Resolution of a DSO vs DC offset

[filip]

Hello,

I don't have a lot of experience with oscilloscopes, but I did some measurements with analog ones.
I am considering buying a DSO (probably a Hantek DSO5062 or an equivalent), but one particular thing worries me about it.

Most of these cheap DSO have 8 bit ADCs.
Let's say that I have a 100kHz 5Vpp square  wave with some ringing and I want to have a closer look at the ringing after the rising edge.

My question is - does the vertical position knob add a generated DC offset voltage between the signal source and the ADC or does it just move the same measurement across the screen?

If the first is true, then I can subtract for example 4.9V (with the vertical position knob) from the input signal and set the measurement range for 200mV, which would give me 0.8mV/bit for an 8 bit ADC - seems fine for watching a < 200mVpp ringing.

But if the second one is true, then ADCs 8bits would be spread across 5Vpp (assuming that the voltage range of the scope matches the waveform) which would mean that each bit is ~20mV and shifting the waveform and decreasing v/div only does a 'software zoom' - does not improve the viewing resolution. That could be a considerable disadvantage of a cheap DSO compared to a quality second hand analog scope.

thanks,
Filip

[alm]

Vertical position usually applies a DC offset before the ADC. Offset range is limited though, and it relies on quick overdrive recovery of the vertical amplifier (which is usually not specified). Analog scopes have the same problem.

[filip]

Thanks for the clarification .

As far as I understand the voltage division stage precedes the DC offset stage, so the problem of limited range offset is partially mitigated, right?


Has anybody tried to determine the behaviour of the input stages of the most popular cheap DSOs (for example Rigol DS1052E, Hantek DSO5202M, Owon SDS6062) in regards to overdrive recovery, offset ranges etc?

[w2aew]

It's going to depend on the scope.  The better ones give you control over both the position and the offset independently, so you can have better control over the usage of the DAC's dynamic range.  Another thing to watch out for is overload recovery, so when you make a measurement like this, it is not affected by the over/under drive of the input.