DC bias (or offset) adjustment availability - Page 1
Forum section: electronics / testgear

author: taydin
author_rank: Frequent Contributor
timestamp: November 25, 2023, 11:20:21 am
content: The Rigol DHO4000 has a DC bias feature in the front end, where a voltage offset can be specified, which will then be subtracted from the input signal. This is a god sent feature for ripple and noise analysis in power supplies, and normally requires expensive power measurement probes that have built in DC offset adjustment. Here in the forum, I am seeing people talk about this feature being available in Siglent scopes as well, but I have looked into the mauals of several models and I can't see this feature being present. The only offset is the regular screen display offset, which is available in pretty much all scopes. So, is this DC bias thing a unique feature of the Rigol DHO4000 only?

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 12:13:39 pm
content: Having separate DC offset and vertical position controls is also common on all but the entry-level Tektronix scopes. I believe that Siglent, like Lecroy, couple DC offset to the vertical position knob. So turning the vertical position knob changes the DC offset in the vertical amplifier, or in an active probe that allows controlling the offset from the scope.Look at the DC offset range of those scopes which will be much larger than the vertical position range on the Rigol.

author: taydin
author_rank: Frequent Contributor
timestamp: November 25, 2023, 12:45:01 pm
content: I'm looking at the channel controls of my SDS1104X-E. There is a DC offset configuration for each channel, but it is tied to the vertical position control knob. I had a Keysight DSO9104A and this scope didn't have a distinct DC offset configuration, either, only vertical position. So if this is so common, can you tell me a scope other than DHO4000 that has separate DC offset for the front end and separate vertical position control?

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 02:38:57 pm
content: Like I said, it's common on all but entry-level Tektronix scopes, like the TBS-2000b series for example. Even my old Tek TDS-3000 scope has offset adjustment separate from the vertical position. Their lowest end scopes may not have offset adjustment at all. I'm not sure about the TBS-1000 series for example.It's a design philosophy thing, like having one vertical knob per channel vs a shared knob. The other scopes like Siglent do have a way to adjust offset, it's just controlled differently.

author: Fungus
author_rank: Super Contributor
timestamp: November 25, 2023, 03:12:38 pm
content: The Rigol DHO800 has it too.(and all the rest of the DHO range...)

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2023, 05:33:04 pm
content: Quote from: taydin on November 25, 2023, 11:20:21 amDC bias and vertical offset are the same thing. Only the representation on the display is different. You won't find two different signals to set the DC offset / vertical offset when you are going to look for them in the analog frontend.

author: rf-loop
author_rank: Super Contributor
timestamp: November 25, 2023, 07:41:31 pm
content: Quote from: taydin on November 25, 2023, 11:20:21 amNo! Not at all. How can even think it.The lack of this DC offset, if this lack exists, is a fatal design failure if we are talking about normal general-purpose oscilloscopes that are not in the special class named: Toys.Offset Range: Rigol DHO4000± 0.5 V <500 μV/div1)± 1 V ≥500 μV/div1)- ≤65 mV/div± 10 V >65 mV/div - ≤270 mV/div± 20 V >270 mV/div - ≤2.75 V/div± 100 V >2.75 V/div - ≤10 V/div1) 100 μV/div, 200 μV/div, and 500 μV/div are a magnification of 1 mV/div setting.Offset range: Siglent SDS2000X HD±1.6 V 500 μV/div - 5 mV/div±4 V 5.1 mV/div - 10 mV/div±8 V 10.2 mV/div - 20 mV/div±16 V 20.5 mV/div - 100 mV/div±80 V 102 mV/div - 200 mV/div±160 V 205 mV/div - 1 V/div±400 V 1.02 V/div - 10 V/divOffset range: Siglent SDS2000X Plus± 2 V 500 μV/div - 100 mV/div± 20 V 102 mV/div - 1 V/div± 200 V 1.02 V/div - 10 V/divOffset Range: Siglent SDS1000X-E± 2 V 500 μV/div - 118 mV/div± 20 V 120 mV/div - 1.18 V/div± 200 V 1.2 V/div - 10 V/divAnd also in many many other models and brands.

