Budget DSO with 1mv/div or better sensitivity - Page 1
Forum section: electronics / testgear

author: pfm
author_rank: Regular Contributor
timestamp: July 28, 2014, 04:54:10 pm
content: I am on the lookout for a dso (~100Mhz) that has better than the standard 2mv/div sensitivity. I am looking for 1mv/div or better. What are my options ? Please advise.I am open to new, used, import, PC based, DIY, anything..Thanks!

author: 8086
author_rank: Super Contributor
timestamp: July 28, 2014, 05:16:25 pm
content: Rigol DS1104Z

author: pfm
author_rank: Regular Contributor
timestamp: July 28, 2014, 08:39:37 pm
content: Thanks, thats a good suggestion. Is it really a 12-bit scope ? atleast to a certain bandwidth and/or sample rate ? am I reading the detailed specs correctly ?Any other brands in more or less the same price range ? like Owon or Instek or Hantek ... ?

author: Mark_O
author_rank: Frequent Contributor
timestamp: July 28, 2014, 10:00:10 pm
content: Quote from: pfm on July 28, 2014, 08:39:37 pmNo. Not at all. Not sure where you'd get that misimpression.QuoteNo. You can eek out a couple more bits by oversampling, if youdon't care aboutare willing to sacrifice bandwidth.QuoteNo. Most of those have 5 mV/div sensitivity, with a few offering 2 mV/div.

author: EEVblog
author_rank: Unknown
timestamp: July 28, 2014, 10:10:42 pm
content: Quote from: pfm on July 28, 2014, 08:39:37 pmNo. Many scopes like this will have a "High Resolution" mode that does boxcar averaging to give you a bit more effective resolution, but they all still use an 8 bit ADC.

author: pfm
author_rank: Regular Contributor
timestamp: July 29, 2014, 12:50:00 am
content: Quote from: Mark_O on July 28, 2014, 10:00:10 pmFrom here - see attached detailed spec sheets of the DS1000Z and DS2000A series.

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2014, 02:05:06 am
content: The Siglent SDS 1000 and 2000 series both have a Vertical range of 2 mV to 10 V per division.

author: echen1024
author_rank: Super Contributor
timestamp: July 29, 2014, 03:57:17 am
content: Rigol DS2000. 500uV vertical range.

author: Mark_O
author_rank: Frequent Contributor
timestamp: July 29, 2014, 04:12:09 am
content: Quote from: pfm on July 29, 2014, 12:50:00 am"High Resolution--12 bits of resolution when ?5 ?s/div @ 1 GSa/s (or ?10 ?s/div @ 500 MSa/s)."Yeah, I can see where that could be confusing. IMO, it's rather deceptive. (It's in the Sampling section.) If you look a bit farther down, you'll see the real story..."Vertical Resolution:Analog channe: 8 bitDigital channel: 1 bit"A couple of my old LeCroy scopes have 16-bit wide memories (but still 8-bit ADCs), so when one of them increases rez by oversampling, it's a true rez increase. OTOH, the Rigol(s) have only 8-bit wide memories, so there's no where to go with any "extra bits".

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: July 29, 2014, 12:35:47 pm
content: Quote from: Mark_O on July 29, 2014, 04:12:09 amLeCroy's ERES mode also works differently than the boxcar averaging employed by Tek & Co:http://teledynelecroy.com/doc/differences-between-eres-and-hires

author: idpromnut
author_rank: Unknown
timestamp: July 29, 2014, 02:47:25 pm
content: Quote from: echen1024 on July 29, 2014, 03:57:17 amWhile these scopes (MY scope!) do indeed go down to 500uV/div, this is ONLY in 1X, i.e. not with a 10X probe attached (in which case it is "only" 5mV/div). Also, I wouldn't lump this scope with various "budget" DSOs… they are cheap, yes, but not 300-500$ cheap, which is what I think the OP was hinting at ("...used, import, PC based, DIY…").

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 29, 2014, 03:39:07 pm
content: another option... buy anything and then buy or build 10X active probe @ 1X, 2mV/div scope setting you'll get 200uV/div resolution.

author: pfm
author_rank: Regular Contributor
timestamp: July 29, 2014, 05:02:24 pm
content: Ok so it seems like these scopes that offer 1mv/div or less must be having better noise performance than the 2mv/div ones ? since both use 8bit ADC only ? Is that what it means ?

