Universal PI Micro

[BJBJ]

BW!

I don't want to discourage you but!
Studying the datasheet I see that they're quoting +/-15ppm INL which
is closer to 16-bit resolution than 24. A bit more digging suggests
that the part is really only accurate to 18 bits at very best under
premium circumstances which a detector is not!

So some of those 24 bits are quite worthless. For instance Audio
rate codecs aren't much better 24-bit ADC/DAC combinations are
pretty cheap these days, but unless you're picking the parts
carefully and surrounding them with high-quality drivers & supplies
you'll never get the kind of accuracy that simply counting bits
would suggest.

Assuming a +/- 1V signal swing, a single bit is around 0.1uV
at 24 bits. This is pretty much impossible to engineer,
especially considering the equivalent circuit SNR has to
be better than 144dB. Even if voltage swing are increased
dramatically this PI enviroment are about to trash AD bits
around.

When recording audio for instance, it's impossible to find a
microphone+preamp combination that comes anywhere close to
that. And that are in studio environment here we are out
in the field!!

So most so-called 24-bit converters are just specmanship.

Even if your signal are heavy filtered/sampled into DC i would be
supriced if you even could squese out 16bit of resolution of the Aduc.
You still have charge injection to deal with who will alter the
sampled value.

Best regards
BJBJ

[BJBJ]

>You really want to see the noise BjBj ? it isn't a pretty sight !

Yes please, i do!

What do you think of my frame sample idea? Bad?

>PS What did you find so hilarious about the nuggets ?

Hilariously large!


We simply dont have these here in mid Scandinavia.
The ice age have grind'ed them down to powder.

Regards
BJBJ

[bugwhiskers]

Hi BjBj,

There is some averaging being done in software and it may well prove to be enough. It is possible to have a switchable non motion mode where the user would hover over a suspected target then some serious averaging to reduce noise could be done.

You really want to see the noise BjBj ? it isn't a pretty sight !


regards
bugwhiskers

PS What did you find so hilarious about the nuggets ?

[BJBJ]

Do you have a scope picture (JPG) of a typical decay curve with this 80mV noise?

What about this idea then, why not using the GPR principle?
Stroboscope the samples period then averaging out the noise?
It give far more detail to the sampled signal but it will require
fast sampling but then you could lower the number of sample
cells, i.e run it in sample frames , one frame of e.g 8 samples
then a storage period to RAM then run another frame.

[bugwhiskers]

Hi BjBj,

The biggest problem with the bus prototype was noise, a whopping 80mV of it in the decay curve.
The earlier you can sample with a PI the better, especially for small nuggets. The more amplification you put on the front end the later the earliest sample time. If a low amplification is used and the overall "gain" is realised through improved resolution of the available signal voltage then it would seem to be the best all round compromise. With 24 bits to work with it is very easy to shove them right a few times (halving the value)to get rid of noise and still have a fairly chubby number to work with.

regards
bugwhiskers

[BJBJ]

BW!
If i may ask, was the performance in your nugget test not satisfying
with the Atmel device in terms of the resolution of the 10bit AD?

Btw, hillarious nuggets!

regards
BJBJ

Hi BjBj,

The resistors show in the schematic that are on the inputs to the AtoD are zero ohms. The caps are 10nF which is what I am using in the bus prototype.

The screen should be up and running by the weekend, a bit slower than anticipated due to paying work commitments and getting used to a different micro's instruction set.

Regarding a better MUX, Maxim make a MAX4051, the specs of which are slightly better but at quite a price premium.

regards
bugwhisker

I was more into the DG range of muxes, however as they are really premium they come with a premium price to, but before switching to premium muxes
it would indeed be interesting to hear what your results show with the Aduc
device.

Regards
BJBJ

[bugwhiskers]

A quick note for BjBj

Hi BjBj,

The resistors show in the schematic that are on the inputs to the AtoD are zero ohms. The caps are 10nF which is what I am using in the bus prototype.

The screen should be up and running by the weekend, a bit slower than anticipated due to paying work commitments and getting used to a different micro's instruction set.

Regarding a better MUX, Maxim make a MAX4051, the specs of which are slightly better but at quite a price premium.

regards
bugwhisker

[bugwhiskers]

Hi nitrous,

The 2 evaluation boards that I ordered a few weeks ago just turned up 10 minutes ago. They are 4 inches by 5 inches and have not been designed to be as small as possible. However, they are well made and cheap and an easy way for users to get into micro controlling their PI experiments. There is also a 4 inch * 1.5 inch prototype area.

The kit came with 2 power plug packs, neither are suitable for Australia but there is provision for using a 9v battery which allow me to get it up and running straight away. The documentation includes a BOM which I couldn't obtain from the Internet.

regards
bugwhiskers

[nitrous]

thanks guy

how big is the board expected to be at this point?
The 'here and now' is at least as interesting as the hereafter!
Who knows what the future might bring.
Thanks for the reply.
Doug

[bugwhiskers]

Hi nitrous,

Thanks for your interest in the project.

There are no plans to make a miniature version of the evaluation board at this stage.

regards
bugwhiskers

[nitrous]

will you have an SMT system available?

Hi there Bugwhiskers

Great project you've developed. Especially since so many others have become involved.
I wonder if you'll have a miniture version of your PI system available.

I would like to make a very small version, perhaps modified with a separate LCD/Audio module. What will the final size be and will you be making SMT versions available? Something that has the main board in the headphones? An other possibility would be to use this board for a pinpointer with appropriate coil design. Again, a small form factor would be very helpful.

Thanks,
Doug

[bugwhiskers]

Hi 1843,

There is a link back in post #104 that shows the software suite or IDE for programming.

regards
bugwhiskers

[1843]

Hi bugwhiskers,
Which program do you use to write your project (u-controller data)?
Regards

[BJBJ]

Unregistered was just me again! I wonder why it went unregistered?

Anyhow,yes it will act as a simple Rc filter but the voltage in and out
are depending on the voltage in and out, the less voltage into the switch
the worser the accuracy.To remedy that a bit one can drive the Mux from
either +10V or +/-5volt or simply switch to a better Mux.

Indeed, there are always a bit of laboratoring with new designs.

Your doing great.Keep up the good work.

Regards
BJBJ