Is this a major advance in PI sensitivity ??

[Midas]

Well that is another point of view ... however there is not a single design posted on this forum that actually uses an ADC ( commercial schematic or user submitted ) ... happy to be proved wrong here.
The design process should begin with the GOAL ( or outcome ) ... which I would imagine is to design a detector with performance at least in the same ballpark as commercial detectors.
Secondly the main functional subsystems that make up the detector system should be identified, how they interact and specified.
Thirdly each of these subsystems should be designed, prototyped and tested.
Fourthly the whole system should be integrated and the proof of concept be verified.
Fifth at this point the required performance gap should be identified AND IT IS AT THIS POINT THAT ANY EXOTIC COMPONENTS ARE UTILISED as each of the subsystems is brought up to the required performance level. If you can avoid it you wont use exotic components as they often suffer from supply problems during the product life cycle.

Finally thats why these forums ... ( although very interesting so dont get me wrong here ) ... never result in a finalised design. Everyone is either simulating or looking up chip catalogues for better and better chips when in reality I think there would be much more satisfaction if all the subsystems that actually make up a detector were actually posted up.

So am I being hypocritical here .... I think not.

I have specified and actually made ( as against talking about it ) a PI controller chip.
I have specified and actually made a frontend ( actually several ) but the two main ones are the differential and the more recent mono (with diff amp to reduce EMI).
I am working on the sampling and amplifier section.
I have specified and actually made a fully FPGA based DSP system that is a full direct sampling system but subject to current patent applications.

All of these systems followed the methodology I outlined above and did not use especially exotic components.

As for the ADC chip ... the detector system model is a wide bandwidth / high frequency system that is modulated by a very low frequency signal ( the target / ground ). The real challenge is to extract the signal not the carrier by demodulating / filtering / amplifying / sampling IN THAT ORDER. Thus the low frequency signal being sampled is more than adequately achieved with a commodity type ADC used in sound cards etc ( in fact most of these chips have 2 x differential inputs at 24 bits and 192 Khz sampling rate. )

The WM8788 is available from Farnell for A$ 2.58 in single quantities... you can get two for $5 !! ( cheaper now than when I last bought them .. wow ).

And just for reference the first motorbike was a bicycle with a motor bolted to it

I mostly agree with your design philosophy, but I'm going to insert a new third step:
Research whether the subsystem you need has already been developed and is available for use, perhaps in another field. Make an honest assessment of whether you think you capable of improving it, if not use it as is. In other words, don't waste time reinventing the wheel.

Other than that my only point was that your 5th step is going to frequently send you all the way back to your 3rd step. Time is by far the biggest investment in this sort of development. So if you can avoid some of these cycles by starting with suitable quality components then even if you do spend a little more than strictly necessary on the bits, your still be well ahead. If you achieve the performance your after and start thinking about production then you can start thinking about economizing on components.

THe WM8788 is indeed a surprisingly cheap performer. I'm a little suspicious of how the datasheet is written though. Nothing is specified at its full data rate or pre-low-pass-filter.

Oh and I said a better motorbike...a moped is never going to be better than an R1.

[Aziz]

Indeed,

my first "motorbike" was made with a sound card. Brrrouuumm, brouuuuum, broooouuuuum, well, it's running a hell. (A two cylinder "motorbike", which means a two-channel ADC).


Moodz is correct. Use the most used chips from the commodity (cheap and available), which can't be bought off from the ******** chip trolls (buying all the available stocks to sell them at a much higher price). They do the job as well.

And sometimes it's better to know how to make a discrete solution.

Aziz

[moodz]

Depends on what your designing I think Moodz. If its truly ground breaking and experimental and the concept can be proved with cheap components then sure use whatever. If your designing something based on well proven concepts then its inefficient to choose components that you know are ultimately going to limit performance, sending you back to the drawing board. Its like sitting down to build a better motorbike and starting by making a bicycle. You get to the stage where its time to put the motor in and have to scrap the frame and the wheels you've already made because they aren't strong enough. But you should have already known that because you've seen other motorbikes..

BTW, What's this $5, 24bit convertor of which you speak? I kind of suspect it's not going to be anywhere near fast enough for direct sampling.