author: taydin
author_rank: Frequent Contributor
timestamp: November 25, 2023, 07:52:18 pm
content: Quote from: nctnico on November 25, 2023, 05:33:04 pmMy understanding is that the DHO4000 DC bias setting removes that amount of DC voltage from the input signal. That way, you can set the DC bias to 9V and then monitor the output of a 9V SMPS using your oscilloscope's input, and only see the ripple/noise and very little DC, whatever is leftover from the adjusted DC bias and the actual output voltage.But the vertical position knob doesn't remove anything. It just moves the image on the screen up and down.

author: rf-loop
author_rank: Super Contributor
timestamp: November 25, 2023, 07:57:07 pm
content: Quote from: taydin on November 25, 2023, 07:52:18 pmYes, same as also other oscilloscopes, as example in my previous msg.Now, please think and rethink before next turn...

author: taydin
author_rank: Frequent Contributor
timestamp: November 25, 2023, 08:05:53 pm
content: Quote from: rf-loop on November 25, 2023, 07:57:07 pmI think you are completely misunderstanding what I'm talking about. Please think and rethink before replying ...

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 08:14:19 pm
content: Quote from: taydin on November 25, 2023, 07:52:18 pmTrue.Quote from: taydin on November 25, 2023, 07:52:18 pmOn the Rigol scope, true. On Siglent and Lecroy scopes, no. The "vertical position knob" is actually an offset knob (and labeled as such on Lecroy scopes). See the offset range that rf-loop posted.

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2023, 08:19:21 pm
content: Quote from: taydin on November 25, 2023, 07:52:18 pmYou may think that but if you set the DC offset without a signal connected, you'll see the signal moving up or down. Just as if you adjusted the offset knob. The only difference is that DC offset adjustment is specified in volts and vertical offset is typically expressed in divisions (in case there are seperate controls). However, the limit of the amount of offset you can apply is limited by the offset range as stated in the datasheet. There are some variations on how this has been implemented in the user interface on various oscilloscopes (which you may like or dislike), but without a DC offset setting, the vertical position provides the same function as a DC offset adjustment.

author: taydin
author_rank: Frequent Contributor
timestamp: November 25, 2023, 08:29:30 pm
content: Ok, let's go based on an actual scenario: I have set my signal generator to generate 1 kHz, 200 mVpp sine wave with an offset of 8 V. I apply that to my SDS1104X-E with the input coupling set to DC. I set the vertical offset with the vertical position knob to -8V and reduce the vertical range. And I am hitting the limit where the range is 500 mV/div. So right now, if I measure RMS, it measures about 8 Vrms, And it measures 8 V average.But, if the front end would have removed the 8V offset, I would be able to go to a much lower vertical range (in case the sine wave amplitude was much lower), and I would be able to measure the ACTUAL RMS and average value of the sine wave that's riding on the DC offset. So I don't understand, how are these two scenarios equivalent?

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 08:45:03 pm
content: Quote from: taydin on November 25, 2023, 08:29:30 pmRead rf-loop's post again. Every scope will have a limited offset range, one that usually goes up with the vertical range. Regardless if DC offset is a separate setting from position or not. What you're looking at, is what the offset range is at the vertical attenuation setting you want to use. For example, for an 8V offset the SDS-1000X-E goes down to 152 mV/div, while the DHO4000 goes down to 65 mV/div, and the SDS-2000X goes down to 10.2 mV/div. Looks to me like the SDS-2000X is the superior scope for this particular measurement.

author: rf-loop
author_rank: Super Contributor
timestamp: November 25, 2023, 08:55:12 pm
content: Quote from: taydin on November 25, 2023, 08:29:30 pmOf course. Take first just DC without this riding 200mVpp sine. And it is right. 8V DC = 8Vrms.Now if there IS real signal what is example 200mVpp sine and its DC offset is 8V this 8V offset need keep in calculation even if it is added with oscilloscope offset because we are measuring input signal...But if you want only this 200mVpp sine RMS what have 8V offset... you need take only AC part. You can add -8V and look it on screen. Now if we talk about AC RMS. Mathematically it is Stdev (some call it AC RMS) and this Stdev you can find in Siglent as also RMS.But if you want RMS it is input signal RMS because it need also count added offset. Because RMS need include also DC part. Only AC RMS do not care this DC offset.But really if you adjust example in Siglent vertical "Position" it is just same as DC Offset uou perhaps try talk. In front end analog circuits it really add DC offset to signal in summing amplifier. Offset DC come out based to offset DAC.But oscilloscope really need take count this internally added offset(- or +) because purpose is measure input signal as perfectly as it is possible. In your example 8V offset 200mVpp sine: 8.0003 Vrms.Oscilloscope know it have subtracted 8V if you have set -8V offset. Original signal what we are measuring have it and it need tell in measurement result.RMS is RMS 8V DC if measure RMS result is 8Vrms. But if you measure AC RMS (aka Stdev) it is 0

