Some questions regarding my Siglent SDS1104X-E - Page 1
Forum section: electronics / testgear

author: KaLi
author_rank: Newbie
timestamp: November 18, 2019, 09:40:37 am
content: Hi there,I recently bought that oscilloscope. Because of job-works I put it aside and last weekend I finally had some time to test it a little.And I found some strange behaviour... maybe that is normal. Some other owner or more advanced oscilloscope user can help me with my question.Test Setup: Channels are set to AC-coupling. Input is 1 MHz sine with 10V amplitude splitted to CH1 and CH3 -- and for the second test to CH2 and CH4 (because CH1/CH2 and CH3/CH4 live on the same amplifier circuits) -- initially I wanted to test/compare the amplification of the channels and observe the noise.The osci-measured Vpp of each sine-pair is okay and comparable. But CH1 has an offset of -55mV vs. CH2 +115mV which sets both curves visually apart, as you can see in the first attached picture. In the second test CH2 is roughly where it should be (0V), but CH4 has an offset of about +100mV -- it's not as visible, but you can see it in the second pictures.Of course I switched cables and eliminated every possible aspect outside of the scope.Because of AC coupling, sine waves and reasonable high frequency -- I would have guessed, that there shouldn't be any offset to the plotted curves at all.Any ideas?BTW: Until now nothing is hacked, the scope is in "just unpacked" condition. ;-)

author: Fungus
author_rank: Super Contributor
timestamp: November 18, 2019, 12:01:29 pm
content: Quote from: KaLi on November 18, 2019, 09:40:37 amHave you done a self-calibration?

author: KaLi
author_rank: Newbie
timestamp: November 18, 2019, 12:50:28 pm
content: Yes... always the "long" self calibration after at least half an hour of heat-up time. The osci is powered on since this morning (Germany) to work as stable as possible.What I learned just now: All "measurement calculations" (measure menu) seem to be made on a kind of visual level from the display pixels. If I change the trigger or shift the timebase the values for the measured "mean" values change... as do the stddev values if I change the volts/div. I really thought there would be a value-buffer from which the calculations as well the display readout would be fed. Not a problem... but good to know.But the difference between the mean-values of CH1 and CH3 stay -- the only shift.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: November 18, 2019, 03:23:54 pm
content: Well, if my understanding is correct, each channel has its own amplifier. What is shared by Ch 1 and 2 (and Ch 3 and 4) is the A/D converter. The A/D converter in this oscilloscope is 8-bit. Assuming the total range for 2 V/div is 16 V (so it can display a full wave on the screen without clipping), one LSB would be 16,000 mV / 256 = 62.5 mV. The range of the A/D converter might be higher though, so an LSB might be higher too.I would not expect a better precision for this instrument, but I could be wrong. I can check mine tonight if it helps, and compare it with other similar (but supposedly better) oscilloscope (Keysight). What probes are you using?

author: tautech
author_rank: Super Contributor
timestamp: November 18, 2019, 07:11:21 pm
content: Additional comments if not already done:Ensure probes are carefully compensated.Assign channel identity rings to probes.Ensure the latest FW is installed....V6.1.33

author: rf-loop
author_rank: Super Contributor
timestamp: November 19, 2019, 04:14:16 am
content: Quote from: Caliaxy on November 18, 2019, 03:23:54 pmUsing 2V/div signal p-p abs max range (ADC full range) is roughly 20V. Display range is 8 div (16V @2V/div). It can easy look if set signal level small*)amount over display vertical range and then stop and zoom out vertically.*) not to amplifiers overdrive levelAlso it need note that different channels may have small differences between linearity and freq response etc. When channels are in AC coupling and compare different channels signal vertical positions, signals shape (symmetry) also need be very equal (much better than eye can detect on the scope screen). Even after then there can be some small error due to internal differences in bias, linearity or is better say that vertical total performance including all differences between channels every parameters (data sheet give tolerances and... these numbers need take seriously without "blue eyes" and yes all what you see on what ever oscilloscope screen is total sum of all partially unknown errors summed to imagined thruth)Also for example measuring ripple over DC it is not always best way to use oscilloscope internally AC coupled. Some times it is better stay DC coupling and use internal vertical shift (it can also lock for fixed position or fixed offset). It depends... This scope AC coupling corner freq is under 2Hz. Some times need less or some times more, so some times it is (if need different corner freq.) better to do externally depending situation and keep scope DC coupled.(also it is good to note that trigger coupling AC/DC is totally different story because it is done in digital side after ADC (because this scope have true full digital side trigger engine ) and it do not care at all if analog side is AC or DC coupled. Trigger engine "know" only ADC output and this Trigger coupling AC or DC is just done with DSP, in this case digital High Pass Filter)