Well that is another point of view ... however there is not a single design posted on this forum that actually uses an ADC ( commercial schematic or user submitted ) ... happy to be proved wrong here.
The design process should begin with the GOAL ( or outcome ) ... which I would imagine is to design a detector with performance at least in the same ballpark as commercial detectors.
Secondly the main functional subsystems that make up the detector system should be identified, how they interact and specified.
Thirdly each of these subsystems should be designed, prototyped and tested.
Fourthly the whole system should be integrated and the proof of concept be verified.
Fifth at this point the required performance gap should be identified AND IT IS AT THIS POINT THAT ANY EXOTIC COMPONENTS ARE UTILISED as each of the subsystems is brought up to the required performance level. If you can avoid it you wont use exotic components as they often suffer from supply problems during the product life cycle.

Finally thats why these forums ... ( although very interesting so dont get me wrong here ) ... never result in a finalised design. Everyone is either simulating or looking up chip catalogues for better and better chips when in reality I think there would be much more satisfaction if all the subsystems that actually make up a detector were actually posted up.

So am I being hypocritical here .... I think not.

I have specified and actually made ( as against talking about it ) a PI controller chip.
I have specified and actually made a frontend ( actually several ) but the two main ones are the differential and the more recent mono (with diff amp to reduce EMI).
I am working on the sampling and amplifier section.
I have specified and actually made a fully FPGA based DSP system that is a full direct sampling system but subject to current patent applications.

All of these systems followed the methodology I outlined above and did not use especially exotic components.

As for the ADC chip ... the detector system model is a wide bandwidth / high frequency system that is modulated by a very low frequency signal ( the target / ground ). The real challenge is to extract the signal not the carrier by demodulating / filtering / amplifying / sampling IN THAT ORDER. Thus the low frequency signal being sampled is more than adequately achieved with a commodity type ADC used in sound cards etc ( in fact most of these chips have 2 x differential inputs at 24 bits and 192 Khz sampling rate. )

The WM8788 is available from Farnell for A$ 2.58 in single quantities... you can get two for $5 !! ( cheaper now than when I last bought them .. wow ).

And just for reference the first motorbike was a bicycle with a motor bolted to it

[Midas]

The usual design process that I find is most efficient is to build it first an then make it better. You don't choose a part that offers something like better dynamic range unless you "need" it. There is always a better part available than the one you have now ... You will never build anything if you are always looking at the grass on the other side of the fence.
PS a 24 bit converter with differential inputs is like $5. More dynamic range than you will need.

Depends on what your designing I think Moodz. If its truly ground breaking and experimental and the concept can be proved with cheap components then sure use whatever. If your designing something based on well proven concepts then its inefficient to choose components that you know are ultimately going to limit performance, sending you back to the drawing board. Its like sitting down to build a better motorbike and starting by making a bicycle. You get to the stage where its time to put the motor in and have to scrap the frame and the wheels you've already made because they aren't strong enough. But you should have already known that because you've seen other motorbikes..

BTW, What's this $5, 24bit convertor of which you speak? I kind of suspect it's not going to be anywhere near fast enough for direct sampling.

[moodz]

I had been thinking about a comment by Carl-NC I read in another thread about 12 bit resolution VS 8 bit, combined with the subject of dynamic range mentioned in this quote:

"In theory, given the same voltage range for single-ended and fully-differential inputs, the fully-differential inputs will have double the dynamic range" from this link:

http://www.maxim-ic.com/app-notes/index.mvp/id/1108

Which lead me to the LME49724 easily located on eBay and low in cost. Problem is then I end up with an ADC such as the AD7450 and again am in a situation where so far only evaluation boards seem to be readily available as well as cost being an issue from a mass production perspective.

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7450/products/product.html

http://www.analog.com/en/evaluation/EVAL-AD7450/eb.html

Therefore I end up stuck in an endless loop of one difficulty or another until I can think of a better part for the ADC. Unless of course I find a cheap source for the AD7450.

The usual design process that I find is most efficient is to build it first an then make it better. You don't choose a part that offers something like better dynamic range unless you "need" it. There is always a better part available than the one you have now ... You will never build anything if you are always looking at the grass on the other side of the fence.
PS a 24 bit converter with differential inputs is like $5. More dynamic range than you will need.