author: rf-loop
author_rank: Super Contributor
timestamp: November 25, 2023, 09:11:35 pm
content: Quote from: alm on November 25, 2023, 08:45:03 pmfor an 8V offset the SDS-1000X-E goes down to 152 mV/div (typemistake perhaps:102mV/div)ETA: correct value is 120mV/divBut SDS2000XHDhave quite good offset properties.Old discontinued SDS2000X can not same, also SDS2000X Plus can not.

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 09:19:11 pm
content: Quote from: rf-loop on November 25, 2023, 09:11:35 pmIf it's a typo, then the typo was made by Siglent. See page 8 of thedatasheet.

author: tautech
author_rank: Super Contributor
timestamp: November 25, 2023, 09:24:01 pm
content: Quote from: alm on November 25, 2023, 09:19:11 pmYou might consider tests by rf-loop have discovered better. Theone can presume is the clue.

author: rf-loop
author_rank: Super Contributor
timestamp: November 25, 2023, 09:58:13 pm
content: Quote from: alm on November 25, 2023, 09:19:11 pmYour data sheet is from 2017.Also it looks like I made error...120mV/div is correct for SDS1000X-E. (102mV/div is SDS2000X Plus)SDS1000X-E current data sheet; Rev. 04D Aug. 2021And in your link it is this very obsolete (look this address):siglentna.com/USA_website_2014/Documents/DataSheet/SDS1000X-E_DataSheet_DS0101E-E03A.pdfAlso if look Siglent NA, there is current version availablethere is: siglentna.com/wp-content/uploads/dlm_uploads/2021/08/SDS1000X-E_DataSheet_EN04D.pdf

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 10:29:30 pm
content: Quote from: rf-loop on November 25, 2023, 09:58:13 pmHey, I can't help it if Siglent NA leaves old data sheets on their website, showing up for me as the first Google hit when I search for SDS1000X-E datasheet. Apparently even content from official Siglent websites can not be trusted.

author: tautech
author_rank: Super Contributor
timestamp: November 25, 2023, 11:17:24 pm
content: Quote from: alm on November 25, 2023, 10:29:30 pmI would trust a Google hit less for being the most up to date version ofanydatasheet.YMMV

author: alm
author_rank: Super Contributor
timestamp: November 25, 2023, 11:31:55 pm
content: Quote from: tautech on November 25, 2023, 11:17:24 pmSiglent salesman blames Google instead of Siglent, what a surprise

author: tautech
author_rank: Super Contributor
timestamp: November 25, 2023, 11:49:16 pm
content: Quote from: alm on November 25, 2023, 11:31:55 pmI use Google all the time but never have a problem getting the right, proper, correct and latest version datasheets here:https://int.siglent.com/download/documents/?CateIdss=

author: rf-loop
author_rank: Super Contributor
timestamp: November 26, 2023, 07:37:44 am
content: Quote from: alm on November 25, 2023, 10:29:30 pmIn this matter, Siglent needs to improve itself.Here was one, albeit small, example of how important it would be for device manufacturers (Siglent in this case, but in general for everyone) to take care of the quality of the documentation and that they are up-to-date. If one want to keep an out-of-date document available, for one reason or another, publisher should stamp it or indicate clearly in some other way that it is "Obsolete".But then, other problem is many sellers or others who copy document some day and share these without even try to keep them up to dated when new versions are available.However, this small detail shouldn't jump over the OP's point. Understanding this offset thing is one of the fundamentals of using an oscilloscope that everyone should understand, including automatic measurements. Including possible differences in different oscilloscopes.In addition, the matter is messed up and complicated by the fact that there is no completely uniform way of expressing things in the user interfaces of the devices.​

