tony sayer wrote in news: snipped-for-privacy@bancom.co.uk:
No. Without disclosing a diagram I have no right to pass on, there isn't. In the posts here I described it in a lot of detail too, so I won't repeat myself.
In article , Lostgallifreyan scribeth thus
OK Fair comment!...
-- Tony Sayer
tony sayer wrote in news: snipped-for-privacy@bancom.co.uk:
Cool. Thought you might be going to argue. :) I can tell you this much... I'm making a device that lets me turn a sound card into a logging tool. Assuming the ADC's have reasonable DC performance, I can use it to map out changes that are unique, too fast for a multimeter, too slow and unrepeatable for an oscilloscope. By using something like Sound Forge, which has really nice keyboard shortcuts for zooming, locating, selecting, etc, it makes fun easy work of sifting through truly enormous amounts of data, so watching for all sorts of drifting in meter circuits can be done. Further, you can do experiments logging to one channel while speaking a commentary into the other, so on playback you can know what you did that caused the responses you see. This could be a very useful diagnostic tool, and very cheap.
All the earlier specifics are basically about trying to do this while not screwing up the sound card performance, so the adapter can be left in circuit.
Out of general interest, Sparkfun Electronics do a nice multichannel logger called the Logomatic V2, which (with Kwan's firmware) can log two serial inputs and 8 (10?) analog inputs at up to 1KHz at 12 bits, but if, for a similar price, you can get two or more channels sampling at 48 KHz at 16 bits, it has to be worth trying... A lot of people have done this sort of thing for laser show control by modifying the outputs, but I haven't seen much to suggest anyone's doing it to inputs.
Should be enough here now, with the other posts, to allow anyone to figure out what to do. Or come up with a better way, in which case, please post it.
What I did was to buy a second-hand HP 35665A for $350. A boat anchor, but a goodie.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Phil Hobbs wrote in news: snipped-for-privacy@electrooptical.net:
A bonny wee beastie. :) I like HP Agilent stuff, I have a nice 1740A scope that people compare favourable with a Tektronix 265B scope but having seen both, I think the HP wins tenfold.
But here, I bet I could pay the equivalent of $350 just getting that HP
35665A shipped to me. Joking.. but it does look heavy. I'll settle for the adapted soundcard because the bang per buck is so good, and I can do it with no added weight of gear, or extra space found for it. And I can use all kinds of software to handle the data easily.Here's a followup to this, because it seems to work, so if anyone's still interested, they might like to know how it went...
First, I noticed noise, about 270µV of it, so I put a LPF filter in the reference voltage (10K and 10µF ceramic). I also changed the buffer amp from LF412 to another of the OPA2277A's I'm using. (The other channel buffers a negative voltage for when my board is to remove a DC offset instead of adding one). I also had to desolder a pin on the ADC and bend it to a conveniently grounded pin next to it, and solder it there to disable an onboard digital HPF.
I now have DC coupling, with noise on an empty channel within 3dB of best unmodified system performance, which is better than I'd hoped. (-78.3dB as opposed to -80.8dB originally).
Most of the existing DC offset is in the rest of the original system, I know this because I can see it change as the device warms up, with all external signals being absent or constant.
The DC offset remaining is around 700 values on a scale of 32768 so I'm ok with that, especially as Sound Forge makes a truly neat way to remove it immediately prior to record. It's so good that when detecting laser power on the meter all this is aimed at testing, I won't need to tweak its own offset, I can just record the output and do that in Sound Forge, as well as any extra filtering I might want.
My conclusion is that modifying a decent studio audio interface for data logging at arbitrary sample rates from 2000 Hz to 96000 Hz is well worth doing. One ideal unit is the Echo Layla24, often found on eBay for less than £100 now. Given the bang per buck, I prefer this to any other method because I can still use it as a viable multichannel audio I/O when I want to.
(Incidentally, DC coupling on those units is even easier, as they don't have DC on either side of the DC blocking caps, so just put a wire link where those are now, and get accurate voltage generation up to around ±13.5V, with fast and accurate changes, from wave file players or other software... All kinds of uses for that, no doubt).
One last point: I can get decent audio band through an OPA2277 despite the modest slew rate, but there are limits. Full scale differential input is possible for sample rates up to 48 KHz, but for 96 KHz only non-balanced input will allow this cleanly, so if the input is differential on a system with a ±V supply, attenuate the signal by 6dB, or choose a faster low noise amp. The low offset might not seem so important now, but the low noise and drift still are.
Lostgallifreyan wrote in news:Xns9DBBACB9E92C9zoodlewurdle@216.196.109.145:
Correction: 'with a ±15v supply'...
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