[Midas]

I had been thinking about a comment by Carl-NC I read in another thread about 12 bit resolution VS 8 bit, combined with the subject of dynamic range mentioned in this quote:

"In theory, given the same voltage range for single-ended and fully-differential inputs, the fully-differential inputs will have double the dynamic range" from this link:

http://www.maxim-ic.com/app-notes/index.mvp/id/1108

Which lead me to the LME49724 easily located on eBay and low in cost. Problem is then I end up with an ADC such as the AD7450 and again am in a situation where so far only evaluation boards seem to be readily available as well as cost being an issue from a mass production perspective.

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7450/products/product.html

http://www.analog.com/en/evaluation/EVAL-AD7450/eb.html

Therefore I end up stuck in an endless loop of one difficulty or another until I can think of a better part for the ADC. Unless of course I find a cheap source for the AD7450.

Yep, to quote Big Bang Theory:
"Sounds like your stuck between a rock and a crazy place"
You have internet access, you have a credit card, hence you have access to state of the art parts from all over the world. You just have to lose your fear of credit card fraud.
Farnell of course has the part your looking for.. its $12.75.. free delivery.

[Dr Vel]

I had been thinking about a comment by Carl-NC I read in another thread about 12 bit resolution VS 8 bit, combined with the subject of dynamic range mentioned in this quote:

"In theory, given the same voltage range for single-ended and fully-differential inputs, the fully-differential inputs will have double the dynamic range" from this link:

http://www.maxim-ic.com/app-notes/index.mvp/id/1108

Which lead me to the LME49724 easily located on eBay and low in cost. Problem is then I end up with an ADC such as the AD7450 and again am in a situation where so far only evaluation boards seem to be readily available as well as cost being an issue from a mass production perspective.

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7450/products/product.html

http://www.analog.com/en/evaluation/EVAL-AD7450/eb.html

Therefore I end up stuck in an endless loop of one difficulty or another until I can think of a better part for the ADC. Unless of course I find a cheap source for the AD7450.

[moodz]

As Midas said, you'll be ok with a major supplier like Farnell ->
http://uk.farnell.com/jsp/search/bro...uestid=1343842

Farnell ( or element14 as they called in Australia ) are fine in my experience ... particularly as they provide free next day delivery for most items. I would rate them twice as reliable as Ebay however everyone has thier own experiences ....

The LME49724 is fully differential whilst the THAT is technically differential input only .. though it can provide a differential output with additional components.

So I guess from what I can see in the spec sheet you could use it and possibly feed into the input of a differential input ADC. Certainly the noise, common mode rejection GBW are sufficient depending on gain you intend to use.

moodz

[Qiaozhi]

Has anyone tried using the LME49724 from National? In my rural area eBay is the best way for me to find parts and the 'THAT' line seems to be nonexistent on eBay. I prefer to use paypal only, after my card was hit 3 times in 2 years buying from various independent sources where I had to give out my card info. Over 2,000 purchases on eBay so far and in 8 years not one hit on my bank account using paypal so while I digress I thought it worth mentioning why I would really rather use only parts I can buy on eBay. The National part is easy to find on ebay and is only $6.90 free shipping ($10.90 for two including shipping in the 2nd link). I have not carefully read all the pages covering this project so I am not sure if anyone else has already mentioned this part number.

http://www.ebay.com/itm/LME49724-High-Performance-Audio-Differential-Opamp-/180699594222?pt=LH_DefaultDomain_0&hash=item2a1288fdee

http://www.ebay.com/itm/LME49724-Hig...item27bce683f3

As Midas said, you'll be ok with a major supplier like Farnell ->

http://uk.farnell.com/jsp/search/browse.jsp;jsessionid=WPXLVIVHUZASGCQLCIPJKBQ?N=0&Ntk=gensearch&Ntt=THAT1510&Ntx=mode+matchallpartial&suggestions=false&ref=globalsearch&_requestid=1343842

[Dr Vel]

I do not see however much difficulty reworking the circuit for it. I like using parts easy to find and low in cost where possible.

