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Why are oscilloscopes so inaccurate?
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David Hess:
The design constraints needed to make wideband amplifiers with a response down to DC are not compatible with DC precision.  This is also why you cannot use an oscilloscope to directly measure settling time to high precision; instead, a rate of change or null measurement must be made.

Further with an 8 bit instrument, once you throw away 1 bit for linearity, that is only 1 part in 128 so about 2% when 1 bit is sign.

Higher resolution oscilloscopes and digitizers exist but for high accuracy, they have to compromise in other areas.

Sampling oscilloscopes avoid these problems by omitting signal conditioning before sampling so they can achieve much higher resolution at high frequencies.


--- Quote from: soldar on August 10, 2019, 11:02:37 am ---How on earth do you measure a voltage on a scope screen with better than 1% accuracy? Does it involve a microscope?
--- End quote ---

You use an oscilloscope that includes a differential comparator.


--- Quote from: magic on August 10, 2019, 12:14:49 pm ---The analog frontend could probably be fitted with a DC servo, frontends of cheap DSOs already use separate paths for high and low frequencies.
--- End quote ---

The problem with a DC servo is that in the event of overload, it ruins the overload recovery time.  This is also a problem with two path designs unless special care is taken.  There is a lot to be said for the performance and simplicity of the old stacked JFET input buffer.


--- Quote ---Thermal stability of variable gain amplifier ICs and ADCs remains a question.
--- End quote ---

Thermal balance is a big problem and especially so in integrated circuits.  Even worse, the methods used to combat it in integrated circuits cannot be used at high frequencies because of parasitic coupling.  Essentially the offset varies with operating point and time.  Very high performance instruments compensate for this in the digital domain.

A more obvious and less complex way to control FET mismatch-induced offset would utilize a matched dual monolithic FET. Readers are invited to speculate on why this approach has unacceptable high frequency error. - Jim Williams, Linear Technology application note 65, page 72.
Alex Eisenhut:

--- Quote from: David Hess on August 10, 2019, 03:16:17 pm ---
You use an oscilloscope that includes a differential comparator.


--- End quote ---

Sounds like a use case for a vintage analog scope...

http://w140.com/tekwiki/wiki/1A5
DaJMasta:
It's been mentioned, but different instruments for different applications.  It's a limitation of the practical design of the frontend and sampling system (low resolution, response down to DC and up being hard to keep linear, etc.), but it's also not something that's needed for the kinds of things scopes are usually known for.

The scope's strength in measurement is time - its triggering and sample rate have to be very low jitter and tightly controlled to give you that nice smooth sinewave on the screen, but in essence, it's not intended to be a measurement tool - a scope is for visualization.  A DMM, a power meter, a frequency counter... all of these are explicitly for measuring whereas the primary function of an oscilloscope is to show detail on a signal as a visualization tool, the measurement of a signal's parameters is sort of secondary addition (take a look at all-analog scopes, measurement was never a big priority until digitization started making it easier).
coppercone2:
look at bandwidth. Measurement usually gets ugly when you widen bandwidth.

You have something that is widely considered perfectly linear (visible "DC", that is where you can track change with your eyeballs) and AC, in which every parameter follows 1/F tendancies. It's like looking at everything through a stroboscope (trying to visualize a waveform of 1MHz on a scope).
SiliconWizard:

--- Quote from: bd139 on August 10, 2019, 11:11:26 am ---Scopes are shit at everything they do. They just do a lot of shit things at the same time which turns out is a lot more useful than things that do one thing really well.

--- End quote ---

Same can be said about most people.
 ;D
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