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PIC ADC sampling time?

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Wond3rboy

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Hi i want to know that can we use an external ADC which samples at high sampling rates and then use it with an 18F series PIC?

Also heres my million dollar question(last time it sparked controversy!):

Can i use low frequencies to measure the depth of atmost 15 inches of limestone(the kind used in highways i-e compacted)? Any help?
 

dknguyen

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Most Helpful Member
Low frequency...what? Lots of can be low frequency,
 
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kchriste

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Hi i want to know that can we use an external ADC which samples at high sampling rates and then use it with an 18F series PIC?
You mean an external ADC because the PIC's ADC isn't fast enough? Yes you can. As long as the PIC has enough MIPs to keep up with the sampling rate and perform a useful function at the same time, it shouldn't be an issue.
Can i use low frequencies to measure the depth of atmost 15 inches of limestone(the kind used in highways i-e compacted)? Any help?
Are you talking about using sound waves to detect limestone? I don't know much about that, but I would look into how you would distinguish limestone from any other rock/clay/etc underground. I would think it would be difficult to tell the difference. Maybe detect the air pockets formed by the erosion of the limestone by water? In the oil patch they use explosives for this. :D
 
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Wond3rboy

Member
No GPRs please..thanks.

Thanks for the replies.

@dknguyen, by low frequencies i meant something that can be sampled by the ADC taking in to account the 2Fmax Nyquist criteria.

@kchriste, detecting airpockets may result in wrong readings since the airpocket may not be at the bottom of the limestone pile on the point where i try to measure it.I am looking in to it.

@Mike and Burt, i was looking at some ADC's offered by TI and saw that all were available in SMD's( i would prefer otherwise though).I wont be able to process and maintain the maximum sampling rate that i can get so i will store them in a table and then either send it a computer for processsing or use the uc to do it.

If you have anyother info then please share.
 

ericgibbs

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hi W3,
Is the 15 inches of limestone, loose rubble or undisturbed sedimentary rock.??

What resolution in measurement is required.?
 
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Wond3rboy

Member
Hi Eric and Nigel, the limestone is 15 inches of compacted stone laid out for a road(they do most work manually here) so i want to be able to measure that the amount laid is the intended amount. The internal ADC of PICs has a max sampling rate of 200Khz and the only 'low freq' waves i found used in soil measurement were around 1Mhz to 500k. For that i will need a high sampling rate. As i said in the previous post that i just need to capture the data(for enough time to get an accurate reading) and it can be processed later.

I was thinking about putting some water and then try to differentiate the moisture level since limestone wont absorb it but the ground(most probably) will.

I was looking at the specs of the 18F46K20 which does 16MIPS(highest for 18F's).
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
The internal ADC of PICs has a max sampling rate of 200Khz and the only 'low freq' waves i found used in soil measurement were around 1Mhz to 500k. For that i will need a high sampling rate.

That's NOT low frequency, that's radio frequencies.

You need to be looking at video FLASH ADC's, and probably only 8 bit ones.

You also need to be looking at a system with lot's of fast RAM - you're talking large amounts of memory needed, 3 MBytes per second of recording or so. It's not really a PIC application.

I would suggest you first evaluate the entire premise using a laptop, and once you have it all working, then worry about making a small standalone version.
 

Wond3rboy

Member
By 'low freq' i meant with respect to the start of the freq used for Depth Detection which are usually GPR stuff.

The measurement is stationary and the amount for time can be automatic(less then 1 sec if readings are accurate).I will try to carry out the LAptop evaluation thing.Thanks.
 

be80be

Well-Known Member
Mike Well I didn't read the pdf should of it said it could do both at microchips chip finder ADC
and 2 DAc is what it said at @4us sorry maybe they linked the wrong chip.
 

Russ Hensel

New Member
Have you got the rest of the hardware together? Look at its output with a scope and see what is going on. If this works then you can move to the signal processing. If it does not then the processing will get you no where.
 

ericgibbs

Well-Known Member
Most Helpful Member
the limestone is 15 inches of compacted stone laid out for a road(they do most work manually here) so i want to be able to measure that the amount laid is the intended amount.

hi W3
If its only a 15inch depth of compacted stone, [30 inch path distance] I would use a more basic method.

All you are looking for is the echo due to the change in density of the stone and subsoil interface, its a time measurement.

A mechanical device for striking the surface and sound detector for picking up the 'strike' and reflected echo.
Start and stop a counter, calibrate by initially placing the sound detector 1 mtr on the road surface away from the 'strike' point.
 

kchriste

New Member
Forum Supporter
As mentioned by Eric, you don't need to ADC sample the actual 1Mhz signal. Just the envelope of the echo, which will be a much lower frequency.
 

Wond3rboy

Member
Hi Eric and Kchriste, sorry for taking too long to reply.Thanks and i am working on it.Will post any question if needed.
 
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