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| SDS1104X-E DC offset in AC coupling |
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| Bionicbone:
Did you see the OS update for hardware 09? |
| Deso:
--- Quote from: Bionicbone on March 24, 2023, 02:50:07 pm ---Did you see the OS update for hardware 09? --- End quote --- Yes. I'm on OS v3 from the factory. --- Quote from: bdunham7 on March 23, 2023, 02:15:52 pm ---I don't know the exact details, but the input amplifier on these has a stage before the AC-coupling so that the input impedance is always 1M + 15pF (or so) regardless of which coupling you chose. This first part will have some offset of its own and that will be blocked by the AC-coupling, so you do see some change in DC offset when changing coupling. You should compare all three modes--DC, AC and GND. --- End quote --- I've looked at Dave's images of the SDS1104X-E frontend and it doesn't look like they have anything active before the AC/DC coupling relay. Moreover, they look extremely close to the Hantek DSO5000P schematic. Of course, there is some difference, but the basic idea and signal flow seem to be the same. So it looks like there are no active parts before AC/DC coupling. What's more, the SDS1102X-E's specification lists an input impedance for DC coupling only. --- Quote from: bdunham7 on March 23, 2023, 02:15:52 pm ---As for the amount of the offset, you can read the specs and see what the tolerances are. --- End quote --- I couldn't figure it out: "Offset Accuracy: ±(1%* Offset+1.5%*8*div+500 uV): ≤1 mv/div" Current situation is that I have 2 divs (1mV) error at 500uV/div out of 8 divs, which is 25%. The error is visible and measurable even at 5mv/div with AC coupling. --- Quote from: bdunham7 on March 23, 2023, 02:15:52 pm ---I When comparing to other instruments, you need to make sure you are comparing at the same scale. Do those other scopes have a 500uV/div setting? --- End quote --- No. But the tests were performed at their lowest possible setting. But to be fair - below in the attachments are the same measurements but this time at 2mv/div on both scopes. Still, the old one beats this one by a lot. |
| bdunham7:
--- Quote from: Deso on March 24, 2023, 07:53:08 pm ---I've looked at Dave's images of the SDS1104X-E frontend and it doesn't look like they have anything active before the AC/DC coupling relay. Moreover, they look extremely close to the Hantek DSO5000P schematic. Of course, there is some difference, but the basic idea and signal flow seem to be the same. So it looks like there are no active parts before AC/DC coupling. What's more, the SDS1102X-E's specification lists an input impedance for DC coupling only. --- End quote --- Well, it would be interesting to know the exact setup here, perhaps someone can enlighten us. However, the input impedance measured directly on mine is 1M in DC and about 1.2M in AC and GND. So something changes but they are not just using a blocking capacitor like you'd see in many older CROs and perhaps other modern models as well. Those would indicate open with a DMM. --- Quote ---I couldn't figure it out: "Offset Accuracy: ±(1%* Offset+1.5%*8*div+500 uV): ≤1 mv/div" Current situation is that I have 2 divs (1mV) error at 500uV/div out of 8 divs, which is 25%. The error is visible and measurable even at 5mv/div with AC coupling. No. But the tests were performed at their lowest possible setting. But to be fair - below in the attachments are the same measurements but this time at 2mv/div on both scopes. Still, the old one beats this one by a lot. --- End quote --- TBH, I have to agree that seems like a lot. Certainly far more than mine. |
| Deso:
So, several days after my offset findings I hit another snag - in trigger setup menu, when switching to AC or LF Rejecting Coupling - the trigger marker line disappears. It is not above/below the current screen and it works, but is not visible at all. It shows only hint with the current threshold voltage. The channel coupling does not matter for this issue. I thought a 5+ year old design should be clean of software bugs. Another thing - the Trigger Source.. it stays always to the channel that have been selected even if this channel is not active anymore. It would be nice to automatically switch to the current active channel if the selected has been disabled. |
| bdunham7:
--- Quote from: Deso on March 29, 2023, 06:17:41 pm ---So, several days after my offset findings I hit another snag - in trigger setup menu, when switching to AC or LF Rejecting Coupling - the trigger marker line disappears. It is not above/below the current screen and it works, but is not visible at all. It shows only hint with the current threshold voltage. The channel coupling does not matter for this issue. I thought a 5+ year old design should be clean of software bugs. --- End quote --- That's neither a bug nor feature, just the way things work. If you displayed a marker at the selected trigger threshold (say 0.0V) with AC-coupled triggering and then looked at an AC signal with a DC bias, you would then complain that the trigger event doesn't occur at the intersection of your signal and the displayed trigger marker. |
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