DC Voltage Measurement Error on the Oscilloscope  - Page 1
Forum section: electronics / testgear

author: vanorudik
author_rank: Newbie
timestamp: September 13, 2023, 08:37:41 am
content: 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 calculateDC 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 , 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

author: Someone
author_rank: Super Contributor
timestamp: September 13, 2023, 08:53:44 am
content: I think your confusion is in combining the two errors/accuracy specifications. |Reading - net offset| takes the offset out of the gain multiplier. To simplify:gain error (typ 1.5%) multiplied by signal away from offset+offset error (0.1 div × input sensitivity + 0.5 mV + offset * 0.5%)=total worst case specified error

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2023, 09:31:40 am
content: That is at the input. There will also be a gain error due to the voltage divider between probe and input (this divider is both resistive and capacitive).I'd get rid of the probes and use a direct connection or a precission divider. But my guess is that there will be higher frequencies involved so frequency dependant behaviour must be considered as well.

author: vanorudik
author_rank: Newbie
timestamp: September 13, 2023, 10:20:33 am
content: Thank you very much for your reply.It makes sense.Did I understand correctly that the "signal away from offset" you mentioned is thedeltabetween theinput voltage valueand the value of thereal signal location on the scope(bottom, center, top of the screen)?

author: Someone
author_rank: Super Contributor
timestamp: September 13, 2023, 11:56:44 am
content: Quote from: vanorudik on September 13, 2023, 10:20:33 amNo, there is nothing about position on screen in those calculations. offset here is the signal offset (not some display offset as some other manufacturers have) which is displayed by the scope as you turn/adjust the offset control.|Reading - net offset| = distance from offset voltage

author: vanorudik
author_rank: Newbie
timestamp: September 13, 2023, 06:18:54 pm
content: Okay, thanks for clarifying.Just wanna make sure, that I understand correctly:The offset (from theoffset errorformula) is the parameter of the input signal, and at constant DC voltage it is always zero?Whereas the "net offset" is the offset of signal from the zero line?

author: harrimansat
author_rank: Regular Contributor
timestamp: April 07, 2025, 07:50:14 pm
content: there´s some tektronix probes with DC gain adjust