Siglent scope question? - Page 1
Forum section: electronics / testgear

author: fastguido
author_rank: Regular Contributor
timestamp: November 14, 2020, 04:55:52 pm
content: I have always used an analog scope in my 50 years of playing with electronics, but decided to move to 2020 and purchased a digital scope, a Siglent 200mhz, four channel (SDS-1204X-E). My question to people that are familiar with this type of scope, is there a way to show a readout of a DC voltage? It seems to be able show everything else, like pk-pk, period ,etc. But for the life of me can't see a way to show a displayed DC voltage? Even the old Sencore SC-61 could display DC voltage. I can't believe that a device that can measure most any waveform can't show this. What I'm I missing?

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2020, 05:14:41 pm
content: Quote from: fastguido on November 14, 2020, 04:55:52 pmLook in the on-screen measurements for "maximum".Quote from: fastguido on November 14, 2020, 04:55:52 pmSome DSOs have this function but oscilloscopes are normally used to measure things like peak-to-peak voltage, RMS voltage, etc., not a static DC voltage.For DC voltage you can use a multimeter (and it'll be a lot more accurate than the nominal 5% accuracy of a basic DSO).

author: fastguido
author_rank: Regular Contributor
timestamp: November 14, 2020, 05:49:42 pm
content: I will check out this "maximum", just doesn't sound like DC to me. I still can't believe that with modern scope that seems like it can measure everything under the sun, can't measure dc on a readout. I was hoping that I could probe a point and know the pk-pk, RMS, etc. and the dc voltage at the same time on a readout

author: 2N3055
author_rank: Super Contributor
timestamp: November 14, 2020, 06:39:11 pm
content: "mean" measurement should be DC component of the signal.."Stdev" is AC RMS, "RMS" is AC+DC RMS..

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2020, 07:47:06 pm
content: Quote from: fastguido on November 14, 2020, 05:49:42 pmIf it's a steady DC voltage then "maximum" is the difference between zero and the trace shown on screen.(or use "minimum" if the voltage is negative...)

author: fastguido
author_rank: Regular Contributor
timestamp: November 14, 2020, 08:05:37 pm
content: Thanks, I will check out those.

author: StillTrying
author_rank: Super Contributor
timestamp: November 14, 2020, 08:13:14 pm
content: Quote from: fastguido on November 14, 2020, 05:49:42 pmIt is, keep the trigger on AUTO or use the trigger source AC LINE to keep the DC trace and auto measurements updating. Of course the resolution isn't very good for DC measurements, there's only 200 levels across the full height of the screen.

author: radiolistener
author_rank: Super Contributor
timestamp: November 15, 2020, 01:58:11 am
content: Quote from: fastguido on November 14, 2020, 04:55:52 pmYes, it can measure AC and DC voltage, include peak, peak-to-peak, mean, RMS.You can do it with Measure button. Just select parameters which you're needs to see on the display and it will be displayed.Here is example.First screenshot - 4.9973 V from lab PSU.Second screenshot - 30.020 V from lab PSU.As you can see it's not so precise like good multimeter, but still usable for voltage measurement.

author: mawyatt
author_rank: Super Contributor
timestamp: November 15, 2020, 03:27:25 pm
content: Quote from: fastguido on November 14, 2020, 04:55:52 pmSimilar background of playing with scopes for ~60 years, although Electronics R&D was my career. Have a couple old analog (Tek 2465) scopes and decided to get a new DSO, ending up with a SDS2102X Plus. Don't know if the basic design and components are the same as SDS1204X-E, but the 2102 has proved to be an exceptional performer, including measuring DC accurately.Since I don't know the exact design (2102) but believe that this has the offset control injected well before the ADC and this offset is from a somewhat precision DAC, so you can place the input trace in DC couple mode near the upper range of the display using the scale and offset. This allows the ADC to "see" an input that utilizes most of it's available range, thus achieving higher effective resolution. What is happing is that the offset DAC is subtracting a "known" value from the input signal before this gets to the ADC, and allowing a smaller V/div scale factor, gain if you will, to be employed without saturating the ADC. Making this work in favor of accurate good resolution DC measurement requires an accurate and stable ADC and DAC as well as the input scaling amplifiers/attenuators in the DSO front end, apparently the SDS2102X Plus has these.An simple interesting measurement experiment setup utilizes various precision DC references (we've developed a few custom designs based upon the LM399, LTZ1000 & others) and a high accurate & resolution DVM (using a new KS34465A and an old Agilent 34401A to verify readings).Here's some of the results achieved using Statistics Mean:KS34465A (34401A) DSO (C1 Mean)V10DC 9.99998 (9.99998) 9.99567VVLM399 7.04192 (7.04192) 7.04170VVREF01 10.00008 (10.00008) 9.99998VVADR 9.99996 (9.99997) 9.99805VV1DC 0.999969 (0.999972) 997.04mvMaybe some folks could run this simple test on SDS1204X-E and other DSOs and report the results. Here's some images of the simple setup, and a screen capture. Hope this helps.Best,

