Hi. I just bought this thinger:
Has anyone here played around with one? I was thinking that with a copy of Sound Forge or similar app, a very very cheap realtime datalogger can be made. The main sticking point would be DC blocking, which would have to be removed.
Any thoughts?
-- ---------------------------------------- http://save.nazanin.googlepages.com/home
I think most A/Ds have high pass filtering in DSP. It may simply not work.
Graham
Pooh Bear wrote in news: snipped-for-privacy@hotmail.com:
Which would be annoying. I guess someone might have made a DirectShow plugin that can full-wave-rectify a signal, that way I could get a unipolar trace by filtering the rectified input from a 20 KHz oscillator whose output amplitude is controlled by the signal I want to log. That defeats the point though, I'm hoping that someone made a sound card that leaves nothing more than removal of DC blocking caps as the only thing to do to get a data logging input.
Incidentally, I know that a bit of code in a hardware DSP might come cheap, but cheaper than providing and mounting two 10 µF caps? Surely not? >:)
I can see why it might be done to save space in a widget like the thing I linked to, but as a standard practise, I'd have thought it was not done. If is, I guess it might explain why it's impossible to modify an Echo Layla 20 bit interface to take DC inputs. In that, there certainly is something blocking DC on the ADC itself.
-- ---------------------------------------- http://save.nazanin.googlepages.com/home
Another issue is power. If I recall properly, power-over-USB provides only a single supply. If you want to measure DC voltages which swing below ground, you're going to either need some way of developing a secondary, negative-rail voltage supply, or need an external bias network (possibly with a buffer) to shift the voltage up into the common-mode range of the single-power-supply USB chip.
Standard USB-for-audio-purposes adapters probably do the latter... capacitor-couple the signal and then use a resistive ladder to shift the centerpoint up to .5 Vcc.
That's probably cheaper than having to provide a second power supply of some sort to the USB chip, so that its common-mode input range extends well below ground.
--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
isn't this a audio output only device ;-)
Stef Mientki
The DC block for sound cards might be on-chip and then you are pretty much stuck. A better option could be one of those TI EX430 kits. They cost $20 and contain an MSP430F2013 which has a 16-bit ADC on board. No DC blocking plus it's freely programmable.
-- Regards, Joerg http://www.analogconsultants.com
Joerg wrote in news:oBIjg.44341$ snipped-for-privacy@newssvr27.news.prodigy.net:
Thankyou. That also answers my unanswered question in another thread, for suggestions for a specific device with ADC and some space for code. You wouldn't know one that also has 16 bit DAC in addition to that lot, would you? I'd want one for making a log convertor for analog voltages, with a gadget that makes the simplest and cheapest board layout possible.
-- ---------------------------------------- http://save.nazanin.googlepages.com/home
Stef Mientki wrote in news:73b97$448f384f$83aef479$ snipped-for-privacy@news1.tudelft.nl:
Nope :) Definitely input, one of those sockets is a stereo mic input. This thing might not be suitable, as people have said, but that would be because it's stuffed full of DSP I don't want and maybe can't bypass (although if it IS the Micronas chip, it is apparently user controllable by some means I don't know yet). I just want a cheap way to get 16 bit voltage conversion in and out, and the USB idea is appealing. If I can get this in something cheap and mass-produced, I will, there's no point in re-inventing the wheel at inordinate expense..
-- ---------------------------------------- http://save.nazanin.googlepages.com/home
Not from the MSP430 series. Atmel or Microchip might have something but I am not familiar with their devices, except for some 8051 family parts.
-- Regards, Joerg http://www.analogconsultants.com
Texas has some of these chips, if I remember well 24 bit ADC + ucontroler (8051?) + 20 bit dac in 1 device (maybe even a few ADC/DACs), I just found out after making a log converter with separate components ;-)
Stef
There may be a quick and dirty option: Find out the highest frequency it can process reliably and with enough dynamic range. Then chop the slow input signal at that rate. A simple CMOS oscillator and a gate or FET can do that. You may lose some bits but hey, if it's good enough it might work.
-- Regards, Joerg http://www.analogconsultants.com
Joerg wrote in news:r0jkg.44845$ snipped-for-privacy@newssvr27.news.prodigy.net:
Nice. :) I think that might just do it.
One other thing I'm thinking is that the HPF on an IC might be disabled, if the IC designer was generous about this.
The Echo Layla 20 bit audio rack unit is a VERY tempting device for laser scanner control and other things, monitoring laser power, general lab monitor and control. Echo Audio kindly sent me the I/O datasheet in PDF. I've found that the ADC is a Cirrus Logic CS5335-KSEP and with a bit of fine surgery pin 1 can be made to control DC offset, and a DC blocking cap can be bypassed. Currently, if I try to bypass the DC block cap, the result is white noise in that channel, I have NO idea why, it really is odd, that one..
If I can solve this, the CS5335 pin 1 will be a nice control, when low, it tracks an onboard op-amp's DC output and cancels it, when high, it makes the offset compensation freeze, so I can short an input, hold low for a second, then raise high to eliminate any offset between the short and the DAC input inside the CS5335, regardless of source, an extremely useful feature. I hope the Micronas or similar IC in those cheap USB thingers can be modified for this, but I suspect they might not provide a means to bypass the HPF, even though they do apparently offer some control.
