Is this a major advance in PI sensitivity ??

[moodz]

Gday Tinkerer ... thanks for your reply .. in a professional design I would use a separate drive for the bias generator as the present circuit configuration is highly dependant on a fast pulse repetition rate ( eg > 4 Khz ) to maintain low ripple in the bias generator outputs.
C6 an C7 are actually 10 uf ... I was surprised too that the impact on gate drive timing was so minimal ... the caps dont really have time to discharge much between pulses ( approx 70 mv across 200us ). The diff amp only draws around 10 ma from the supply.

I do hope to get onto descrimination on the FPGA project .... I just need to finalise the dspic code for the Model T demonstrator ( which wont have descrimination )

Regards,

moodz.

[samu]

Metal isolation?

Hi,

I´ve mounted the circuit and it works pretty well. Now, I´m trying to program a good software for the pic.

What I´m planning to do, is to use a large coil installed in a large metal box. I tried it and the result was logical, the circuit detects the box. I can compensate the box signal, but then the sensibility decreases a lot, I guess due to a kind of saturation and makes the MD not useful.

I don´t know if there is a way to magnetically isolate the metal box from the coil so the eddy currents are not induced or avoid. I tried to connect the box to ground (big fail), maybe there is a material to do this, or a way to stop eddy currents.

I think it would be nice if we can shield the coil in every direction but the one in which metals are expected.

Thanks to everyone.

[ashkelon]

coil capacitance big problem cousing the noise

http://uploadpic.org/v.php?img=A0cMhpLrEq

[moodz]

coil capacitance big problem cousing the noise

http://uploadpic.org/v.php?img=A0cMhpLrEq

If the coil is balanced the capacitance will be the same on each coil and noise will be balanced and cancel ... the amount of capacitance is relatively immaterial. The high frequency noise is coupled via gate leakage from the TX mosfet on one side of the coil.

moodz

[mickstv]

coil capacitance big problem cousing the noise

http://uploadpic.org/v.php?img=A0cMhpLrEq

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

[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 ?

[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.

[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

[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-Hig...item2a1288fdee

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

[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-Hig...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]

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.

[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-Hig...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/bro...uestid=1343842

[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

[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-d...s/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.

[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-d...s/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.