Author Topic: Quality PC-Based oscilloscope?  (Read 43560 times)

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Offline HiredMind

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Quality PC-Based oscilloscope?
« on: January 19, 2013, 04:19:04 am »
Hi All,

I know most PC-based oscilloscopes are pretty much garbage.  But I'm wondering, does anyone make a pc-based scope with pro-level electronics on par with Tek/Agilent/Rigol, etc?   To give an example: If I can't find an adequate PC-based one, my next choice would probably be a Rigol DS2072)

Since I'm a programmer I have only needed a scope to make sure microcontrollers were doing what they were told, baud rates were set correctly, etc.   But the projects I'm working on are getting faster, and I have some ideas for some projects that will require the ability to see accurate analog waveforms.   

I know what you're saying: Why not just get a regular DSO?   Well as I said I'm a programmer, so I spend most of my working life in front of 4 gigantic monitors and screaming fast development PCs.  And I'm much faster with a mouse than those buttons and knobs on a regular scope.  I'd love to have the quality capture abilities of a high-end scope without paying for the tiny scope display.


« Last Edit: January 19, 2013, 04:23:41 am by HiredMind »
 

Offline mianchen

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Re: Quality PC-Based oscilloscope?
« Reply #1 on: January 19, 2013, 04:28:30 am »
picoscope seems to be everywhere. and smartscope? they both have adverts on eevblog's main page.
 

Offline Bored@Work

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Re: Quality PC-Based oscilloscope?
« Reply #2 on: January 19, 2013, 05:15:53 pm »
Tiny screens? Well, if you pay enough you get oscilloscopes with large screens, too.

But let's stay with the entry level ones. They start with 8 bit DACs. That's 256 possible values. How much vertical resolution do you think you need to get 256 different values all nicely displayed?

But even then, you get entry level oscilloscopes with remote interfaces so you can operate them from a PC. Although the PC software is usually rubbish. You also get some with a monitor output.

And while you might be faster with a mouse, there is a big difference between having to adjust a value with a stupid slider (I hate sliders, even in normal applications), compared to a real knob. And really, when you prod around in some circuit do you think it will work out that you sit in front of your four screen tanning bed, reaching over to the circuit with one hand to probe something, watching your screens and the circuit  at the same time, cruising around with the mouse and typing something with your third hand?
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Offline saturation

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Re: Quality PC-Based oscilloscope?
« Reply #3 on: January 19, 2013, 06:29:59 pm »
As B@W said, you can control modern test instruments that have bus/network options, with a PC or use defacto standard Labview environment as a controller.  National Instruments is a segment leader. 

http://www.ni.com/digitizers/

Agilent has a few instruments here:

http://www.home.agilent.com/en/pc-1418982/oscilloscope?pm=SC&nid=-34492.0&cc=US&lc=eng

When monitoring data status, dedicated instruments provide a secondary portal, if not a control override, to confirm or control the data input shown via PC.  However, if your need includes data collection, clearly some PC approach is optimal, and if your need is fully focused on data collection over much time, a bus based device may save lab space and possibly cost. 

Dedicated instruments are faster to use in tasks were data collection is rarely required, like in hardware design, debug and repair and overall its popularity still shows as you can see from the offering of the big T&M makers like Agilent.


Best Wishes,

 Saturation
 

Offline frenky

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Re: Quality PC-Based oscilloscope?
« Reply #4 on: January 19, 2013, 06:52:02 pm »
Hi.

I bought USB scope Hantek 5200A some years ago and was very happy with it until it stopped working.  :--

You can get it on Aliexpress for 340$ with DHL delivery.

Teardown photos:
https://sites.google.com/site/colorcodeshtml/home/DSO-5200A_top_500K.jpg
https://sites.google.com/site/colorcodeshtml/home/DSO-5200A_bottom_500K.jpg

The specs:
http://www.hantek.com/english/produce_list.asp?unid=66

« Last Edit: January 19, 2013, 07:09:48 pm by frenky »
 

Offline Lightages

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Re: Quality PC-Based oscilloscope?
« Reply #5 on: January 19, 2013, 07:19:55 pm »
HiredMind:

Are you sure what you really want is not a logic analyzer? A scope is still useful for seeing the signal, but if you need to be looking a bus signals and logic......
 