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2014, 05:11:45 pm
content: Keep in mind that oscilloscope input sensitivity is limited by bandwidth because of noise. High resolution modes used by DSOs exploit this in a useful way by increasing resolution and lowering bandwidth (and noise) at the same time.Some DSOs can also increase input sensitivity digitally by averaging multiple acquisitions without sacrificing bandwidth. I have always been a little leery of using averaging like this but it does work on the oscilloscopes which support it.Neither of the above techniques do anything to improve drift or non-linearity.Some modern DSOs cheat by treating their ADC as a programmable gain amplifier and sacrificing resolution similar to how digital zoom on a camera works.The old Tektronix 2210 (rare) and 2211 (common) combination analog/digital oscilloscopes support 500uV/div sensitivity with bandwidth reduced to 5 MHz. For them the x10 vertical gain switch serves double duty by replacing the bandwidth limit switch. As DSOs they are pretty limited but they do operate in real time. There is also the 4 channel 2214 but they are rare and perhaps difficult to service.A Tektronix AM502 differential amplifier may be used with any DSO to provide sensitivities down to 10uV/div but bandwidth is limited to 1 MHz maximum. The distantly related but currently produced LeCroy DA1855A differential amplifier and its ancestors made by Preample may be used as a 100 MHz x10 amplifier on any oscilloscope.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2014, 05:43:29 pm
content: Quote from: pfm on July 29, 2014, 05:02:24 pmNo, there is some variation do to design and implementation but the physics dominates.Analog oscilloscopes face the same limitation do to noise which is why they have maximum sensitivities of about 2mV/div at 100 MHz and even that is pushing it. If you want better sensitivity without averaging multiple acquisitions, then the bandwidth must be limited. As an extreme example (*), a Tektronix 7A13 vertical amplifier supports 1mV/div at 100 MHz and displays a little less than *one half division* of RMS noise. It has a switchable 5 MHz bandwidth limit though which reduces the displayed noise to a very usable level.The integrated noise calculation for a 1 MOhm input with a shunt capacitance of about 20 pF and 100 MHz bandwidth agrees pretty closely with the above. Higher bandwidth oscilloscopes with high impedance inputs have lower input capacitance further increasing noise which limits their sensitivity further without bandwidth limiting.It is worth noting that DSOs which lack index graded displays will show peak to peak noise about 5 times greater than the RMS noise.(*) The 7A13 is a differential amplifier so it starts with double the noise. It also has a complex input amplifier designed for wide input voltage range, high overload capability, and fast overload recovery which further increases its input noise.

author: iDevice
author_rank: Regular Contributor
timestamp: July 30, 2014, 11:56:39 am
content: Thought about a 7K mainframe with a 7A22 plugin ? (10?V/div)That's what I use for very low level signals like transducers etc.This plugin alone is one of the main reason to get an old 7k on the bay, it's unmatched today, at least in the few hundred $ price range for the whole setup.

author: robrenz
author_rank: Super Contributor
timestamp: July 30, 2014, 03:20:30 pm
content: +1Here is an exampleof a 7603/7A22 feeding a Rigol 1052 from the signal out. I used the variable settings of the 7A22 to get calibrated values on the DSO.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2014, 03:27:35 pm
content: Quote from: iDevice on July 30, 2014, 11:56:39 amThe 7A22 is pretty slow at 1 MHz maximum and the only 7000 mainframes with digital storage are not what most people would think of as DSOs. An AM502 would provide the same capabilities as a 7A22 with any DSO.Many times I have used a 7A13 (1mV/div at 100 MHz or 5 MHz) with an external DSO connected to the mainframe's vertical output. This configuration actually allows better than 1mV/div because the output from the mainframe is 25mV/div so the DSO may be set to 5mV/div yielding 200uV/div but performance is limited by noise which will have to be reduced by the DSO. A 7A22 could be used the same way with an external DSO but it is slow.If you have an unlimited budget, then LeCroy makes some 12 bit DSOs which have an input sensitivity down to 1mV/div. Some USB based oscilloscopes like from Picoscope are 12-bits or more but I do not know how they really perform.The least expensive options I know of are the 2210 and 2211 that I mentioned but they are pretty primitive DSOs and limited to 5 MHz at high sensitivity.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2014, 03:46:04 pm
content: Quote from: robrenz on July 30, 2014, 03:20:30 pmI noticed in that discussion thread that the noise issues with the 7000 vertical output is mentioned. I modified my 7603 to improve the common mode rejection of the differential to single ended output amplifier which drives the vertical output lowering the noise by about 25 dB. I just increased the value the tail resistor for the transconductance amplifier andused the -50 volt supply for bias instead of the -15 volt supplyused the +50 volt supply for bias instead of the +15 volt supply so the bias current is the same but a transistor current source should work even better. The 76x3 mainframes are particularly bad about noise on the vertical output but the larger mainframes are better in this regard.

author: robrenz
author_rank: Super Contributor
timestamp: July 30, 2014, 04:02:31 pm
content: Would you consider showing that mod in greater detail with component values so a wannabe EE like me could execute it?Edit: added schematic and component layout (rotated to match your picture)

