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Lets suppose we are talking about a reasonable modern scope such as a Rigol DS1054Z.

I want too look at the ripple at the top of a 5v square wave. I'll change the volts/division and offset so that the top of the square wave is centered on screen. In this scenario the bottom of the wave form is way off screen.

Is this bad practice? What does the analog front end do when the waveform is off screen?

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  • \$\begingroup\$ Amplifier saturation may be well-behaved at low frequencies, so it is easy to see when you've pushed gain too high. But at high frequencies saturation may occur more gradually, and it is harder to tell if you're still in the linear amplifier region. \$\endgroup\$ – glen_geek Feb 19 '17 at 16:50
  • \$\begingroup\$ The front end will be protected in any decent quality scope. Don't worry it will come back. \$\endgroup\$ – RoyC Feb 19 '17 at 17:41
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In general it is not bad practice to offset the waveform to measure a small part of it. As other posters have mentioned it can affect the accuracy of measurement.

Good scopes can often do 8-10 screen heights of offset but I've used some that suffer problems with only 3-4 screens of offset.

There is usually a specification for the amount of allowed offset. In the case of the Rigol DS1054Z it is described as Offset Range:

Rigol Allowable Offset

This is the allowable offset that will still meet the specifications described elsewhere in the specification.

Usually the sort of effects I see are the baseline wander but you may also see changes in dynamic response.

Make sure that the probe is compensated correctly if you are offsetting a significant amount or you may get an error in voltage measurements - I've been bitten by that a couple of times wondering why voltages are not what I expect.

kevin

Rigol DS1054Z specification

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Is this bad practice?

I don't think so, unless you push the voltage far above the rated limits and stress the input clamping diodes for a long time.

What does the analog front end do when the waveform is off screen?

Probably the input amp will be saturated against one of its power rails. But I won't worry too much about it, because the input amp is down the signal chain, after the clamping diodes. In any decent oscilloscope design this should be an operating condition that has been accounted for.

Take a look at this, it may interest you: schematics, full post.

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    \$\begingroup\$ This. It is perfectly normal and widely used, if you respect the input voltage limits for the scope. \$\endgroup\$ – Vladimir Cravero Feb 19 '17 at 16:19

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