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Question asked by glowitz on Jan 11, 2012
Latest reply on Jan 16, 2012 by flanguasco
I'm getting to know my new MSOX3024 scope and have started some basic tests, leading to this question about the ARB DC Offset. Specifically, if you set the ARB to DC mode, you can adjust the control **** to set a fixed DC output level. When set to Hi Z mode, this is +/- 2.5V (resolution depends on range, but varies from 100 uV to 10 mV). That said, at the zero setting, I get a 100 uV offset error - and this grows to nearly 13 mV of error at the extremes of +/- 2.5V. I can't seem to find the specs for the integrated ARB -- maybe I missed it in the manual....but is that the expected behavior?

You are asking what I'm using to measure the output. Well, I measured it three ways. First, I measured using my new Agilent 6.5 Digit DMM (34411A). When I saw the error, I took out my Fluke 233. While the resolution of the Fluke is less than the Agilent, it essentially matched the Agilent DMM exactly. It showed the same 13 mV error at the extremes, as well as the 100 uV offset at zero volts. Finally, I measured it using the scope input (channel 1) itself. Looking at the line, the scope matched the DMM readings very closely.

Therefore, I can only conclude that the ARB either isn't that accurate -- or the output is not calibrated properly. This license was added post sale by the distributor, so I don't know if a recalibration is required or if this is tested prior to shipment to the distributor -- even if the ARB license isn't enabled.

I also wonder if the DC offset is part of the core ARB DAC or whether this is a separate DAC just for DC offset, independent of the ARB.

In any case, my expectation is that when I set the ARB to 2.5V, I should measure 2.5 V to within a fraction of a millivolt. Similarly, when it's set to zero, I would expect it to be pretty dead-on close to zero. Attached is a PDF of my measurements. I tried a similar set of measurements using the 50 ohm mode, and it just cut the levels in half -- but still showed the same offset issue.

My test setup was simply to use a dual BNC splitter on the output of the ARB -- one channel going to the Agilent DMM, and the other channel going to the Fluke or Channel 1 of the Scope. The attached measurements were Hi-Z from end to end. Again, going to 50 ohms didn't eliminate the issue, just cut the levels in half.

The attached results show a DC offset accuracy of approximately 0.5% at the extremes, getting worse as you approach zero (in terms of DC offset % error vs. target DC offset).

I'm curious what others who have this option are seeing and what can be done to calibrate this without having to ship it back to the service center, which would be a hassle for me.

I do have some additional questions related to the ARB and FFT measurements that I will post in a separate thread later.

Thanks in advance for your ideas. I love the product -- just trying to understand what to expect and where the limits are.