width, which also produces a non-linear frequency response, so the 0.35 rule does not apply. One advantage of this besides predictable frequency response, is instant overload recovery. Overload never gets past the sampler so nothing needs to recover. (1) Nobody is saying how modern integrated ADCs do their sampling for high bandwidth. An internal 50 ohm termination results in a 25 ohm source impedance to charge the sampling storage element which sure doesn't seem like enough for the bandwidth they achieve. (2) I am describing the sampler needed for random sampling which is the more general case. The sampler in a sequential sampling oscilloscope can operate with a gain of 1 by repetitively sampling the same point of a waveform. --- End quote --- The key phrase there is "it is *not* the same type of sampling". I entirely agree that - to put it overly simply - one type of sampling captures the average voltage during a period, and another captures the average voltage at an instant in a period. The two types' very different aperture times lead to differing characteristics, advantages and disadvantages. Nonetheless they do both provide representations of the input voltage at discrete time intervals - and that is the key distinguishing feature of sampling vs analogue processing. Classic examples of radically different aperture times found in sampling scopes are a diode ring (aperture time << sampling interval) and capacitor in a CCD (aperture time = sampling interval). |
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