Offline nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #6 on: January 19, 2013, 07:30:19 pm »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline zaoka

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Re: Quality PC-Based oscilloscope?
« Reply #7 on: January 19, 2013, 09:57:34 pm »
Picoscope has 5 year warranty.
 

Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #8 on: January 20, 2013, 12:58:55 am »
Are you sure what you really want is not a logic analyzer? A scope is still useful for seeing the signal, but if you need to be looking a bus signals and logic......

Actually I have one: an MSO-19 from Link Instruments.  It's been fine for my needs so far, as long as I make sure I stay within it's limitations.  But I will be needing a real O-scope soon, and so far my frontrunner is the Rigol DS2072.  But if I can get the same capabilities in a PC-based unit, that's my preference.
 

Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #9 on: January 20, 2013, 01:12:45 am »
Tiny screens? Well, if you pay enough you get oscilloscopes with large screens, too.

I've never seen any DSO with a 32" monitor.  Have you?

But let's stay with the entry level ones. They start with 8 bit DACs. That's 256 possible values. How much vertical resolution do you think you need to get 256 different values all nicely displayed?

The point of asking the question was to _not_ stick with entry level ones.  I mentioned the Rigol DS2072 - that's not an entry level scope.  It's not the $23,000 Tektronix 24 channel LA that I used at work a few years ago, but it's not entry level. (And I'm not looking to sell my car to buy a scope either  :-DD ).

And while you might be faster with a mouse, there is a big difference between having to adjust a value with a stupid slider (I hate sliders, even in normal applications), compared to a real knob.

Not for me.  After 20 years at a keyboard/mouse, that combination is basically an extension of my hands at this point.

And really, when you prod around in some circuit do you think it will work out that you sit in front of your four screen tanning bed, reaching over to the circuit with one hand to probe something, watching your screens and the circuit  at the same time, cruising around with the mouse and typing something with your third hand?

So, keyboard&mouse/Scope/Circuit is easier to control than keyboard&mouse/Circuit?   Sorry I just don't buy that.  In any case, having worked with both configurations, I prefer the latter. 
 

Offline nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #10 on: January 20, 2013, 01:35:52 am »
IMHO it depends greatly on the user interface whether a mouse/keyboard combo works better than the controls. Because a scope has many buttons with specific functions you have a bunch of hot-keys in a well choosen layout and there is ofcourse the on-screen menu. The more expensive scopes have touchscreens which make live even more easier.

I have played a bit with Picoscope but I still prefer a real scope. The waveform update rate isn't fantastic and I recall they choose the ranges in a weird way. Instead of 1-2-5 V / div they choose 1-2-5V peak-peak which gives you an odd reading when you are used to a normal scope. If you are looking at a 5V signal you need to select 10V peak-peak which throws away half the resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.
« Last Edit: January 20, 2013, 01:44:29 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Lightages

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Re: Quality PC-Based oscilloscope?
« Reply #11 on: January 20, 2013, 01:53:37 am »
I am a computer nerd. One of my best friends is a computer nerd. We both agree there is nothing like a direct hands on control that is where you are looking and "feeling". You can say it isn't so but believe me if you use a full "normal" hardware oscilloscope you will never want to touch a virtual one again for quick and intuitive control.

Get the Rigol DS2072 or similar and don't look back. When you are doing logic analysis it is one thing, but doing things that an oscilloscope can do needs a real interface not a virtual one. If someone made a USB interface with the knobs and buttons to control a virtual scope that would be different.
 

Offline helloworld922

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Re: Quality PC-Based oscilloscope?
« Reply #12 on: January 20, 2013, 02:15:42 am »
Quote
I've never seen any DSO with a 32" monitor.  Have you?

Some O-scopes have video output, or some sort of PC interface so you can hook the o-scope up to your PC and view waveforms on there.

Personally I've also been considering the DS2000 series because it seems like a very good scope for the price. If I had a second pick I might go with a PicoScope PC-based O-scope (maybe a 2207 or a 2208).