author: DL2XY
author_rank: Regular Contributor
timestamp: November 15, 2020, 04:38:30 pm
content: QuoteHere it is. Scope measurement and multimeter on "Mean", since 121GW leads injected about 40mVpp impuls noise.SDG6052 SDS2504 121GW===========================10.000V 9.9896V 9.989V5.000V 5.0028V 4.9999V2.000V 1.9999V 1.9981V1.000V 1.0012V 0.9996V0.000V -0.0004V 0.0001V-1.000V -1.0021V -0.9991V-2.000V -2.0026V -2.0006V-5.000V -5.0072V -4.9994V-10.000V -10.004V -9.983V

author: fastguido
author_rank: Regular Contributor
timestamp: November 15, 2020, 04:42:56 pm
content: Quote from: radiolistener on November 15, 2020, 01:58:11 amSo "Mean Measurement" is DC voltage? In the manual it states "Average of all data values" who would of thought that is DC, I guess am not that smart.

author: DL2XY
author_rank: Regular Contributor
timestamp: November 15, 2020, 05:12:29 pm
content: Yes, Mean is the DC-Part of any mixed Voltage, including pure DC of course.Stddev is the AC-Part, expressed as RMS value.Pk-Pk is the AC-Part, expressed as peak to peak value.RMS is the effective Voltage, wich would produce the same power as a pure DC-Voltage of that value.Considering in real world there is no pure DC (some noise is omnipresent) it may be a big advantage to use the a scope for precise elaboration of DC-Voltages.

author: Fungus
author_rank: Super Contributor
timestamp: November 15, 2020, 05:16:32 pm
content: Quote from: fastguido on November 15, 2020, 04:42:56 pmNo, it's the arithmetic mean of all the values in the trace being shown on screen.If you can arrange it so that the trace is a flat line representing your voltage then the arithmetic mean of that line is the value you were interested in.

author: 2N3055
author_rank: Super Contributor
timestamp: November 15, 2020, 05:45:52 pm
content: Quote from: Fungus on November 15, 2020, 05:16:32 pmWhich is exactly DC component of anything on the screen, including DC voltage...Or shall I say, if you were to connect multimeter set to DC to the same signal, that is what you will get...

author: fastguido
author_rank: Regular Contributor
timestamp: November 15, 2020, 05:57:55 pm
content: Quote from: Fungus on November 15, 2020, 05:16:32 pmGetting confused, If you have straight line, pure dc on the scope, is there a mean then? Isn't it just straight dc?

author: Fungus
author_rank: Super Contributor
timestamp: November 15, 2020, 06:31:26 pm
content: Quote from: fastguido on November 15, 2020, 05:57:55 pmIf the 'scope is AC coupled then the trace will be at zero and mean should be 0.0.If it's DC coupled then you see the voltage.

author: tautech
author_rank: Super Contributor
timestamp: November 15, 2020, 06:50:34 pm
content: Quote from: fastguido on November 15, 2020, 05:57:55 pmMean is the total measurement of that flat line trace and any variations in itabove0V.There is always some little noise in the trace and that accounts for for small errors in measurement.

author: fastguido
author_rank: Regular Contributor
timestamp: November 15, 2020, 07:37:51 pm
content: So if there is variation, some noise on top of the dc, then mean is dc plus the amount of noise?

author: tautech
author_rank: Super Contributor
timestamp: November 15, 2020, 07:44:24 pm
content: Quote from: fastguido on November 15, 2020, 07:37:51 pm+the system noise. It can be lessened with averaging.

author: Fungus
author_rank: Super Contributor
timestamp: November 15, 2020, 10:19:21 pm
content: Quote from: fastguido on November 15, 2020, 07:37:51 pmMean will be half way between minimum and maximum for truly random noise.It's probably what you want for "DC voltage".

