Electronics > Projects, Designs, and Technical Stuff
Why are oscilloscopes so inaccurate?
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Gyro:

--- Quote from: Circlotron on August 10, 2019, 10:58:25 am ---
--- Quote from: Rerouter on August 10, 2019, 10:35:21 am ---in most scopes your only dealing with an 8 bit ADC, now you can use the channel offset in a lot of cases to squeeze out extra precision,

--- End quote ---
So does that mean that if you shift the trace to the bottom of the screen and only use the upward portion of the trace, you are only getting 7-bit resolution?  :scared:

--- End quote ---

No, it's actually much worse than that...

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.

and

2. Most DSO LCD screens don't have sufficient vertical pixels to display the full 256 levels coming from the ADC, especially when you have multiple channels displayed plus the general crap of annunciators, measurements etc. clogging up the margins. Check your LCD resolution and see how many bits of resolution  it's actually displaying on a typical waveform - you will probably be surprised.

EG. The Rigol 1054Z has 480 vertical screen pixels (480x800). Now split that between 4 traces and the surrounding crud and you've probably got less than 100 pixels per channel, ie. under 7 bits.
wraper:

--- Quote from: Gyro on August 10, 2019, 01:11:57 pm ---2. Most DSO LCD screens don't have sufficient vertical pixels to display the full 256 levels coming from the ADC, especially when you have multiple channels displayed plus the general crap of annunciators, measurements etc. clogging up the margins. Check your LCD resolution and see how many bits of resolution  it's actually displaying on a typical waveform - you will probably be surprised.

EG. The Rigol 1054Z has 480 vertical screen pixels (480x800). Now split that between 4 traces and the surrounding crud and you've probably got less than 100 pixels per channel, ie. under 7 bits.

--- End quote ---
Not that you can accurately see actual voltage from the trace even if LCD resolution is high. But all modern scopes have measurement functions so you can see precise number in digits.
Gyro:

--- Quote ---Not that you can accurately see actual voltage from the trace even if LCD resolution is high. But all modern scopes have measurement functions so you can see precise number in digits.
--- End quote ---

True, though you do have to remember the aforementioned 8 bit ADC limitation in your definition of precise.
drummerdimitri:
Lots of good info on here thanks for the clarification.  :clap:
SiliconWizard:

--- Quote from: mikerj on August 10, 2019, 10:57:49 am ---Why is my DMM useless for showing high frequency time domain behaviour?  |O

--- End quote ---

Hehe, indeed.

The question was worth asking though - that's a very common "trap" for many beginners and even fresh engineers... I think it's already a good sign that the OP actually *noticed* the inaccuracy and asked the question. Many wouldn't even have cared.

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