Reasons for getting the Rigol:

1. It has a higher max sample rate (2 GS/s single channel, 1 GS/s dual channel compared to 1 GS/s single channel, 500 MS/s double channel)
2. Larger memory depth (14 MPts vs. 32 kPts standard, upgrade-able to 56 MPts).
3. I'm not actually tied to my PC if I need to bring my scope to do field work. Of course, you could just as easily bring a laptop and the Picoscope around.
4. I kind of like the knobs/buttons on the actual scope, but that's a personal preference. With the Rigol you can of course interface with the PC and write your own software if you want to control it from the PC. The Picoscope only has a PC-based interface.
5. Much higher waveforms capture rate on the Rigol
8. Higher voltage input ranges and most likely better over voltage protection. The rigol is CAT II 100Vrms rated, while the Picoscope only states 100V max (AC+DC offset) without any CAT rating.

Reasons to get the Picoscope:

1. Availability of Linux drivers (if that's what you use, I mainly use Windows so it doesn't matter either way for me)
2. Nice and small package, you might be able to tuck it into your laptop bag or a backpack and bring it with you to do field work.
3. Much nicer PC interface without having to write your own code.
4. Comes with a waveform generator (I'm not sure if the 2207 comes with the AWG function, but it looks like it does)
5. Higher bandwidth. Even upgrading to the 200 MHz 2208 is under $1000 USD while the equivalent bandwidth Rigol is ~$1600 USD.
6. Several features are standard such as serial decoding and advanced triggers which on the Rigol are a few hundred bucks extra.
7. Following marmad's experiences with the Rigol DS2000 series so far, it seems like there are quite a few bugs in the Rigol firmware. I'm not sure how many of them are "critical", but it is a newer product than the Picoscope offering. I can't comment on the reliability of the Picoscope software, though since I haven't heard anything about it.
8. For what it's worth, Picotech is a British company with a claimed 20 years experience in the business. Of course Rigol's been around quite a while, too (~15 years) and from what I've been researching about them don't necessarily provide poor quality scopes, even if they are based out of Beijing.

IMO which ever route you go you'll probably end up with a good quality scope. I still have quite a bit of time to save up money before I come to a decision on which route I want to take. It sounds like ultimately it's going to be you who's using the scope, not a co-worker, Dave, or anyone else here. If there's a compelling reason for you to not like one or the other, even if it goes against the grain of what others prefer, so be it. Just be prepared to live with the decision you make (or return the scope if you find you really don't like it).
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #13 on: January 20, 2013, 03:28:52 am »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works. There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.

Tektronix was able to achieve a 250MHz BW into a 1M 20pF input all the way back in the 1970's (see Tek 475A), so 15pF is very reasonable. If you really understand scope inputs, then you recognize that the input impedance is not a simple capacitive load. They typically have series resisters that make them look much more like 50 ohms at high frequencies. Scopes with 1M inputs are typically equalized to achieve their specified bandwidth with a 50 ohm source, terminated with an external 50 ohm load at the input. Take a look at a cal procedure.

@HiredMind:

Don't let any jaded old goat steer you away from PC-based instruments. They can be quite good (though not all are). If you already spend much of your time in front of a PC (which describes many modern engineers), they can be very convenient to use. I would just say, look at the software *very* carefully before you choose. I've used many PC-based instruments, and will tell you for sure that some of the support software is truly awful. When evaluating different products, don't just click around in the GUI and assume it will do the job. Go through several real-world setups to get a good idea of how convenient they will be to use. Good luck with your decision.
 

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Re: Quality PC-Based oscilloscope?
« Reply #14 on: January 20, 2013, 04:00:33 am »
I'm sceptical about those 12 bits. Common scopes with 8 bits of vertical resolution might have an ENOB of around 5 at their rated BW. You'd need a lot of oversampling to get this close to 12 bits.

