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Why are oscilloscopes so inaccurate?

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drummerdimitri:
I've recently noticed that the voltage measurement of an AC signal on my scope change a lot with the time division setting.

Is there a norm to set the time division to X seconds or show X amount of cycles for the most accurate readings?

David Hess:

--- Quote from: drummerdimitri on August 24, 2019, 03:21:42 pm ---I've recently noticed that the voltage measurement of an AC signal on my scope change a lot with the time division setting.

Is there a norm to set the time division to X seconds or show X amount of cycles for the most accurate readings?
--- End quote ---

That should not happen unless the AC measurement is made over the entire record and only a fraction or small number of cycles are captured.  Usually the default is to make the measurement over 1 or a whole number of cycles if they can be detected.

nctnico:

--- Quote from: drummerdimitri on August 24, 2019, 03:21:42 pm ---I've recently noticed that the voltage measurement of an AC signal on my scope change a lot with the time division setting.

Is there a norm to set the time division to X seconds or show X amount of cycles for the most accurate readings?

--- End quote ---
No. Some oscilloscopes do measurements on a decimated (downsampled) set of data to speed up measurements. If you have such an oscilloscope: As a result the accuracy goes down if you have many cycles on the display. As a rule of thumb you should be able to see the shape of the signals for the measurement to be accurate. Ofcourse you should always have at least one full cycle on the display.

Bassman59:

--- Quote from: Gyro on August 10, 2019, 01:11:57 pm ---1. Unless your signal is just the right amplitude to fill the entire input range of the ADC (ie, just before clipping) then it won't be giving you the full 8 bits anyway.
--- End quote ---

What? No.

TheUnnamedNewbie:
Another thing to keep in mind: Even if you have the most accurate scope in the world, have you stopped to consider the accuracy impact of your probes, connectors, coax, etc?

If you want to measure to a fraction of a percent, a small amount of impedance mismatch here, a bit of parasitic inductance and capacitance there, and boom, you lost your fraction of a percent. Think about the fact that you have to manually go in and tweak that 10x probe. How on earth are you ever supposed to get accurate measurements out of that?

This is a similar reason why things like spectrum analyzers can't measure to extreme accuracy.

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