Products > Test Equipment
SDS1104X-E Unexpected Behaviour
BillyO:
--- Quote from: bdunham7 on November 24, 2022, 11:45:34 pm ---Do you mean a signal with a large DC bias? Or something else?
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Well, large is relative, but in this case I'd say yes. The DC offset was 10V while the AC component was 4V. However, the variance of interest was less than .5V. I just built a small summing circuit to take the DC component out then adjust the measurements accordingly. So it was easy enough to work around although a bit of a pain. A better measurement solution would have been a fast response chart recorder. They have offset adjustments as a matter of rule, but they are also not within my reach. Another solution would be to get a data logging DMM.
Anyway, I think we're drifting off topic a bit and obviously nobody cares. I'll treat this "Offset" adjustment as though it were an appendix. Not very useful, but hanging around nonetheless .. and that's just fine. :palm:
It's kind of like having a control in your car that is labeled "Brake adjustment" but when you use it to make an adjustment it just applies the brakes. :-//
bdunham7:
--- Quote from: BillyO on November 25, 2022, 02:00:11 pm ---Well, large is relative, but in this case I'd say yes. The DC offset was 10V while the AC component was 4V. However, the variance of interest was less than .5V. I just built a small summing circuit to take the DC component out then adjust the measurements accordingly. So it was easy enough to work around although a bit of a pain. A better measurement solution would have been a fast response chart recorder. They have offset adjustments as a matter of rule, but they are also not within my reach. Another solution would be to get a data logging DMM.
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If you have a 4Vp-p AC signal with a +10V DC bias, you just set the position (or offset) to -10V and the vertical scale to 500mV/div and it should appear life-sized on your screen. That gives you a whole division to look at your 0.5V variance.
BillyO:
I did actually try that, but it seemed to bugger up the mean and RMS measurement values. Perhaps that's a real bug.
bdunham7:
https://www.eevblog.com/forum/blog/eevblog-1223-whats-all-this-ac-rms-stuff-anyhow/
The scope's RMS measurement will give you AC+DC TRMS, for the AC TRMS you use the STDEV measurement. For a 'mean', I'd have to know what mean you mean want but there's probably a way to do it.
Simon:
--- Quote from: bdunham7 on November 24, 2022, 11:01:09 pm ---
--- Quote from: BillyO on November 24, 2022, 05:02:49 pm ---There is actually. I can think of 2 (maybe rare) use cases, but important if you need it. Maybe Mr. loop, with all his superior knowledge of metrology, could expand on that?
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On a CRO or other scope where the vertical position has to operate within reasonable limits, the addition of an adjustable DC bias on the input that allows you to offset the input by many divisions worth does have some uses, like looking at PSU ripple and more. This was an uncommon feature on CROs. But a DSO like the Siglent SDS1104X-E already has this feature built-in and incorporated into the vertical amplifier system. Even if you did have some second offset system in the vertical amp, it would just add to or subtract from the first one. The only other 'offset' I can think of that you could do is some sort of digital trim, but that would have to be pretty limited. What use case do you have that isn't met by the system in the 1104X-E?
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This use case is dealt with by selecting how the position of the screen reacts to changing the V/div, it can go around the centre of the screen or around 0V these do not coincide when you look at a voltage far from ground reference.
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