Scope accuracy question ...

[jpb]

I know I shouldn’t post this because I going to end up looking silly, but too late now.  Isn’t the accuracy of a scope reading dependent on the offset you set as well as the resolution/accuracy of the ADC within the scope?
E.g. If you add an offset and zoom into the point of interest don’t you get a more accurate reading than just having the full waveform on the screen. Doesn’t doing this increased the gain to the ADC and shift ADC’s 0V point up (or down). I’m definitely no expert on this but I thought the scopes worked by adding the offset voltage to the ADC input so that they could cover a larger range of voltages with greater accuracy.

On the WaveJet you can set relatively large offsets so for instance you can have a +/- 10V offset at 50.2mV/div (at 50mV/div it drops to +/- 1V). Unfortunately from the point of view of measuring dc the offset itself is not much more accurate.

The spec for the DC Gain is +/-(1.5% + 0.5% full scale) while the  Offset Accuracy is  +/-(1% + 0.5% of full scale + 1mV). It does gain you something at around 10V if you measure it at 2V/div then the error is specified to be better than 230 mV, if you measure it at 100 mV/div then the offset error is 105mV and the measurement error is another 4mV so the total is 109mV.

But it is a good point as it gets around the 8 bit limit to some extent.

[MasterOfNone]

So by using the offset, even though you gain a lot more precision (i.e. you can zoom in on small spikes etc),  you don’t gain as much on accurate because of the offset accuracy.   

[jpb]

So by using the offset, even though you gain a lot more precision (i.e. you can zoom in on small spikes etc),  you don’t gain as much on accurate because of the offset accuracy.   

You actually gain quite a bit. Both the offset and the gain have an error term that is 0.5% of full scale but using the offset allows full scale to be much smaller so you gain there. The offset error is 0.5% less but there is an extra 1mV error so for voltages where 0.5% is greater than 1mV, i.e. greater than 200 mV you also gain on this.

The conclusion is that if your signal allows it, using the offset is a good idea though the mechanics is a pain - I've played with it on the WaveJet and the offset changes every time you change the vertical range. That is if you set -10V on the 1V/div range and change to 500mV/div then the offset drops to -5V and you have to readjust by knob twiddling to get it back to -10V. Perhaps Agilent and other scopes behave more sensibly in this respect.

[vk6zgo]

OK,at DC & low frequencies,the DMM wins,but the digital voltage readout on DSOs will give you a useable reading way after your DMM has run out of frequency response.

Obviously,there are meters with wide bandwidth,but they tend to be specialist devices,like the old HP410C of analog days.

[Wuerstchenhund]

I know I shouldn’t post this because I going to end up looking silly, but too late now.

You're not end up looking silly, and it's a valid question.

Isn’t the accuracy of a scope reading dependent on the offset you set as well as the resolution/accuracy of the ADC within the scope?
E.g. If you add an offset and zoom into the point of interest don’t you get a more accurate reading than just having the full waveform on the screen. Doesn’t doing this increased the gain to the ADC and shift ADC’s 0V point up (or down). I’m definitely no expert on this but I thought the scopes worked by adding the offset voltage to the ADC input so that they could cover a larger range of voltages with greater accuracy.

This was true for some very old scopes (i.e. if I remember right the Philips PM3320 from 25 years ago did this) and as you said did affect the measurement accuracy. However, on most of today's scopes Vertical Offset is usually a software implementation and doesn't affect the measurement.

Edit: My above comment was wrong. You're right of course (and that makes your question even less silly), Offset accuracy does affect the measurement accuracy.

[Wuerstchenhund]

I've played with it on the WaveJet and the offset changes every time you change the vertical range. That is if you set -10V on the 1V/div range and change to 500mV/div then the offset drops to -5V and you have to readjust by knob twiddling to get it back to -10V. Perhaps Agilent and other scopes behave more sensibly in this respect.

No, most don't. The behaviour you see is certainly annoying but it's the standard behavior as it mirrors the Offset on an analogue scope.

[alm]

Vertical position is usually software, but offset (if available) should be implemented in hardware. Otherwise it's pointless, since you're still limited by the dynamic range of the ADC.

The scope may have 0.5 % DCV accuracy, but don't forget that most 10x probes are +/- 1% attenuation accuracy, so the spec would only be valid for 1x probes.

[Wuerstchenhund]

Vertical position is usually software, but offset (if available) should be implemented in hardware. Otherwise it's pointless, since you're still limited by the dynamic range of the ADC.

In some of the scopes where I could/did check at this level I'm pretty sure this was implemented in software only. But these were quite old scopes, and as you say it's not a good solution as it limits the available dynamic range.

Edit: I checked a few Service Manuals of some newer scopes, and on all of them Offset is still acting as ADC BIAS. So I was wrong. Sorry for the confusion.

[jpb]

Vertical position is usually software, but offset (if available) should be implemented in hardware. Otherwise it's pointless, since you're still limited by the dynamic range of the ADC.

In some of the scopes where I could/did check at this level I'm pretty sure this was implemented in software only. But these were quite old scopes, and as you say it's not a good solution as it limits the available dynamic range.

Edit: I checked a few Service Manuals of some newer scopes, and on all of them Offset is still acting as ADC BIAS. So I was wrong. Sorry for the confusion.

I was about to comment on this but it crossed with the last three replies so I deleted my post.

What I was going to say was that the potential offset on the WaveJet at least is up to 50 times the full scale (+/- 10V at 50.2mV/div) so if it was just software the dynamic range would drop hugely.

Given that it changes with range it must be a D2A providing bias at the inside of the attenuators i.e. at the ADC as you say.

[Wuerstchenhund]

I was about to comment on this but it crossed with the last three replies so I deleted my post.

What I was going to say was that the potential offset on the WaveJet at least is up to 50 times the full scale (+/- 10V at 50.2mV/div) so if it was just software the dynamic range would drop hugely.

Given that it changes with range it must be a D2A providing bias at the inside of the attenuators i.e. at the ADC as you say.

The longer I think about the more I think I may have indeed mixed it up with vertical position.

I guess I'm getting old.

[jpb]

Looking at the WaveRunner service manual (the only one I could find on-line), the dc offset comes from a 16bit D2A.