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Siglent SDS 1202X-E - Firmware Update
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sonpul:
My CH1 is not very good either. Need to warm up for 30 minutes and calibrate. When cold, CH1 will have a large deviation until it warms up for 30 minutes.
tautech:

--- Quote from: sonpul on June 22, 2023, 07:35:00 am ---My CH1 is not very good either. Need to warm up for 30 minutes and calibrate. When cold, CH1 will have a large deviation until it warms up for 30 minutes.

--- End quote ---
Set AutoCal to ON, it's on P1 in the Utility menu.  ;)
sonpul:
The AutoCal setting is always set to ON. This has never had an effect on temperature drift.
AleXis6:

--- Quote from: tautech on June 21, 2023, 09:55:56 pm ---Please run SelfCal.

--- End quote ---
I did 5 times.
may be the quality/resistance of earth wire can influence
Performa01:

--- Quote from: AleXis6 on June 21, 2023, 08:26:18 pm ---my scope is not so good, especially Ch1

--- End quote ---
So you have an offset error of 300 µV. Have you ever checked the spec. sheet? Here's what it says about the offset error:

±(1%* Offset+1.5%*8*div+500 uV): ≤1 mv/div

Since there are folks who might be confused by more complex error margin specifications like this, let's exercise it step by step:
1 % of the offset. You set the offset to zero, so this error component can only be zero as well.
1.5 % * 8 * div. This means 1.5 % of the full scale (screen) voltage. At 500 µV/div it is 1.5 % * 8 * 500 µV = 1.5 % * 4 mV = 60 µV. Up to now the observed offset would be too high, but now it comes:
+ 500 µV. That makes for a total of 560 µV. What you see is certainly less than that, so your scope is well within spec.

These 500 µV are there for a reason. It is not because Siglent is mean and doesn't want their customers to have perfectly zero-aligned traces when offset is set to zero and there is no input signal. It is in the way how the offset compensation works. This is not just a software operation, There is a DAC that produces a voltage that is used to compensate any offset in the frontend, no matter where it comes from. If you feed a signal with a significant DC portion in it (and for some reason you don't want to use AC-coupling), then you might want to eliminate it and the offset can become rather high. In the 500 µV/div range you can compensate up to +/-2 V this way. On the other hand you also want to compensate tiny offset voltages, stemming from imbalances of the input stage of the OP-Amp that handles the LF-path of the split path input buffer.

Now it comes: this DAC has a limited resolution. I don't remember the exact numbers, but I assume it is a 12 bit DAC. If that's the case, then one LSB equals 977 µV. This means that the worst case compensation error can be up to +/-490 µV, even for a freshly auto-calibrated scope.

Yes, I know. Everybody wants perfection, even from the cheapest entry level product. I am the same, believe me. But there are always compromises. And even when it may look irritating, an offset error of 300 µV won't ruin your measurements for sure! ;)
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