author: Fungus
author_rank: Super Contributor
timestamp: November 19, 2019, 05:20:07 am
content: Also remember that these aren't precision measuring instruments, they're for seeing shapes. Accuracy is only about 5% for voltage.

author: KaLi
author_rank: Newbie
timestamp: November 19, 2019, 01:12:48 pm
content: Hi,I've used BNC cables for this test... and I made this also with 10kHz with the same result (yesterday). I've connected everything via some BNC-T connectors where I also had connected my trusty multimeter (to compare measured DC voltages and some RMS voltages). The CH1/CH3 or CH2/CH4 got the same signal out of one BNC-T -- and I switched the connectors to check if the cable made a problem. This was my setup for various tests.I have to say: I had the scope powered on (with some restarts and several auto-calibrations) yesterday around the day and had the same "wrong" results yesterday evening. This result was very consistent, otherwise I wouldn't have started this thread.BUT... today this problem has vanished. This morning the oscilloscope worked correctly and the curves match (within the 32values/DIV of the ADC - some quantization noise nevertheless).Of course I'm well aware of the limitations of this 500 EUR instrument. But I like to check my equipment to know the limitations and to not run into false expectations of its capabilities. And this offset-problem nagged on me, because the scope has no user-calibration settings for offsets, amplifications, etc. -- so the autocalibration has to work correctly. I would think, even with its 500 EUR the oscilloscope should be good for quantitative comparison of signals and not only seeing shapes ;-)

author: Fungus
author_rank: Super Contributor
timestamp: November 19, 2019, 01:29:27 pm
content: Quote from: KaLi on November 19, 2019, 01:12:48 pmWithin a few percent, sure.It will vary with temperature, too. Remember to recalibrate when the seasons change...

author: Performa01
author_rank: Super Contributor
timestamp: November 19, 2019, 03:13:36 pm
content: Digital measurements always have an uncertainty of +/-1 LSB.In the SDS1000X-E series, one vertical division is equivalent to 25 LSB - not 32. You get some headroom beyond the visible screen area.At 2V/div vertical gain, one LSB is 2V/25 = 80mV.What I can see from your screenshots is a 176mV difference for channels 1-3, which is about 2 LSB and can be considered the worst case (-1 LSB, +1 LSB) and some 100mV difference for channels 2-4 (about 1.25 LSB).So even the results from yesterday were well within specification (3.0% +/-242mV at 2V/div). The mean value of the signal is supposed to be zero, so the 3% do not apply, but the specified offset error of +/-242mV (+/-3 LSB) remains. Other 8 bit scopes have similar specs, even when they cost a lot more than 500$.Another thing is the offset change with timebase and/or trigger level. This always happens even with a perfectly symmetrical waveform if the number of periods within one acquisition is not an exact integer number. Only then the mean value would cancel out to zero no matter of the signal phase.This scope always measures from the full record length, up to 14Mpts.As you can see in the screenshots, the record length is 7kpts and represents a time span of 14 * 500ns = 7µs, which should result in precisely 7 periods of a 1MHz signal. The trigger frequency counter measures 999.986kHz. So there are slightly less than 7 full periods, yet not a big deal.At 200ns/div the captured time span would be 2.8µs and the error would be much bigger. With this setting, the DC offset would be extremely sensitive to the signal phase, hence change with the trigger level setting noticeably. Keeping the trigger point in the center of the x-axis and trigger level zero (for a symmetrical AC coupled signal) as well as using slower timebase should help to minimize errors for DC offset (mean) level measurements.Apart from all this, the even numbered harmonics in a signal also cause a corresponding DC offset, but this should be blocked by the AC coupled inputs.