12 and 16-bit oscilloscopes
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From: Mike Monett on 19 Dec 2006 04:11 Hi Tom, > Hi Mike, Joel and all, > OK, the AD7660 is a 16-bit successive approximation ADC that's > pretty decent. The AD7760 is the 24-bit delta-sigma part. I'm not > quite seeing the 120dB SNR in the data sheet for the AD7760 - > looks like 112dB to me. At 78kHz bandwidth, that's about > -160dBfs/Hz. ADI claims 120 dB dynamic range at 78 kHz, but only 112 dB SNR. From the spec: FEATURES 120 dB dynamic range at 78 kHz output data rate 100 dB dynamic range at 2.5 MHz output data rate 112 dB SNR at 78 kHz output data rate 100 dB SNR at 2.5 MHz output data rate So it seems their claims are OK. Tom, how do you get the -160dBfs/Hz figure for 78kHz? Can you give the calculations? I can see how the normal SNR calculation gives wideband SNR. For example, 16 bits is SNR = 20 * log10(2^n) = 20 * log10(2^16) = 96.3295 dB But how do you include the bandwidth in this calculation? > That's good, certainly, though it's not all that different from a > 130Ms/s part that has a 78dB SNR: such a part has a noise level at > about -156dBfs/Hz. > If it's really for a scope, my main interest in more bits would be > to avoid input ranging, and that's not all that difficult at low > frequencies (1MHz these days is "practically DC".). But if it's > for spectral analysis, that's a different story. Then distortion > and noise are important (more important than the number of bits, > which is largely a red herring). > My personal interest would be in fewer bits and much faster > sampling: maybe a Virtex 5 connected to two or four 500Ms/s > converters, into a deep buffer memory. > Cheers, > Tom What's a Virtex 5? It sounds expensive. You are probably the kind who gets ic's with gold-plated pins, where the rest of us have to be content with tin/lead:) The AD7760 might be good starting point for entry level. At $35.00 it's not too expensive, even for a scope with multiple channels. The 120 dB might give 6 digits at DC, so it could be a combined DVM and scope. The Binary Sampler could give a bandwidth past 5 GHz, so that could give a very inexpensive system with pretty good capability. Once you have a system running, the wideband sampler could be used for working on a faster digitizing system. Certainly 1 Gs/s is quite feasible. I believe Maxim had articles on interleaving slow ram to use with their fast digitizers. That could provide a starting point for the pc interface. So it might be possible to bootstrap a system for very reasonable cost. This is significant from several viewpoints. Sure, you can get cheap systems on ebay. But what do you do when they break? Modern scopes are not repairable, certainly not with the resources available in a home lab or small business. After some point, even Tucker won't quote on repairing them. So they become useless junk. But if you build your own system, you certainly can repair it if it breaks. Another advantage is you can add more features to the system when it is needed. Say you need more channels. That is simply duplicating what you already have working. The software is open to anything you can imagine. Once you have solved the problem of transferring data quickly to the PC, analysis can be very fast. You can take also advantage of newer chips with improved performance, so your system can evolve with new technology. The beauty of this approach is you are not dependent on trying to keep commercial equipment running. Decades ago, I had no problems buying complete labs of new HP and Tektronix equipment. But over the years, something happened to the reliability. The Tek and HP equipment started breaking down, and repair costs were astronomical. I began to get the impression it would be faster to make my own lab equipment rather than spend the time packing and shipping equipment back for repair. Now that inexpensive ic's are available with excellent performance, it seems to be very reasonable to build your own equipment. And with the tremendous resources available in this newsgroup, how can you resist? Regards, Mike Monett Antiviral, Antibacterial Silver Solution: http://silversol.freewebpage.org/index.htm SPICE Analysis of Crystal Oscillators: http://silversol.freewebpage.org/spice/xtal/clapp.htm Noise-Rejecting Wideband Sampler: http://www3.sympatico.ca/add.automation/sampler/intro.htm