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2014, 09:26:42 pm
content: I was just looking up the schematic to post but refreshed the discussion thread here before posting.Change R608 from 82.5 ohms to about 1.8 kohms and attach it to the +50 volt regulated supply instead of the +15 volt supply from R610. Leave R610 and C610 alone because they provide bias to the cascode circuit built around Q620. +50 volts is not brought down to the output board so I added a wire wrapped around the existing cable from the power supply. I also used a 47 ohm resistor in series with the +50 supply and a 0.01 uF capacitor to ground to filter high frequency noise and provide decoupling. These changes lower the common mode gain of the Q606 and Q618 differential pair significantly and calculations agreed with the before and after measurements of about 25 dB of improvement. The value of R608 may be trimmed to adjust the output to zero when the trace is centered but I did not find this necessary.Further improvement could be made by replacing R608 with a current source but I tried a simple resistor first and found it acceptable until I can get some better transistors. Any current source with a transistor cascode output and series resistor should work well but the +50 volt supply will still be needed to gain enough voltage compliance.The line related ripple shown in the oscillograph that I posted is part of the common mode signal and ultimately comes from the power supply's output ripple but improving the regulation of the power supply will not help because artifacts from the sweep which was disabled for this measurement will show up anyway. In normal applications, the displayed ripple in the photo is insignificant compared to the broadband noise. It is only visible in this case because the DSO is removing almost all of the broadband noise by averaging multiple acquisitions.If a plug-in like a 7A13 with 1mV/div sensitivity was being used, then the displayed sensitivity on the DSO would be either 200uV/div with a 50 ohm termination at full bandwidth or 10uV/div with no termination at reduced bandwidth. Broadband noise however would be significant without filtering and drift would be an issue at these sensitivities.I asked around and apparently all of the 76x3 mainframes perform similarly having the same ripple which makes sense because they all use the same design but as pointed out above, this level of ripple is not significant under normal conditions. The 4 bay 7000 mainframes that I have checked have a much cleaner vertical output signal.

author: engineer_in_shorts
author_rank: Regular Contributor
timestamp: July 30, 2014, 10:21:12 pm
content: I have a little differential voltage amplifier for these applications. Come from rs.

author: pfm
author_rank: Regular Contributor
timestamp: July 30, 2014, 10:39:26 pm
content: Let me see if I understood this right. If one were to use a very low noise low distortion external amplifier with enough bandwidth then you could use the scope at 1X setting and divide the min sensitivity of the scope by the gain of that amplifier. So a 10X amplifier would essentially allow the 2mv/div to be used as a 200uV/div. Ofcourse the conversion would have to be done manually. Is that correct ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 31, 2014, 12:28:41 am
content: Quote from: pfm on July 30, 2014, 10:39:26 pmyes 100% correct. leaving out details such as noise, triggering problem , accuracy, dynamic range etc. some such as noise already discussed above.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2014, 02:03:37 am
content: Quote from: pfm on July 30, 2014, 10:39:26 pmNoise will limit the maximum sensitivity you can achieve for a given bandwidth and input impedance. There is some improvement to be made with a modern low noise input stage but not a lot.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 31, 2014, 02:22:08 am
content: dso averaging function will also help to remove random noise, but without proper or external triggering, its useless. with noise riding on a signal, if the signal is high enough to rise noise to certain level, built in dso triggering and averaging can still be usefull.

author: robrenz
author_rank: Super Contributor
timestamp: August 01, 2014, 12:31:26 pm
content: @ David HessThank you for taking the time to document that for me.It is now on my to do list

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 03:10:24 pm
content: Just keep in mind that the original design of the vertical output circuit is not bad for what it was intended for. The only reason the ripple became visible in my case is that I was using DSO averaging and external triggering to remove the broadband noise. Under normal conditions, the broadband noise almost completely covers the ripple which comes from both the power supply and the sweep.I stopped fooling with it once I made the simple change with the resistor and +50 volt supply because it worked so well at that point. At some point in the future, I will add the transistor current source and maybe cascode and make it adjustable so I can calibrate the offset.

author: pfm
author_rank: Regular Contributor
timestamp: August 01, 2014, 05:17:43 pm
content: does dso averaging/resolution enhancement techniques remove noise from the original signal as well ? If the signal itself has noise which needs to be measured (and not hiden/removed) and displayed as well, then after dso averaging would it get removed in other words give an incorrect result ?

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 06:14:19 pm
content: Averaging removes asynchronous components of the signal including noise because they change over multiple acquisitions. Resolution enhancement just low pass filters the signal.What you see in the oscillographs that I posted is ripple from the linear power supply that is synchronous to the line frequency which was used as the trigger. The noise which would have completely covered it up was removed by averaging multiple acquisitions.If the sweep had been running then there would have been ripple from that as well and if the sweep was synchronous to the line frequency which is easy enough to arrange, it would have shown up clearly. Or I could have triggered off of the sweep and used averaging to remove both the ripple from the power supply and any broadband noise.Since all of the ripple in the vertical output could be controlled by increasing the common mode rejection of the differential to single ended converter circuit, I just picked the easiest one to measure and used that as my benchmark.

author: Mark_O
author_rank: Frequent Contributor
timestamp: August 01, 2014, 10:27:52 pm
content: Quote from: pfm on July 30, 2014, 10:39:26 pmNo need for manual conversion on the Rigols. Even the cheapest 1000Z series unit provides Probe Attenuation coefficients of 0.01X-1000X, in 1-2-5 steps. Many Agilents also provide 0.1X-1000X, in 1-2-5 steps, which supports up to 10X pre-amplification.[Siglent unfortunately is only 1X, 5X, 10X, 50X..., though it would be pretty trivial for them to support other factors. It's just scaling. But I have some home-made 20X 50-ohm probes that would be a PITA to use on them.]