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Siglent SDS1104X-E Quick Cal Question
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Performa01:
Like the majority of digital oscilloscopes, the SDS1000X-E has an 8-bit acquisition system.

One division on the display happens to be equivalent to 25 LSB of the ADC.

So what vertical gain did you use to measure the 3 Vpp? It probably was at the optimum of 500 mV/div, then one LSB is equivalent to 20 mV. This would explain why you can see non-integrative measurements like peak-peak only in steps of 20 mV.

Why it measures 3.08 V or 3.10 V and not closer to 3.00 V (+/- 0.02 V), as suggested by your other measurements, that is another question. But we'll try to find an answer...

I pretty much expect that it would measure more accuratey with a direct coax connection or x1 probes.

Have you noticed that the measurement error on a x10 probe depends on the accuracy of both the probe and scope input impedance?
For most passive high impedance probes, accuracy of the input impedance is not specified. Your guess about the worst case error is as good as mine.
Then the input impedance of the scope itself. The specification says 2% - and that is the minimum additional error margin for such measurements.

Out of curiosity I've checked an old PP510 probe with the 12-bit SDS2000X HD. For this instrument, the input impedance accuracy is still only 2%, but at least the DC accuracy is specified as 0.5%, so it can easily compete with the average 3.5 digit DMM.

in x1 mode, we get 3.018 V amplitude; so the voltage of the calibrator output is only off by 0.6%. Even though its tolerance is not specified, this output is clearly produced by some 3.0 V logic circuit and modern voltage regulators are just accurate like that.

In x10 mode, we get 301.25 mV amplitude, so one might come to the wrong conclusion that the error is now even lower at only 0.417%. No, it is not, because the true calibration signal amplitude is not 3.00 V! Yet the error because of tolerances in the impedances of both probe and scope do not appear to be nearly as bad as expected.

Have a look at the attached screenshots for more information on this test.

In any case, this little experiment should tell us several things:

* For calculating the total error margin, we need to take the probes into account
* Passive high impedance probes often do not have any specifications that allow the estimation of the additional error margin, but the tolerance of the scope input impedance is already high enough to rule out the guarantee for precise measurements with any probe other than x1
* Measurements statistics are your friend whenever accuracy is important. Always use the statistic mean values over a number of measurements
* Most important: select the right measurement for the task! Peak-Peak is not going to give the right answer to the question about the amplitude of a square wave, because it will include overshots, ringing and noise

TomKatt:
Point taken - I will test that out later this afternoon.

The 3.10 measurement was derived from an original default state with a compensated probe set to 10X connected to the calibration terminals and then pressing the AutoSet button, which set the scope to  500mV /d @ 200us /d.  Pressing Measure brought up the PP volt parameter at the bottom, which indicated 3.10 - 3.08.  The scope had been running in excess of 30 minutes prior to testing.  Basically, I followed the Probe Compensation procedure as described in the operation manual.

But I should test other amplitude and X1 settings to see if that influences the result.

Thanks both for the wisdom and insight.

Edit - although the SDS1104X-E may be among the less expensive scopes in it's class, I suspect that it is likely more accurate than any of the gear I own.  Which is what kind of surprised me and I will not be shocked to discover my interpretation was all PEBKAC.

Dbl Edit - I should probably also use the probe ground spring instead of the alligator clip "antenna"...
bdunham7:

--- Quote from: TomKatt on June 28, 2022, 02:59:48 pm ---The 3.10 measurement was derived from an original default state with a compensated probe set to 10X connected to the calibration terminals and then pressing the AutoSet button, which set the scope to  500mV /d @ 200us /d.  Pressing Measure brought up the PP volt parameter at the bottom, which indicated 3.10 - 3.08.  The scope had been running in excess of 30 minutes prior to testing.  Basically, I followed the Probe Compensation procedure as described in the operation manual.
Edit - although the SDS1104X-E may be among the less expensive scopes in it's class, I suspect that it is likely more accurate than any of the gear I own. 

--- End quote ---

Using the 10X probe is OK except it does give you one more potential source of a scaled error and results in more overall noise.  But I suspect that the vast majority of your 'error' is simply that the Pk-Pk value includes overshoot plus any noise within a BW of 200MHz, along with the fact that the minimum resolution (ADC step) is 20mV.  This is why you don't see 3.09V.  There are 500,000 samples in each of those 1cm or 1div lines at the top and bottom of the square wave.  Try zooming in on one down to 5ns/div or so.  As far as accuracy it depends on what you are measuring.  For DC, your DMM will be leaps and bounds more accurate.  For duty cycle, pulsewidth, time, etc, the scope is virtually perfect for most applications.


--- Quote ---I should probably also use the probe ground spring instead of the alligator clip "antenna"...

--- End quote ---

Not really, not for this.  In fact, when you are hooked to the calibrator, you don't need any ground connection at all.  But in general, no matter what you do noise will be with you always--so it is better to first learn how to deal with it using scope settings. 
Performa01:
Even though noise shouldn't be a major issue at 50 mV/div (=500 mV/div @ x10), it still affects measurements once we get in the realm of <1% error.

My former test was in normal acquisition mode and with no bandwidth limit, i.e. the full 570 MHz -3 dB bandwidth of the SDS2504X HD.

Now I have repeated it with ERES 2.0 acquisition mode, which provides additional four bits of resolution and a 20 MHz bandwidth limit.

Lo and behold, the amplitude is now almost spot on at 3.00944 V! This is just 0.32% higher than the nominal 3 V, for which no accuracy specification exists. The peak-peak measurement at 3.027 V is still off by about 0.6% - but this is certainly very dependent on the quality of the state of the probe compensation.

In x10 mode, we can see a lower voltage again, hinting on a probe factor slightly less than 10. The measurement of the 299.614 mV amplitude hints on a probe error of about 0.44% and the peak-peak voltage is way off at 302.557 V, which corresponds to an error of almost 1%!


bdunham7:

--- Quote from: Performa01 on June 28, 2022, 05:02:04 pm ---Even though noise shouldn't be a major issue at 50 mV/div (=500 mV/div @ x10), it still affects measurements once we get in the realm of <1% error.

--- End quote ---

After the OP has a chance to run his own test, I'll post a screenshot of me doing it on my scope.  At the settings discussed, a 0.1V error is 5 LSB combined on Pk-Pk readings.  Actual noise from all sources would only need to be 2-3 LSB peak to account for what he is seeing.  Also, it does look like Siglent has intentionally made the calibrator output pretty much exactly 0-3V.
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