Best OpAmp for Rx signal...?

[Carl-NC]

I agree with Pork, if you want to do really fast (sub-10us) sampling, use a 2-stage preamp. In some cases you can help overdrive recovery with a diode across the feedback R.

- Carl

[Tinkerer]

Hi Tinkerer,

I have AD797 and LM318 and I will also look to some similar experiments as yourself. I will let you know of my results with DD Coils and Concentrical Coils, in the next weeks. I checked your waveforms at 3us, they are very interesting. You have some oscillation in circuit, but I think that doesn't matter.

Regards,
Nicolae

Hi Nicolae,

it would be nice if you could use similar coil parameters, so we can compare apples with apples.
You are right about some oscillations. Eventually I will look for the cause and eliminate them. In the meantime they do not seem to impair the results too much.

All the best

Tinkerer

[nick_f]

How about OPA637? GBW = 80MHz, noise = 4.5nV/ÖHz at 10kHz, Slew-rate = 135V/us.
It doesn't come cheap though. Futurlec have them for US$21.90.

Regards,
Nicolae

[CrotalusAtrox]

Has anyone considered an ADA4898-1?

PJ

[MFC]

Hi Dear Geo,
Thanks for your answer...

I want to use one opamp for amplification all Rx signals even every weak received signals in one PI metal detector to reach more depth and sensitivity...
we know most PI metal detectors usually work in 100Hz frequency or
under 1kHz ...
What is the best choice for amplification Rx signals in this MD...

what gain is enough for amplification Rx signals in one PI MD?

It is clear that a good opamp must have low noise and high gain...
what is the best solution...?!!!

with best regards... vistac2000

Hi look you can use the NE5534 for preamplifier, it's exelent and cheap. But the diametter of Coil and Shape determines in the deep detection and sensivity. A good solution for the coil is the Spiral Flat Coils Round, Square or both shapes. Remember this: Low capacitance and hight Q is the most important for any PI detector.

if you have a doubt contact with me: Maikel Fuentes Claro (MFC) email: [email protected]

Good Look

[bugwhiskers]

Better noise specs than the AD797 and a lot cheaper.

http://www.national.com/ds/LM/LME49990.pdf

regards
bugwhiskers

[mario]

Speeking from a specs point of view (I have not used the 49990s), those specs are comparable to the 797/1028s and not necessarily exceptional. They are, however, much cheaper, so thank you for your contribution. For some reason, AD op amps seem to be the most expensive in the industry.

I have searched long and hard for replacements and have not been very successful. Which is disappointing because the AD797 was released back in 1997!

Something mentioned earlier, but not emphasized, has much to do with the designs implemented around those parts.

It's best to compliment the design around a particular part and not "drop" them in like most people seem to suggest. Although most of these parts are in deed pin for pin compatible, they do in deed use different complimentary components to achieve their stated specifications -like slew rates.

It is something overlooked, and for which I'm also at fault.

Also, one last thing I would like to mention: most of the parts suggested are very similar in performance. I doubt most users will ever experience any significant depth/sensitivity by using any one particular part over another. The same user will not miss that 5 gram nugget by using a detector that has a 797/1028/5534 installed. The more experienced detectorists will even make the mediocre detectors stand out.

We all seem to be caught up in technology and seem to forget that, when detecting, experience with your equipment matters just as much.

[porkluvr]

One thing that I think is overlooked in the race to find lowest noise, some PI designs use a series resistor before the preamp, while others (notably Minelab SD2000) have no series resistance between the coil and preamp input, but instead have a blanking gate that effectively disconnects the preamp while the flyback voltage is at dangerous a level.

Those detectors that use a blanking gate will have inherently less input resistance during the listening interval, and, (according to op-amp noise theory, as I understand it) can thus better make use of some of the more expensive low noise opamps such as AD797 or LT1028.

Using a very-low noise opamp for a detectors that uses series resistance, is likely to be a waste of money because you are limited in your ability to get system noise much lower than what that series resistance will contribute, despite your best effort. Thus the wide use of NE5534, despite its being an 'old' part number.

I am not an engineer so I will leave a more thorough explanation, or else debunking of a rant, to someone who is more technically adept than I.

[Carl-NC]

You're right, Pork, using a 1nv/rtHz opamp does little good if you have a 1k series input R, which is 4nv/rtHz. The resulting equivalent input noise is still 4.1nv/rtHz.

Folks who want seriously low noise need to get rid of the series R and implement a blanking gate.

[Mechanic]

You're right, Pork, using a 1nv/rtHz opamp does little good if you have a 1k series input R, which is 4nv/rtHz. The resulting equivalent input noise is still 4.1nv/rtHz.

Folks who want seriously low noise need to get rid of the series R and implement a blanking gate.

Yep that is correct! No point using a low noise opamp if the input path has high resistance.

Mario, as for the noise masking targets, yes it can do it when the target is near the detection threshold. The best way I know to decide if one part is better than another is to do a noise test before and after. The best way I know to do this is to disconnect the coil receive wire at the back of the coil plug and connect it to coil ground at the back of the coil plug. Then hook up your scope to the output of the opamp after the dc blocking cap, which should be the output of the filtering stage where it goes to the audio stage. Set your scope up to .5second/div and anywhere down to 5mv/div depending on the noise level. Take note of the peak to peak value of the signal seen and also the frequency seen. Any improvement in the cct will be easily seen here. As far as noise picked up by the coil goes there is not too much that can be done about that except better shielding(well theres probably heaps more that can be done!). I find mostly when I'm out bush that emi is not too much of a problem and that the noise that I can hear on the base of the threshold is due to the noise floor of the detector. Quieten that down a bit and fainter targets become easier to find.

Cheers Mick

[Alexismex]

Hello Forum,
Nobody tell that it is also important the design of the printed circuit from the connection of the coil and.... the input of the OP !!! the type and quality of resistor used at the input of the Op.......

[taliesin]

op amp

http://www.partsconnexion.com/t/Inde...pam1.html.drop one of these in

[Reg]

Bugwhiskers is right, the LM#49990 is about the best and requires no additional components to work right. The AD797 has to have limiters or it will saturate.

One other op amp that seems tow work well is the OPA228. I have used both with success.

Reg