Crazy data acquisition specs? - Page 1
Forum section: electronics / testgear

author: drZoidberg
author_rank: Contributor
timestamp: May 30, 2013, 08:36:53 pm
content: Hello!So I'm looking at this open source data acquisition board's analog specifications, and they look really good. Maybe a little too good..The name of the board is OpenDAQ (link below).Can anyone understand how they are getting 0.01% absolute accuracy from a 2% 100ppm/C voltage reference?Also, they claim that it has 10Gohm input resistance and 35nA input bias current. Doesn't that require a 350V voltage source somewhere? Or am I getting this ohms law stuff all wrong?Is there some clever way that they can accomplish this, using the components you see in the schematic?The specs and schematics are in the manual (2nd link below).Can anyone make any sense of this?Link to the product:http://www.open-daq.com/Link to the specs and schematics (near bottom of manual)www.open-daq.com/shops/opendaq/uploads/opendaq%20manual.pdf

author: Matje
author_rank: Regular Contributor
timestamp: May 30, 2013, 10:55:54 pm
content: Quote from: drZoidberg on May 30, 2013, 08:36:53 pmThey don't.The Spec says 0.01% FSR (meaning Full Scale Range or so), at 4V. Since their ADC has 16 bits (i.e. about 32000 steps) this is just the resolution of the ADC, with some fuzzing. The practical accuracy will be way below that, due to the ref source, noise etc.Quote from: drZoidberg on May 30, 2013, 08:36:53 pmI do not see 10GOhms, only 1GOhms in the manual? I don't think input resistance and bias current have anything to do with each other, so no 350V are required.

author: thm_w
author_rank: Super Contributor
timestamp: May 30, 2013, 11:00:41 pm
content: The bias current and input impedance are straight from INA826 datasheet:http://www.ti.com/lit/ds/symlink/ina826.pdfQuoteThe fact that they used the typical figure of 35nA for the spec instead of maximum makes me hesitant to trust their other numbers.

author: Harvs
author_rank: Super Contributor
timestamp: May 30, 2013, 11:47:48 pm
content: I think there's an issue with English translation going one with it (I'm just assuming this is designed/made by someone in Asia due to the way it's written.) At a quick scan there's a few places where they've said accuracy but actually should have written resolution.Secondly I don't think they understand that if one component has a spec, it doesn't mean that their circuit will achieve that spec.Not a particularly impressive design on the analog front. And I certainly wouldn't be claiming 20G input impedance when you have the input directly facing a garden variety analog mux.Nope I think you can safely say the specs on that sheet are meaningless.

author: JimmyMz
author_rank: Regular Contributor
timestamp: May 31, 2013, 12:02:13 am
content: Quote from: Harvs on May 30, 2013, 11:47:48 pmI read near the beginning of the schematic, "manufacturers address," and it says Spain.

author: drZoidberg
author_rank: Contributor
timestamp: May 31, 2013, 07:10:03 am
content: Thanks a lot guys.Yes, I agree with the resolution/accuracy thing. I'm working on something similar and was worried that these guys were really that much better than me. But now that I hear what you guys have to say, I'm starting to get optimistic again =) Still, it's a bit demoralizing to know that if I ever finish my project, I will have to compete with people who are lying on their specs (intentional or not). I imagine that an average Joe can easily get fooled by this spec sheet, and end up buying the amazingly much "better" board.(Just to be clear, I'm also working on an open source DAQ)There was also one other thing that looked very weird to me. That was the inductor on the USB ground pin (). I can understand the inductor between the shield and the ground plane. This is to avoid RF interference from being injected into the ground plane. But isolating the ground pin from the USB drivers ground pin, surely that can't be optimal? Or does the differential signal somehow remove the need for low impedance grounding?Quote from: Matje on May 30, 2013, 10:55:54 pmYes, of course. The circuit uses active components for biasing the input stage (constant current source), this is how they get the high input impedance while still having the high bias current.