[Midas]

Has anyone tried using the LME49724 from National? In my rural area eBay is the best way for me to find parts and the 'THAT' line seems to be nonexistent on eBay. I prefer to use paypal only, after my card was hit 3 times in 2 years buying from various independent sources where I had to give out my card info. Over 2,000 purchases on eBay so far and in 8 years not one hit on my bank account using paypal so while I digress I thought it worth mentioning why I would really rather use only parts I can buy on eBay. The National part is easy to find on ebay and is only $6.90 free shipping ($10.90 for two including shipping in the 2nd link). I have not carefully read all the pages covering this project so I am not sure if anyone else has already mentioned this part number.

http://www.ebay.com/itm/LME49724-High-Performance-Audio-Differential-Opamp-/180699594222?pt=LH_DefaultDomain_0&hash=item2a1288fdee

http://www.ebay.com/itm/LME49724-Hig...item27bce683f3

Pretty restrictive policy you have there. I would think you very safe with major electronic parts suppliers. Then even in the unlikely circumstance you do get defrauded credit card companies pretty much take it on the chin for you as long as you inform them without too much delay. Or you can actually get free samples from THAT without your credit card.
But anyway the LME49724 looks like a fine part. Differential output though, not single like the THAT1510. Not neccearily a bad thing but its not a direct substitution.

[Dr Vel]

Has anyone tried using the LME49724 from National? In my rural area eBay is the best way for me to find parts and the 'THAT' line seems to be nonexistent on eBay. I prefer to use paypal only, after my card was hit 3 times in 2 years buying from various independent sources where I had to give out my card info. Over 2,000 purchases on eBay so far and in 8 years not one hit on my bank account using paypal so while I digress I thought it worth mentioning why I would really rather use only parts I can buy on eBay. The National part is easy to find on ebay and is only $6.90 free shipping ($10.90 for two including shipping in the 2nd link). I have not carefully read all the pages covering this project so I am not sure if anyone else has already mentioned this part number.

http://www.ebay.com/itm/LME49724-High-Performance-Audio-Differential-Opamp-/180699594222?pt=LH_DefaultDomain_0&hash=item2a1288fdee

http://www.ebay.com/itm/LME49724-High-Performance-Audio-Differential-Opamp-x2-/170672948211?pt=LH_DefaultDomain_0&hash=item27bce683f3

[mickstv]

I agree mick, thats not bad sensivity at all, well done. Whats sort of time from end TX to first sample are you getting? The best I've got with a standard round coil (also caniballised cat5 TP) is 16us. Admittedly my mosfet and driver is pretty ordinary.
I think detecting a .4g nugget would require a pretty damn fast coil with a traditional PI so I'm curious to what difference the differential arrangement makes.

Hi Midas, the first sample is at about 10-12us with the coil I'm using, which is a spider coil. The only amp in circuit at the moment is the THAT amp and I'm only running one sample circuit at the moment but will be adding two more sample circuits shortly.

Forgot to mention I'm monitoring the output of the sample circuit only with a CRO at the moment.



Mick

[Midas]

I've been building a modified version of this frontend design and haven't noticed any problem with noise on either input or the output of the diff amp, the diff amp is running at about 500 and thats with an LCD and computer within 2 metres of the test bench. The coil I'm using at the moment is a spider coil made out of a twisted pair removed from a length of cat5.


I've built one sample circuit so far and it has no post sample amplification yet, whilst monitoring the output of the sample circuit with a cro set to 100mv per div i can detect the presence of a 0.4 gram nugget at about 10cm in air which I think is pretty good.

MIck

I agree mick, thats not bad sensivity at all, well done. Whats sort of time from end TX to first sample are you getting? The best I've got with a standard round coil (also caniballised cat5 TP) is 16us. Admittedly my mosfet and driver is pretty ordinary.
I think detecting a .4g nugget would require a pretty damn fast coil with a traditional PI so I'm curious to what difference the differential arrangement makes.

[ashkelon]

MICKSTV
This scope noise picture belong to other md circuit (PD3200). if you read sample directly with ADC, this noise change everything.

If this DSPIC project working free of noise i want to make. does that1510 dif. opamp eliminate the all noise ?