From: John Brookes <[email protected]> Reply-To: <[email protected]> Date: Saturday, May 10, 2014 at 7:28 AM To: <[email protected]> Subject: Re: [beagleboard] BeagleBoard Data Acquisition Platform
> My dear Penguin, > > Yes, the desired SPS rate is rather low, as I was feeling limited by the > actual possibilities here. I am a pattern recognizer > bird myself, and looking primarily at 2 problems: > 1. Non Invasive medical diagnostics. These use NIR, electronic noses, and > perhaps EEG, EMG. > 2. The quantum mechanical nature of consciousness. These onvolve detecting > signals like 1 above - coupled with AI, like 1. > > Both of these aims ask for rather many channels with rather low speeds. Why? > Because the combinatorial pattern recognizer asks for > a unique signature across many channels. Example (rough): Suppose you can > detect a signal in the NIR at .8 microns. It has 32 levels, > IOW a crappy signal. That's 2 exp 5. Now suppose you have 8 of these channels. > Suddenly, the degree of freedom is 2 exp 40. IOW > the signal is more unique, and noise has a harder time jamming. > > That's the justification for many channels. Now, I wonder what your desire for > high speed is. What would you use it for? I looked at the > microchip pic24fj128gc010. That can do only 20 MSPS, though. (from memory) > OTOH, I have a Tektronix scope that can do 10 GSPS on 4 channels. > So I am not aware of ADC chips that fast - It's an interesting problem though > - How would you design such a thing? http://www.ti.com/tool/adc12d1800rb Regards, John > > > Let's see - 120 MSPS would entail 240 MB/s at 16 bit res. (just for argument > here.). That seems well within SATA 3 and USB 3 type rates these days. > But this is not a vanilla ADC chip. > > A SAR-type design (speaking very roughly) will require 240x16x10exp6 > comparisons per sec. That's a comparator speed of 4x10exp9 per sec - > A comparator can get a speed (realistically) of 10 nanosec. Thats 10 exp 8 > comparisons per sec. So 4x10exp9/10exp8 = 40 comparators. > > Seems doable, but I have shied away from it as being beyond my practicality > (and needs) - since I am not skilled in high speed circuit design, as well as > the issue of handling and data storage at the CPU level. > > I estimate that, in order to get that rate, you would have to build a desk top > super computer, made up of DSP board multiprocessors. (idea to get away from > circuit design.) > Then the DSP memory would be connected to 1TB SSD drives to collect data. At > this rate, you are going to accumulate a gB in 4 seconds, so a 1TB SSD will be > good for > an hour of data collection. > > What do you want this speed for? High speed events? > JB > > > > On Sat, May 10, 2014 at 3:00 AM, sa_Penguin <[email protected]> wrote: >> 32 channels is - a lot. At 16-bit, too. On the other hand, the actual sample >> rate is quite low: 200K sample/sec >> This sounds like the backbone of a 32-channel audio mixing desk. Which is >> fine, if that's what you are into... >> >> Personally, I'd prefer 2 [or even 1] ADC channel, with a MUCH higher sample >> rate. Say: 120 M sample/sec. >> Sure there are faster ADC's - but original [parallel] ATA cables were rated >> to 133MHz, so I'm aiming for a spec >> that reduces the need for matching length tracks etc. >> >> You'd probably need an FPGA to interface that with a Beagleboard, or >> Beaglebone Black [my device]. >> >> -- Alan >> >> >> On Wednesday, 12 May 2010 05:04:07 UTC+9:30, Ben Gamari wrote: >>> Hey all, >>> >>> For those who care, I have drawn up designs for the second iteration of my >>> BeagleBoard-based data acquisition platform[1]. >>> >>> This new design features 32 DAC channels and 32 ADC channels, both with >>> 16-bit >>> resolution. The ADC sampling rate is a little lower than I would have liked >>> at 100ksamples/second (with the SPI bus running at 2MHz), but this should be >>> more than enough for most tasks. The DACs on the other hand can run at up to >>> 20MHz (limited by the level shifters). Additionally, the board now exposes >>> 8 >>> GPIO pins behind a level shifter, making it possible to directly interface >>> with >>> standard 5V TTL levels. >>> >>> The ADC part I'm using is TI's ADS8344 and the DAC is TI's DAC8568. The >>> level >>> shifters are TI's TXB0108 and the demultiplexer used for chip select is TI's >>> SN74AHC139. Altogether, the board is quite expensive. Each of the four DACs >>> are >>> $25.00 and each of the four ADCs are $10. Thus, a fully populated board is >>> about $150 in parts alone. Far more expensive than I was hoping for, but it >>> seems that these prices are pretty common in the world of converters. >>> >>> The board is designed to fit on a BeagleBoard XM-style expansion connector >>> and >>> thus sits beneath the BeagleBoard. >>> >>> One issue I encountered with the last design[2] was the large in-rush of >>> current at startup which seems to cause the BeagleBoard to brown-out. This >>> makes it necessary to remove the board while starting up the BeagleBoard. >>> While >>> I'm not certain of the cause of this, I suspect that the largish filter >>> capacitors (330uF IIRC) on the voltage rails might be at least in part to >>> blame. Anyone have any thoughts on this? I've reduced the value of these to >>> 100uF, but it would be nice to have a slightly more certain solution. >>> >>> If anyone has any comments, I would love to hear them. I think this design >>> is >>> orders of magnitude better than the original, but there is no doubt still >>> room >>> for improvement. In particular, I would love to hear suggestions about the >>> PCB >>> layout. I took some steps to ensure good analog characteristics (e.g. >>> maintaining continuity in the ground plane), but I'm sure there are other >>> things that could be improved. Moreover, the reference supply is little more >>> than a RC filter. Is this sufficient or could there be a better option here >>> (perhaps an active voltage reference or Zener regulator)? >>> >>> Anyways, I look forward to hearing any feedback that folks have. Thanks for >>> listening. >>> >>> Cheers, >>> >>> - Ben >>> >>> >>> [1] http://goldnerlab.physics.umass.edu/wiki/BeagleBoardDaq >>> <http://goldnerlab.physics.umass.edu/wiki/BeagleBoardDaq> >>> [2] http://goldnerlab.physics.umass.edu/wiki/BeagleBoardDaq/Version1 >>> <http://goldnerlab.physics.umass.edu/wiki/BeagleBoardDaq/Version1> >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "Beagle Board" group. >>> To post to this group, send email to [email protected]. >>> To unsubscribe from this group, send email to >>> [email protected]. >>> For more options, visit this group at >>> http://groups.google.com/group/beagleboard?hl=en >>> <http://groups.google.com/group/beagleboard?hl=en> . >>> >> -- >> For more options, visit http://beagleboard.org/discuss >> --- >> You received this message because you are subscribed to a topic in the Google >> Groups "BeagleBoard" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/beagleboard/TMxuNnQ4zOQ/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> [email protected]. >> For more options, visit https://groups.google.com/d/optout. > > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. 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