bug on DSO5102P? - different measures when changing the coupling (AC vs DC)

fmnardini

Hello all!

I am Franco Maria, engineer from Italy. I recently bought (on Amazon) a Hantek DSO5102P. Today, I measured the main voltage (220Vrms, 50Hz) in my office (using an insulation transformer, obviously) and what I see on the screen changes significantly in case I use AC or DC coupling.

In details, when using:

DC coupling: I read 592Vpp, -296Vmin, 296Vmax (returned by the measure func)
AC coupling: I read 664Vpp, -332Vmin, 332Vmax (returned by the measure func)

The testing condition is the same, i.e., probe connected to the output of the insulation transformer. The only thing that changes in the experiment is coupling, AC vs DC.

I know that AC coupling exploits HPF and so, 50Hz could be attenuated. However, I am experiencing exactly the opposite. When using AC coupling, I see increased voltages.

I did some experiments by adding in parallel to the probe two TRMS recently calibrated testers (Brymen 869s and Fluke 117). They both return the same Vrms value returned by the oscilloscope when in AC coupling. In DC coupling, Vrms, Vpp, Vmin and Vmax returned are WRONG! (-10% more or less than the correct value).
I am attaching two photos showing the results of the experiment.

I also did several experiments more:

1) I tested with four different probes (Hantek branded and others, both 1x and 10x): problem always present
2) I flashed the latest firmware available on your website (dated 2021-02-25) and after I run self-calibration. Nothing changed.
3) I tested several different voltages (220Vrms, 30Vrms). Error in DC is always more or less 10% of the AC readings.
4) I tested AC vs DC with the square wave from the internal generator: problem NOT present
5) I then tested a square wave sweeping from 10Hz to 1KHz. For frequencies greater than 200Hz the problem seems no more present.
6) I also tested a sin wave sweep from 10Hz to 1KHz. Again, from 200Hz on, the problem seems no more present.

What's going on here guys?

Moreover, a friend of mine owns a DSO4202C. I asked him to run the same test on the main voltage (220Vrms, 50Hz). The result is the same. The error rate is different (less than 10%). However, DC coupling readings are far less than the correct ones provided again by AC coupling.

I am available if you want me to test some other ideas. I really hope you can help me. I need a reliable DSO in my work!

Thanks a lot in advance for the help!

best,

Franco Maria (from Tuscany, Italy)