Hi all,
I have discovered a weird phenomena with my SDM3055X dated from March this year.
Software 3.01.01.07, hardware 02-02-00-05-00.
I'm measuring a variable DC supply with an output between 0-200V with the DMM.
I use a RIGOL DS2072A to look at the ripple of the supply, that's how I found out.
The DC supply is based on the Harrison/HP/Agilent method, so the "+" is actually the common ground, and the "-" is the varying negative going supply.
I'm connecting my DMM with the Red lead to the "+" of the supply, and the Black lead to the "-" of the supply.
Sounds complicated, but is maybe important. With this setup I'm measuring a positive voltage on the DMM.
With the DC supply off, I see pulses appearing on the DSO as soon as I switch on the DMM, so even before it finishes to boot.
It get's even stranger. When I now turn on the DC supply and adjust the output to 150V and higher, pulses appear on the DSO as well.
When I disconnect only the black lead from the DMM, everything is clean again, same when it is off.
Disconnecting only the red lead makes no difference.
Now for the grand finale...
When I reverse the two DMM leads to connect to the supply, and now measure a negative voltage, everything is clean.
The DSO and the DMM are connected to the same powerblock and so have the same earth ground.
The DC supply is grounded through the DSO probe connection. I have a 240VAC 50Hz mains.
[UPDATE] What I forgot to mention is that my DC supply has no capacitor on the output. This is important because just adding a 1uF film capacitor fixes the problem. This is why most of you problably won't see it when there is some capacitance involved.
I'm puzzled by this is. Luckily I found the cause because I already spend several hours trying to find the problem in the DC supply.
Has anybody seen this, can verify this or provide an explanation?