author: rf-loop
author_rank: Super Contributor
timestamp: November 16, 2020, 03:28:18 am
content: Quote from: fastguido on November 15, 2020, 05:57:55 pmThere is not straight line, pure dc on the scope. And if we are tight whole world do not even exist pure DC.But back to practice.Make one tiny exercise.Set scope for Ch1 and 100us/div (for keep 1GSa/s and 1.4M memory) and 500mV/div. Connect pure clean1.5V DC (example battery) to scope input BNC and do it so that it can not get polluted with any external noise.Look this trace. It looks straight. Ok.Then Stop scope.Zoom in vertically using V/div and zoom in horizontally using t/div.Turn also display display to dots mode if it was not already.Tell me where is DC. Tell me where is straight line.Simply. Oscilloscope do not see any DC and also it do not show DC to you. Usual DMM can lie to you that there exist some DC. It is lie and you believe it. Yes in practive we can think it is "DC" (what do not even exist in this world - except in students "ideal" books).And if you want look through whole memory length more accurately take it out from scope (.csv) and I am sure you can not find constant value there over whole memory. (it takes while when it load it to USB flash and this was reason why I previously as these scope t/div settings - for get lot samples but still amount what can produce .csv file for computer what you can look and handle there.) and open it then on computer.DC with some random noise including scope added random noise.Best is use "Mean" or RMS (Root Mean Square).If you are interested about this noise what is mixed with this DC. It can measure using Stdev. It remove this average DC out and show RMS value for this noise over DC.Now if your DC (with scope "eyes") include some peaks etc what are not symmetrical around this "DC" then Mean may give wrong result. In this case of course can use average, mean, rms, or what ever but some times it is best that human eye and brain analyze it without blind trust numbers.There is one method what also can use with your scope.Example for 1.5V you can use example 50mV/div or some other. Then use Offset for get it visible just so that it is on base level (GND level). Read offset value, it is your "DC" value if you did all ok. If scope is well calibrated you can find that with Offset you have much more resolution than just scope main ADC 8 bit. Now with this more vertical sensitive trace you can also, with your own eyes, look what is there this "DC" level and see there also these possible "riding" peaks or some other "noise". So you can use this vertical zoom for get more resolution for things around this "DC" and also you have Offset DAC whole resolution what is Lot of more than 8 bit for know this "DC" level.

author: radiolistener
author_rank: Super Contributor
timestamp: November 16, 2020, 08:08:09 am
content: Quote from: Fungus on November 15, 2020, 05:16:32 pmBut when you're measure arithmetic mean of a signal, you will get DC value.Mean value is what the multimeter shows you, when you choose DC measurement.

author: radiolistener
author_rank: Super Contributor
timestamp: November 16, 2020, 08:14:06 am
content: Quote from: fastguido on November 15, 2020, 07:37:51 pmNo, mean is always DC voltage.But you're needs to make sure that you're using DC coupling mode for the input.Because when you use AC coupling mode, DC value will be removed from the signal and then mean will show you value close to zero (some amount of DC still will be there for a complex signal).

author: fastguido
author_rank: Regular Contributor
timestamp: November 16, 2020, 03:21:36 pm
content: So if mean is always DC voltage, then why is it not labeled that way? Also is it possible to display, say two or more channels of "mean" on the readout, it appears only one channel at a time?

author: tautech
author_rank: Super Contributor
timestamp: November 16, 2020, 07:10:15 pm
content: Quote from: fastguido on November 16, 2020, 03:21:36 pmIt doesn't.For your use case engage Mean, Max and Min and compare the measured result with a DC voltage then the probe compensation signal. The difference in each should be clear if applied to a waveform or a static voltage.QuoteYes of course you can, you just need to assign any measurement to the channel you need it measuring.

author: radiolistener
author_rank: Super Contributor
timestamp: November 17, 2020, 02:09:55 am
content: Quote from: fastguido on November 16, 2020, 03:21:36 pmBecause DC doesn't exists in real world. All signals are actually AC in real world.You can say that some AC is DC only for a limited time period. For multimeter that time period is fixed. For oscilloscope that time period depends on time scale selection.To avoid confusion it is named "mean" to display the actual function which is used to obtain it. Because "DC" can't tell anything what is it exactly.Since you're planning to measure "DC", it doesn't matter for you. Because such difference is important for AC signals which amplitude is high enough.