All this sounds tedious, but it beats having to build from scratch every time, especially when 8 ins, 10 outs, at 20 bits or better (plus 2-channel S/PDIF I/O at 24 bit), can frequently be found on eBay for £70 or so. Beside the prospect of adapting that, all ideas of self-build look as appetising as a pair of used boots on a plate.
Your fast-chopping idea is nice though, if it works, it won't matter what the input processes are, at 20 bits I might still get to keep an accurate
16 bit log. These digital multi-channel things put out sync signals too, so I guess if I can derive something from that I can reduce bit-loss by making the chopper sample-accurate.-- ---------------------------------------- http://save.nazanin.googlepages.com/home
Stef Mientki wrote in news:15ce0$44905281$83aef479$ snipped-for-privacy@news1.tudelft.nl:
That one sounds cool. Anything that takes the pain away... :) As I mentioned in the big post a few minutes ago, it's usually far more attractive to adapt existing commercial devices, given that there are so many cheap ones, and surely some might be spectacularly useful if this works.
I've looked at log convertors with analog parts, and decided not to try it that way, though one analog log amp still IC interests me, the AD8307. An
8-pin DIL. The datasheet talks high, wide and plentiful about RF though, I don't know enough to know if it could do DC for 3 octaves or so of accurate freq-pitch scale conversion, and no-one's been able to tell me yet so this one is still a mystery to me.-- ---------------------------------------- http://save.nazanin.googlepages.com/home
My ISP's news server is dropping posts, some of which later pop up (like yours right now). Would have answered sooner but couldn't see it.
Basically this is modulating your input signal onto a carrier, the chopping frequency being the carrier.
They usually aren't. They are after that one big market of audio capture and anything else is peanuts to them from a business point of view.
Look at the DC level on the other side of the cap with a scope. If it isn't zero at all times (doubt that it is) then a bypass would force it down and one or more bias levels could go out of whack. Once they are it can take a longer time than expected to return to normal.
True. If you want to cheat and can spring about $400 then this one might be a nice option:
You can also synchronize with a nifty software PLL that "learns" when to expect the sample. But that can run astray if the signal is too low for a long time. As with building your own hardware that'll be a hassle you just might not need right now.
-- Regards, Joerg http://www.analogconsultants.com
Sorry about delay. :) That was a good post, so I wanted to return some effort.
I might also try the chopper idea, as the 'word clock' output is regular and always present. I wouldn't try to decode it in software if it's regular enough, I'd just filter it to get a simple regular pattern and lock an analog PLL to it if possible.
Please let me know if the proposed modification in the PNG file looks ok, or might be bettered (bearing in mind the need to minimise to extreme any mods made on the original board).
As mentioned in the PNG file's text, the ADC does allow direct coupling by bypassing the DSP HPF it uses to block DC. Actually it either monitors it and perpetually readjusts the output to compensate, or it freezes the offset at the last known value. I mentioned that in some more detail in my previous post in this subthread:
Well, it's my news server that does that sometimes.
It was a bit hard to see, maybe next time you could post a zoomed out schematic (browsers are often bad at zooming). And add designators (R1, R2...). :-)
I'd drop the 10M input dividers to 1M, still shouldn't mess up your bias. A concern: This opamp may not like it if you feed it just 5V single supply. Also, IIRC it's input common mode range must stay about
1V away from its negative rail and if you grounded the neg supply this is cutting it really close. You might want to make that new input divider asymmetrical to push this a little higher.-- Regards, Joerg http://www.analogconsultants.com
I should have mentioned, the main circuit's op-amps are on a dual rail 15V supply. Sorry, that was definitely a gross oversight of mine, not showing that. About the resistors, I agree, I'd considered that too. I often reduce to 1M in situations where it first occurs to try the highest I have, things are usually more resilient to induced noise that way, and any error is more predictable. I did try shrinking the image but it made a pig's ear of the text, probably another dodgy decision to put it there, but nm.. My main concern is whether there might be some horrible gotcha to look out for when using the non-inverting summing amp in this way. Also, the possiblity that some vastly superior answer might be staring me in the face while I can't see it. :)
Watch out for power sequencing. If your 15V supplies come on before that of the CS5335 or if your opamp bias points aren't immediately stable when those 15V rails come up the top right 150ohm resistor might source up to 100mA into the AINL+ input. This could cause it to latch up or fry. I don't know how much it could stomach, might be worth to think about raising that resistors value.
-- Regards, Joerg http://www.analogconsultants.com
I'll trust that one :) That's a network the unit's maker applied right from the IC maker's datasheet, two 150R resistors and a 2n2 cap. I'll tap the same supply the main op-amp's getting, for my addition (though I'll maybe modify the PSU slightly to carry the extra power, as some bits already get hot). I think it will be ok, as the 2.2 V won't be doing anything it didn't already do before, as the CS5335 sees it. I'm more cautious about that reference itself, I think the CSS5335 sources it at a very low current, hence my initial choice of
10M for resistances on the adder input. I'm thinking of using the LF412, one for each pair of channels, four IC's in all.ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.