From: Joel Kolstad on 19 Dec 2006 12:11 "Mike Monett" <No(a)email.adr> wrote in message news:Xns989E2A7476538Noemailadr(a)208.49.80.251... > Another advantage is you can add more features to the system when it > is needed. Say you need more channels. That is simply duplicating > what you already have working. Au contraire! *Synchronizing* the channels requires a shared timebase distributed to each module or some other solution, and letting *any* module trigger also requires some shared data lines. Perhaps you didn't mean to imply otherwise, but I've caught myself at times thinking, "Yeah, if I bought a 2 channel USB-based DSO, I could just buy 2 and get 4 channels..." -- which doesn't work with any commercial offering I'm aware of, due to the the need to share timebase/triggering data. > Now that inexpensive ic's are available with excellent performance, > it seems to be very reasonable to build your own equipment. As I mentioned somewhere else, these days I think the bulk of the time is actually in the software, at least for something like a "generic" scope where you just have simple triggering methods and otherwise are just collecting data. ---Joel > > And with the tremendous resources available in this newsgroup, how > can you resist? > > Regards, > > Mike Monett > > Antiviral, Antibacterial Silver Solution: > http://silversol.freewebpage.org/index.htm > SPICE Analysis of Crystal Oscillators: > http://silversol.freewebpage.org/spice/xtal/clapp.htm > Noise-Rejecting Wideband Sampler: > http://www3.sympatico.ca/add.automation/sampler/intro.htm
From: Tom Bruhns on 19 Dec 2006 12:33 John Devereux wrote: > "Tom Bruhns" <k7itm(a)msn.com> writes: .... > > I'm not quite seeing the 120dB SNR in the data sheet for the > > AD7760--looks like 112dB to me. At 78kHz bandwidth, that's about > > -160dBfs/Hz. That's good, certainly, though it's not all that > > different from a 130Ms/s part that has a 78dB SNR: such a part has a > > noise level at about -156dBfs/Hz. > > Interesting; your implication is that one can obtain equivalent > performance at lower frequencies by signal averaging, yes? So just > pick the device with the highest sampling rate and low dBfs/Hz? Yes, with respect to noise anyway. After all, a delta-sigma does it, but in spades, by using feedback (the "modulator") to move the noise from low frequencies to high, and then "averaging" (decimation filtering, actually). ADCs designed for low frequencies are likely going to have better distortion performance than the high speed ones. Of course, the distortion of the high frequency ones is usually worse at high frequencies than at low, but still, even at low, they won't be quite as good. The advantage, of course, is that you can also look at the higher frequencies, which is tough with a delta-sigma narrow-bandwidth converter (except in narrow aliased bands...) > > > If it's really for a scope, my main interest in more bits would be to > > avoid input ranging, and that's not all that difficult at low > > frequencies (1MHz these days is "practically DC"...). But if it's for > > spectral analysis, that's a different story. Then distortion and noise > > are important (more important than the number of bits, which is largely > > a red herring). > > > > My personal interest would be in fewer bits and much faster sampling: > > maybe a Virtex 5 connected to two or four 500Ms/s converters, into a > > deep buffer memory. > > My motivation was based on experience with a TDS3054 scope. I love it, > but one thing it does not do is improve resolution at the lower sample > rates. For one-shot events, you are stuck with the same ~7 bit > resolution at 10kHz that it uses at 500MHz. It would be nice to have > something optimized for low frequency use. Yes, IF the 7 bits is really linear, then band-limiting filtering will help things out considerably. The ADC in the HP3563 that Fred mentioned is only 13 bits, but is dithered and has very good linearity, so you can accurately measure signals well below one part in 2^13. I suppose doing decimation on a 130Ms/s data stream with a PC real-time might be possible, but would be pushing things. You have to deal with getting data in, as well as implementing a multiple pole filter that probably is going to take a few dozen instructions per sample. Much nicer to have that with the ADC, along with a big capture buffer. Cheers, Tom
From: Tom Bruhns on 19 Dec 2006 12:44 Mike Monett wrote: > Hi Tom, .... > > Tom, how do you get the -160dBfs/Hz figure for 78kHz? Can you give > the calculations? > > I can see how the normal SNR calculation gives wideband SNR. For > example, 16 bits is > > SNR = 20 * log10(2^n) > = 20 * log10(2^16) > = 96.3295 dB > > But how do you include the bandwidth in this calculation? White noise (assumed, but not strictly accurate, especially at frequencies low enough to be in the 1/f noise region) is equal power per unit bandwidth. 