2ch >1GHz PCIe (or similar) ADCs on a reasonable budget. Ideas? - Page 1
Forum section: electronics / projects

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 10, 2019, 12:23:16 am
content: I'm looking at old mass spectrometers on ebay and some of them just have a direct CH1/CH2 channel out. While the entire instrument is probably not worth refurbishing due to software incompatibilities and stuff, I'm wondering: how would I go about acquiring a suitable ADC to get the most ouf ot it?Sure, I could look at the waveforms with a scope and save them, but that'll be pretty cumbersome. All google results for GSPS digitizers don't even list prices (and at that point I might just as well buy a more complete mass spec). Ideas? Maybe some alternate search terms or equipment that contains digitizers?

author: ogden
author_rank: Super Contributor
timestamp: August 10, 2019, 12:50:19 am
content: Quote from: KaneTW on August 10, 2019, 12:23:16 amRest assured -it will cost a fortune. DSO with Ethernet readout seems to be much better idea.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 10, 2019, 01:33:46 am
content: Yeah. Your specs are not completely clear though. What do you mean 1GHz? Do you mean 1GHz of analog bandwidth?And what do you mean by GSPS ADC? Is it 1GSPS? 2GSPS? 5GSPS? Higher? That won't cost the same at all.Anyway, you'll probably find something more affordable than a dedicated PCIe card with an USB oscilloscope or indeed a full-blown DSO.PCI/PCIe acquisition cards tend to be very expensive. If you're lucky to find an affordable one second hand or something, it's likely it will be rather old, and thus have no drivers and app to use it on any recent computer... so pretty much dead beef.And giving us a little more details about what those spectrometers output exactly may help us suggesting alternatives as well. You may not need that wide a bandwidth and/or that high a sampling rate.

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 10, 2019, 07:14:00 am
content: I don't have access to a mass spec but I presume it's an output from the detector, so it'll be detected intensity over time. Specs for the particular model I'm looking at recommend anywhere from a 2GSPS, 500MHz analog bandwidth capture card to a 4GSPS, 1.5GHz scope.http://www.chem.cmu.edu/cma/DE-STR%20Manual.pdfBallparking some values, I'm thinking there'll be about 1.3ns between two adjacent peaks if two molecules are 0.1 m/z apart, and 130ps if two molecules are 0.01m/z apart.At the same time, an acquisition would take up to around 100us.The data is usually fairly peaky, but also has a fair bit of DC component.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 10, 2019, 01:53:19 pm
content: To get an idea about the price of the Acquiris boards the manual suggests:https://www.electronicproducts.com/Board_Level_Products/Digitizer_boards_cap_1-GHz_analog_bandwidth.aspxThat's around $10000-$15000. Not sure you expected to spend that much.Something that is not quite clear to me (I still don't know exactly how those spectrometers work) is whether you really need high sampling rate acquisition (meaning do you really need single shot acquisitions?) Some kind of "sampling oscilloscope" (or a regular scope with modest max sampling rate used in equivalent time sampling mode) may be enough? (And would cost much less.)Something like this for instance:http://www.analogarts.com/products/usb-oscilloscope/sa985-1-ghz-bandwidth-oscilloscopeIt's $650, and seems to come with pretty decent software.

author: magic
author_rank: Super Contributor
timestamp: August 10, 2019, 08:01:38 pm
content: Yeah, those boards are expensive. I once naively checked if one wouldn't be cheaper than a full DSOI suspect the production volume just isn't there.I think I have seen faster sampling scopes than 1G being mentioned here on the forum and still at reasonable prices. You would need to search around. Of course an SS only works if you can retrigger the signal thousands of times and it's always identical.Maybe there are some high end scopes with useful ETS, but on the cheap ones it's a joke because the input amp isn't any faster than the ADC.

