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.