The 50 Ohm generator terminated into 50 Ohm represents a 25 Ohm source impedance. Is the impedance of your DUT 25 Ohm or less? 10x high-Z passive probes will present a similar load. Passive high-Z probing is still feasible at 250 MHz, but loading is starting to become an issue. A 50 Ohm input impedance is a welcome option, although a good feed-through terminator ($50 each) might also work.
 

Offline mazurov

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Re: Quality PC-Based oscilloscope?
« Reply #15 on: January 20, 2013, 04:36:13 am »
Agilent makes 2700 series USB oscilloscopes -> http://www.home.agilent.com/en/pc-1418982/usb-modular-oscilloscope?nid=-34492.0.00&cc=US&lc=eng , they are very nice, but expensive comparing to other PC-based scopes ( but have much better characteristics ).

I do a lot of microcontroller work and the scope is rarely necessary; you'll need one to check your oscillator and to take a look (once) at high-speed signals, if you have any. In both cases, any old 350-500MHz analog instrument will do. The rest of general troubleshooting is done using $30 Open Bench Logic sniffer LA.
 

Offline seattle

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Re: Quality PC-Based oscilloscope?
« Reply #16 on: January 20, 2013, 05:14:46 am »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works. There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.


It takes 4X oversampling to pick up an extra bit, so there are some pretty steep severe bandwidth limitations compared to a native 12-bit converter running at 100Msps.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #17 on: January 20, 2013, 05:36:35 am »
I'm sceptical about those 12 bits. Common scopes with 8 bits of vertical resolution might have an ENOB of around 5 at their rated BW. You'd need a lot of oversampling to get this close to 12 bits.

Sure the ENOB will be lower than the hardware's resolution. A good scope with 200MHz bandwidth would be capable of 6 bits or better. I've seen 7.1 bits claimed by Agilent for their lower bandwidth scopes. But that wasn't the discussion. With an appropriate level of oversampling, an 8 bit converter can give you 12 bits exactly as well as it can give you 8 bits without said oversampling. It's only a matter of how many samples you average for each resulting value. For each factor of 4, you get 1 more bit of resolution.

The 50 Ohm generator terminated into 50 Ohm represents a 25 Ohm source impedance.

Yes, it results in a 25 ohm source whether the termination is internal or external to the scope. We'd like to think that a scope with a selectable internal 50 ohm load would have better response, but it's unlikely for scopes with a bandwidth in the 200MHz range. Why? Because scopes in this range typically just take the existing 1M input and switch a 50 ohm load on top of it. That means the capacitance of the 1M input is still there in parallel with the 50 ohm load, just as it would be if you used an external 50 ohm load. The result is far from a perfect 50 ohm termination. Measure the return loss of your 50 ohm scope input some time if you doubt it. They're typically atrocious at rated bandwidth.
 

Offline free_electron

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Re: Quality PC-Based oscilloscope?
« Reply #18 on: January 20, 2013, 05:46:56 am »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
You need to go and explain that to Agilent... They still sell scopes with 1:10 passive probes rated for 500MHz ... With 1M input on the scope. The probe is 10Mohm input. n2873a for example. Or N2890. These probes have rise times of 700pS so they are more than capable of doing that...

Even their older discontinued probes like the 116x series or the  :10073 did that 20 years ago.

I agree the chingchangchong probes sold for 20$ on fleabay hardly clamber above 100MHz .. But a real probe doesn't blink at 500MHz ... Then again, these real probes cost more than the base Rigol oscilloscope.... Buy 4 probes, get scope free  ;D
« Last Edit: January 20, 2013, 05:54:26 am by free_electron »
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Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #19 on: January 20, 2013, 11:09:08 am »
Agilent makes 2700 series USB oscilloscopes -> http://www.home.agilent.com/en/pc-1418982/usb-modular-oscilloscope?nid=-34492.0.00&cc=US&lc=eng , they are very nice, but expensive comparing to other PC-based scopes ( but have much better characteristics ).

Oh wow - I've been trawling the Agilent website off and on for weeks and didn't even notice them.  Thanks Mazurov, that looks very promising, I'll check them out.
 

