Hi there,

I have Rohde & Schwartz RTM3004 Oscilloscope.
I am supplying constant DC voltages to 4 channels of the scope from HMP Power Supply.
These are the voltages: 0.5, 1, 1.5, 2, 2.5, 3, 3.5 ,4 V. I also checked these voltages with the high quality DMM.
I use the Mean Value option in the scope to measure these voltages.
What I am experiencing is that I get different measurement results of the same supplied voltage on different channels with different probes.
I have also noticed that the location of the signal on the scope's display also affects the results.
For example, the measurement for a signal at the bottom of the screen may be slightly (or significantly) different from that for the same signal at the center or top of the screen.
I decided to record the differences between supplied and measured voltage for each channel for each state (bottom, center, top of the screen).
I calibrated both the scope and the probes before I started recording the differences, and I still got up to 15-20 mV difference.
I have crossed probes and channels to see if there is a problem with the specific channel/probe, but it does not seem to be a consistent behavior.
The 15-20 mV offset is large for my purposes.
I am trying to understand if these voltage offsets are acceptable and within specification of the product or it is not and if so, how can I fix it.
I referred to page 5 on the specification document of this product:
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/RTM3000_dat-sw_en_5214-9144-22_v1100.pdfIf I got this correctly, I want to calculate
DC measurement accuracy, for that I have to calculate:
DC measurement accuracy =
DC Gain Accuracy × |
Read - Net Offset| +
Offset AccuracyI calculated Offset Range at 1 Mohm = ±22.5 [V], and the Offset Accuracy = 0.0525. (Does that mean 5.25%?)
But I am not sure about two parameters:
DC Gain Accuracy and |Read - Net Offset|
• The first parameter is DC gain accuracy.
We use an input accuracy (vertical scale) of 500 mV/div. According to the specification, Offset and Position should be = 0. However, in my case, the value of offset depends on what we need to measure but is usually not 0.
So, I am not sure if ±1.5% is the correct DC gain accuracy in my case.
• The second parameter I do not know is the |Reading - net offset|.
It looks that this parameter is not mentioned at any other place in the Document.
Can someone help me with this issue?
Thanks a lot