author: rf-loop
author_rank: Super Contributor
timestamp: November 26, 2023, 09:55:08 am
content: Quote from: taydin on November 25, 2023, 08:29:30 pmAt this time I have not SDS1000X-E for real test but yes it have offset range what it have.If you have example 200mVpp sinewave what have 8V offset. It can "remove" this 8V only if vertical V/div is 120mV/div or more (based to data sheet).It is its limit. Some other oscilloscope have different limits.How it "remove" this 8V.Input circuit have this (principle):When you adjust offset (position) there is DAC what produce this Offset voltage what is summed to input signal. If input signal offset is 8V and you add -8V this input signal offset is "removed" and result is zero. Now you can see signal center of image vertical. Note that in digital oscilloscopes ADC range is nearly same as displayed vertical range on screen. (In SDS1000X-E displayed vertical range is 8 div and ADC range is bit over 10 div. So always you adjust vertical "position" you add some voltage to input signal (+ or - add or subtract).But think now this your example and measurements.(and yes input DC coupled)200mVpp sine with 8V dc offset. (7.9V low peak and 8.1V high peak) Its mean value is 8V and its RMS is 8.000312 V (if signal is perfect clean theoretical signal)And because lack of enough resolution both display perhaps 8.00V and it is just right.But now, if we are interested about this Sinewave part without DC offset.Oh well it is simple and also SDS1000X-E can do it just right.Now need measure AC RMS. (DC removed.)Just ask automatic measurement do it. Do not measure RMS if you do not want get RMS (8Vrms in this case)There it is: Stdev. (StDev is AC RMS)In measurement menu. This is for it. It remove this DC offset.But for keep measured signal with DC offset in visible on screen (ADC range) there user add offset voltage to signal using adjustment named as "Position".In this example I have used Cycle RMS and Cycle AC RMS aka StdevIf cycles in this case do not match with data length we get easy very wrong results. (in this particular case can also use just RMS or Stdev for whole data length)Here example how other model do it.Look these values. All they are ok but naturally user need understand limits of true accuracy. (note that this real input signal is dirty, not oscilloscope) just for example because many times we have dirty signals instead of school books theory signals)Input around 200mVpp sine with around 8V offset.One note.If anyone wants a feature where this 8V offset is subtracted from the signal so that the display shows the signal center is 0V on a general purpose oscilloscope (DC coupled) and then some designer does it, I hope this designer clears his desk and exits through cashier and then never come back. Just to avoid a complete mess.We have to keep the truth visible from the input signal as strictly as possible as main principle.If there is a DC offset, we need to know it until we don't want it. In this case, we can also switch to the AC input coupling, which removes the DC offset.Now if the user has done that, he is also responsible for where and how the measured values are applied.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2023, 02:55:09 pm
content: Quote from: nctnico on November 25, 2023, 05:33:04 pmThey might be two different signals. It will depend on the design.Offset and vertical position do the same thing, and the later may be used as the former like Tektronix likes to do, (1) but offset is added before some or all of the attenuation, and the vertical position is effectively added later, (2) or implemented digitally. This gives the offset more range than the vertical position, and a range which depends on how much vertical attenuation is in place after the point where offset is added. In extreme cases, the offset control can have a position range of 10s of thousands of divisions.(1) On older DSOs Tektronix made the displayed signal only part of the digitizer range for greater overload capability and to prevent vertical aliasing in the display. Typically 25 counts per vertical division are used, so 8 vertical divisions represent 200 counts and there are 55 counts not shown. This prevents a minor overload from affecting anything, and signal processing can correctly process a further division above and below what is shown on the display. Vertical aliasing is prevented because there is a direct correspondence between the digitizer counts and screen pixels. I do not know what Tektronix does on their current DSOs.(2) I have seen some designs where the vertical position was added early in the signal chain with variable attenuation to make up for later vertical signal attenuation, and of course some use a real offset control for position, instead of the reverse like Tektronix.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2023, 03:03:02 pm
content: Quote from: rf-loop on November 26, 2023, 07:37:44 amI think that is sometimes deliberate. Misleading documentation and controls can hide a lack of features when comparisons are made to a competitor. For instance delayed triggering on a Tektronix DSO is completely different from delayed triggering on a Rigol instrument, but had the same meaning for decades between different manufacturers before Rigol changed it to obfuscate their lack of this feature.