Online Marco

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Re: Quality PC-Based oscilloscope?
« Reply #20 on: January 20, 2013, 01:02:15 pm »
That means the capacitance of the 1M input is still there in parallel with the 50 ohm load, just as it would be if you used an external 50 ohm load. The result is far from a perfect 50 ohm termination. Measure the return loss of your 50 ohm scope input some time if you doubt it. They're typically atrocious at rated bandwidth.
Would it be possible to bodge some sort of RLC terminator which avoids this?
« Last Edit: January 20, 2013, 01:09:23 pm by Marco »
 

Offline nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #21 on: January 20, 2013, 01:49:40 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works.
I put that to the test on my Tektronix scope and as expected it looks fine for a sine wave but a triangle or sawtooth get distorted due to inperfections of the noise and non-linearities in the ADC. If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise. The reality is that you would need extremely good noise and a comparator with an extremely low input offset and enormous gain. So unless you have perfect noise and use an ADC where the comparators are better then required for the ADCs resolution you have a chance of actually getting more resolution from an ADC.

The problem is that people teaching sampling theory tell nice stories about how you can shape noise into bits totally forget about the practical application of that theory. Some theories just don't work well in real world situations!
Quote
There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.
Which are called delta-sigma or pipelined ADCs. And there is also the succesive approximation ADC which basically is a comparator.

Quote
Tektronix was able to achieve a 250MHz BW into a 1M 20pF input all the way back in the 1970's (see Tek 475A), so 15pF is very reasonable. If you really understand scope inputs, then you recognize that the input impedance is not a simple capacitive load. They typically have series resisters that make them look much more like 50 ohms at high frequencies. Scopes with 1M inputs are typically equalized to achieve their specified bandwidth with a 50 ohm source, terminated with an external 50 ohm load at the input. Take a look at a cal procedure.
The people at Picoscope think they can do the same with standard 1:10 1M Ohm probes. Just look at their brochure. 15pf at 250MHz is around 42 Ohms so you may get lucky with the Picoscope and 50 Ohm terminators at the input but if you are into RF then I'd suggest equipment with 50 Ohm inputs. My rule of thumb is to go for 50 Ohm cabling and proper HF probes (active or passive divider) for measuring anything over 100MHz.
« Last Edit: January 20, 2013, 02:27:26 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #22 on: January 20, 2013, 02:17:53 pm »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
You need to go and explain that to Agilent... They still sell scopes with 1:10 passive probes rated for 500MHz ... With 1M input on the scope. The probe is 10Mohm input. n2873a for example. Or N2890. These probes have rise times of 700pS so they are more than capable of doing that...
No, they are not. The probe capacitance has too much influence on the signal. I measured the same signal (a 40MHz square wave from a 50 Ohm source) with a 1:10 probe (top) and a passive divider probe (bottom). I must say that the top signal will look more like a square wave with the probe you mentioned but the fact is the signal will still be distorted due to probe loading. The n2873a has a capacitance of 9.5pf. My passive probe's capacitance is less than 1pf.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline helloworld922

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Re: Quality PC-Based oscilloscope?
« Reply #23 on: January 20, 2013, 07:56:34 pm »
Quote
If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise

The key factor which makes oversampling work is that the noise has to be enough to trigger at least 1 bit digitization changes. If your signal has +/-1mV noise and your minimum ADC resolution is 4mV (what it is for a 1V reference with 8-bit resolution), you won't get increased accuracy no matter how many samples you read.

Also your effective sampling rate drops because you need multiple smaller samples to try and recreate the higher-accuracy "sample". To get a 1 GS/s 8-bit effective resolution using a 1-bit ADC and appropriate noise, you would need to sample at 65.536 Terra samples per second! With a 1 GS/s 1-bit ADC, you only get ~15.259 KS/s at 8-bits effective accuracy..
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #24 on: January 20, 2013, 08:00:13 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works.
I put that to the test on my Tektronix scope and as expected it looks fine for a sine wave but a triangle or sawtooth get distorted due to inperfections of the noise and non-linearities in the ADC.

So did I. Images below for 8 bit normal sampling and 12 bit oversampling. No distortion noted. It just cleans up the noise as you'd expect for any form of averaging.
 


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