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 11, 2019, 02:57:42 am
content: Yeah, the Acquiris boards are too pricy, although I've seen old PCI versions with 2GSPS/500MHz on ebay for 300ish.I have a 350MHz/2.5GS scope available, so I could go that route. That's enough for low-resolution stuff.From my understanding (not an expert either) time-of-flight mass spectrometers do a bunch of runs and average them. I could loan a high-bandwidth scope for testing, and then see where to move from there once I have some data.So far I've only seen time of flight specs use single-shots. I'm not sure if an ETS will work, but in theory it should.(Now that I think about it, the trigger rate is probably going to be in the 20Hz region---too slow for an ETS to get a good signal in a reasonable timeframe)

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 11, 2019, 03:25:04 pm
content: Quote from: KaneTW on August 11, 2019, 02:57:42 amWell, as I mentioned above. I suspect those "cheap" boards are untested and won't even be supported on any recent hardware and software...Quote from: KaneTW on August 11, 2019, 02:57:42 amObviously start with that, at least to see what you're dealing with. It would at least help selecting a more appropriate equipment if needed.Quote from: KaneTW on August 11, 2019, 02:57:42 amThe 3 above sentences look a bit contradictory to me (but I may miss something!)If you need "a bunch of runs and average them", that's a very typical use of a sampling oscilloscope (/ETS mode). Sounds contradictory with the single-shot acquisitions in itself.The trigger rate is not relevant? If you need a bunch of averaged runs, you'll need them whatever the trigger rate is. It's just going to take a while.I don't know how many runs would typically need to be averaged, so I don't know whether using single-shot acquisitions and averaging them would make any difference with ETS in terms of total acquisition time. Some real figures would help.

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 11, 2019, 04:06:06 pm
content: There's only so much sample and a tiny bit of the sample is vaporized with each trigger, so you can't do too many captures. I'll try looking around for some real data. I haven't seen any references to sampling oscilloscopes so far, though, so maybe I'm missing something.

author: ogden
author_rank: Super Contributor
timestamp: August 11, 2019, 05:25:02 pm
content: Mass spectrometer is obscure for many. I know that there exist (existed) spectrometers that do not need high speed sampling:Do you have pointers to info about instrument you are aiming at? Could be nice to learn something new along the path.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 11, 2019, 06:51:07 pm
content: Quote from: KaneTW on August 11, 2019, 04:06:06 pmI can understand the point. Just still not sure how it mixes with the need to average a bunch of runs...Anyway, this certainly sounds like an interesting project. How much did you pay (or are you going to) for the spectrometer itself?Will be interesting to see what you get out of it.

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 11, 2019, 07:23:46 pm
content: Quote from: ogden on August 11, 2019, 05:25:02 pmThere's a used "for-parts" 1k EUR Applied Biosystems Voyager DE-PRO on eBay. I'm not sure if I want to buy it at all---I'd have to find compatible replacement parts for the PCI parallel I/O cards and I'm not sure if it was properly decommissioned. I posted a link to the manual further up (http://www.chem.cmu.edu/cma/DE-STR%20Manual.pdf)It's a time-of-flight device with direct (amplified) detector outputs, from what I can tell.In the end it's not quite an ideal device, because I'd like to be able to use it in conjunction with a LC system. But it's certainly a step up from nothing at all and looks like an interesting project.

author: ogden
author_rank: Super Contributor
timestamp: August 11, 2019, 10:06:50 pm
content: Page 88 of manual mentions oscilloscope as sampler option:

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 12, 2019, 11:48:09 am
content: Yeah, saw those. These would be ideal but hard to find or expensive.

author: KaneTW
author_rank: Frequent Contributor
timestamp: August 12, 2019, 03:57:27 pm
content: I've done some simulations and with a 350MHz/2.5GSPS scope I can actually get a pretty good resolution; around 0.015 Dalton on a 500 Dalton molecule, yielding a resolution of about 30000. Higher bandwidth doesn't seem to help much, and you can drop down to about 250MHz without compromising the signal.Some further testing reveals that the resolution is proportional to the sample rate, with a bandwidth requirement of at least 1/10th. (I wonder if that's a theorem somewhere)E: Bandwidth does seem to matter once you have close-by peaks where one peak is significantly larger (~10x) than the other.