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Siglent SDS1104X-E anomaly on what should be a simple task.
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bdunham7:
I recently hauled out a project I've been planning and that is making a replacement LED display for the Fluke 8842A.  That's another story--in short a drop-in replacement is impractical--but I ran into what I think is a shortcoming of the Siglent scope.  I haven't tried to reproduce it with other test signals, so I'll just show you what I have.  What I'm doing is taking the segment and digit scan signals from the VFD drivers of the 8842A and using them to drive a set of 7-segment blue LED displays.  The display has 8 'digits' and 11 'segments' total, so 8 lines x 11 lines.  I'm just looking at 3 digits and the 7 segment lines for now.  The VFD drivers put out a +30V signal on both the segment and digit lines and when both are on, the segment for that digit lights.  This obviously won't work for an LED, so I'm using the segment scan lines to drive the segments of common cathode displays in parallel and the digit scan lines to drive a darlington array which then grounds each common cathode in sequence.  I'm using dropping resistors at this point, but I'll need to use a lower voltage supply for the system to work.  I needed to know the maximum voltage drop across the darlington array, which I decided to determine by connecting a scope between ground and the common cathode of one digit.  The specs say about 1 volt, but I was considering using a 5 volt supply, so I needed to know fairly closely.

I connected Mr. Siglent with one probe to the incoming digit line and triggered off of that, then the other probe to the common cathode connection.  The signal is noisy when the array transistor is off because of cross-feeds from other digits, but when on it seems to go down near zero.  So I started changing the V/div setting and at 2V/div, it indicated a half a division above ground.  I changed nothing except I twisted the V knob 1 click, the scope went to 1V/div and I heard a relay click--but the signal in question was now 4 division above ground. I rechecked everything, thinking I had some strange ground loop or capacitive coupling or bad probe (the probe was the Siglent PP510 and it is falling apart), so I tried another probe, a different channel, moving the grounds.  I also tried both 1X and 10X, nothing changed and the problem was repeatable.

So, I tried some other scopes.  The problem didn't occur on my Tek TPS2024 and even my ancient Scopemeter gave me the correct result.  Well, both of those are isolated, so perhaps still a ground?  Now this is not a fast signal, the scan lines stay on for ~1 millisecond, which makes it difficult to even see the signal with an analog CRO.  But, I still have a Tek 2221, 60MHz 20MSa/s, and it is common ground just like the Siglent or any other scope.  Even that worked properly. 

I think I know what the issue is now, thanks to a comment David Hess made a while back, and I'm sure Siglent will claim it is user error.  However, what I'm doing is not an unusual use of a scope and if this is a limitation, it would be good to know about it. 

Photos are a rough sketch of what I'm doing (on an actual envelope), the Siglent at 4 different V/div settings and then the Scopemeter, Tek TPS and Tek 2221 on just the final screen.






















masterx81:
You are overdriving the input amplifier.
A good read:
https://www.evaluationengineering.com/instrumentation/oscilloscopes/article/13002369/overcoming-overdrive-recovery
Every scope have it's characteristics.
Common "problem" on dso scopes.
tautech:

--- Quote from: masterx81 on February 06, 2021, 07:19:15 am ---You are overdriving the input amplifier.
A good read:
https://www.evaluationengineering.com/instrumentation/oscilloscopes/article/13002369/overcoming-overdrive-recovery
Every scope have it's characteristics.
Common "problem" on dso scopes.

--- End quote ---
Certainly overdriven in the last screenshot and the giveaway is the long vertical tails in this screenshot:



For the first screenshot the complete waveform is entirely on the display so another explanation is needed and the possibilities are; the unmatched probes, channel crosstalk, probe capacitive loading, circuit PSU not up to the task or inadequate decoupling yet the blip on the wider pulse at minimum amplitude is likely to have any impact at all as it seems well removed from any threshold levels.



It is my view the scope is correctly displaying something and further investigation should discover what yet if it has no effect on the operation of the DUT why bother.
bdunham7:

--- Quote from: tautech on February 06, 2021, 08:32:03 am ---For the first screenshot the complete waveform is entirely on the display so another explanation is needed and the possibilities are; the unmatched probes, channel crosstalk, probe capacitive loading, circuit PSU not up to the task or inadequate decoupling yet the blip on the wider pulse at minimum amplitude is likely to have any impact at all as it seems well removed from any threshold levels.
It is my view the scope is correctly displaying something and further investigation should discover what yet if it has no effect on the operation of the DUT why bother.

--- End quote ---

Sorry if that wasn't clear, but the first screenshot is not a problem at all--nothing is wrong.  I just put the arrow there to show the area that I was trying to measure.  The messy waveform is just a side effect of how I set it up to measure that specific voltage drop.  The only time the scope is properly connected is when the green trace is high and the driver is on, the rest of the time it is bleed-through into a disconnected high impedance circuit.  So the entire waveform during the time the green trace is low is just garbage.  Of course that garbage is what is overdriving the scope, so I can't ignore it.


--- Quote ---Certainly overdriven in the last screenshot and the giveaway is the long vertical tails in this screenshot:

--- End quote ---

You can't see it in a still photo, but all of those bits hanging by those tails were dancing up and down like marionettes (puppets on strings).  I actually don't see a lot of evidence that the waveform was badly distorted and the area being measured has the appropriate shape--flat in the middle and low at the extremes--but it is offset upwards by 3 volts.  So an entire millisecond goes by and there's no sign of any recovery.
bdunham7:

--- Quote from: masterx81 on February 06, 2021, 07:19:15 am ---You are overdriving the input amplifier.
Every scope have it's characteristics.
Common "problem" on dso scopes.

--- End quote ---

Yes I am, and while every scope has those characteristics, almost none seem to specify them.  The only reference I could find that did was a statement from someone at Cleverscope stating that their recovery time was 1 sample, but no further specifics.  The way I amplified my area of interest is not at all an uncommon way to do it, thus my initial confusion.  I expected it to work because I've done things the same way with other scopes. As I showed, the other DSOs either didn't overload at that same level or they exhibited much faster recovery times, if that is exactly what is going on here. 

So according to Tektronix, they don't specify a recover time from an overdrive situation, they say that if you are concerned about it, you have to characterize the scope yourself.  So it seems that the main characteristics would be 1) at what level the overdrive condition occurs and 2) how long it takes to recover.  It seems to me that 'more than a millisecond' is a pretty long recovery time for a 200MHz scope.
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