78kHz bandwidth is 10*log(78000) decibles more power than a 1Hz bandwidth. Subtract 10*log(78000) from the 112dB SNR. > > > That's good, certainly, though it's not all that different from a > > 130Ms/s part that has a 78dB SNR: such a part has a noise level at > > about -156dBfs/Hz. > > > If it's really for a scope, my main interest in more bits would be > > to avoid input ranging, and that's not all that difficult at low > > frequencies (1MHz these days is "practically DC".). But if it's > > for spectral analysis, that's a different story. Then distortion > > and noise are important (more important than the number of bits, > > which is largely a red herring). > > > My personal interest would be in fewer bits and much faster > > sampling: maybe a Virtex 5 connected to two or four 500Ms/s > > converters, into a deep buffer memory. > > > Cheers, > > Tom > > What's a Virtex 5? It sounds expensive. You are probably the kind > who gets ic's with gold-plated pins, where the rest of us have to be > content with tin/lead:) New (and fast) Xilinx FPGA. You can do a LOT with a much cheaper Xilinx Spartan3E. > > The AD7760 might be good starting point for entry level. At $35.00 > it's not too expensive, even for a scope with multiple channels. The > 120 dB might give 6 digits at DC, so it could be a combined DVM and > scope. The Binary Sampler could give a bandwidth past 5 GHz, so that > could give a very inexpensive system with pretty good capability. > > Once you have a system running, the wideband sampler could be used > for working on a faster digitizing system. Certainly 1 Gs/s is > quite feasible. > > I believe Maxim had articles on interleaving slow ram to use with > their fast digitizers. That could provide a starting point for the > pc interface. So it might be possible to bootstrap a system for very > reasonable cost. An FPGA can accumulate several samples and put them in parallel into a standard (cheap) DDR2 memory module. > > This is significant from several viewpoints. Sure, you can get cheap > systems on ebay. But what do you do when they break? Modern scopes > are not repairable, certainly not with the resources available in a > home lab or small business. After some point, even Tucker won't > quote on repairing them. So they become useless junk. But if you > build your own system, you certainly can repair it if it breaks. > > Another advantage is you can add more features to the system when it > is needed. Say you need more channels. That is simply duplicating > what you already have working. The software is open to anything you > can imagine. Once you have solved the problem of transferring data > quickly to the PC, analysis can be very fast. > > You can take also advantage of newer chips with improved > performance, so your system can evolve with new technology. > > The beauty of this approach is you are not dependent on trying to > keep commercial equipment running. Decades ago, I had no problems > buying complete labs of new HP and Tektronix equipment. > > But over the years, something happened to the reliability. The Tek > and HP equipment started breaking down, and repair costs were > astronomical. I began to get the impression it would be faster to > make my own lab equipment rather than spend the time packing and > shipping equipment back for repair. > > Now that inexpensive ic's are available with excellent performance, > it seems to be very reasonable to build your own equipment. > > And with the tremendous resources available in this newsgroup, how > can you resist? There's truth to what you write, though I'd temper it with what the real costs are at various points of use. What's right for you likely isn't for a big company. There are certainly still a lot of challenges to building your own equipment. How do you deal with big BGA packages? And that's just the tip of the iceburg. That said, I'll be taking my little $100 L/C meter in to work today because I don't trust the big expensive one there when measuring inductors... Cheers, Tom > > Regards, > > Mike Monett > > Antiviral, Antibacterial Silver Solution: > http://silversol.freewebpage.org/index.htm > SPICE Analysis of Crystal Oscillators: > http://silversol.freewebpage.org/spice/xtal/clapp.htm > Noise-Rejecting Wideband Sampler: > http://www3.sympatico.ca/add.automation/sampler/intro.htm
From: Frank Miles on 19 Dec 2006 12:48
In article <Xns989E18B5E84E2Noemailadr(a)208.49.80.251>, Mike Monett <No(a)email.adr> wrote: [snip] > DOS also gives the fastest possible performance, since you own the > cpu and don't have to give up cpu cycles for multitasking. That's > the reason I mentioned it - the response while sifting through a > megabyte of data was practically instantaneous from the keyboard, > but it would be a bit slower in Windows. [snip] Don't you have gaps where the various interrupts take control away? Perhaps your application doesn't have this problem, but if the gaps occur at "critical times" - say, in the middle of a single-shot acquisition - this would seem to be a problem. Do you turn off interrupts, and if so for how long? Are there any significant penalties for turning them off for an "extended period"? -f -- |