Detection distance for a US nickel and quarter

[liudengyuan]

Why not use an instrumentation amplifier?

[liudengyuan]

Or why not directly sample the fully differential amplifier?

[Altra]

Or why not directly sample the fully differential amplifier?

See jpg in first post

https://www.geotech1.com/forums/show...t-model-T-quot

[liudengyuan]

If we want to form a power supply of plus or minus five volts, should we use 7905 or 7805 except for the charge pump chip?

[liudengyuan]

Our op amp uses a ? 5v power supply, and the microcontroller uses 3.3 or 5v, so how do the op amp and the microcontroller connect to the level?

[Carl-NC]

Why not use an instrumentation amplifier?

Many IA's are not fast enough for PI. Usually you can do as good or better making your own.

Or why not directly sample the fully differential amplifier?

You can do that, if you have a fast enough ADC with enough bits.

If we want to form a power supply of plus or minus five volts, should we use 7905 or 7805 except for the charge pump chip?

That will work. Download a bunch of project schematics to see various solutions.

Our op amp uses a ? 5v power supply, and the microcontroller uses 3.3 or 5v, so how do the op amp and the microcontroller connect to the level?

You may need level shifters, depends on the situation. Again, look at some micro-based projects to see what's been done.

[Ferric Toes]

By the end of today I will have a detector using long sample pulses ready to test in the field outdoors. Timing is 350uS TX pulse; 2A peak current; p.r.f. 520; first delay 15uS, sample 1 & 2, 150uS; intersample delay 500uS (end of 1 to start of 2). Workshop range test on quarter looks good. Equivalent size cupro-nickel shilling a bit less range, but still good. Tests so far on 8in coil, but need to use 10inch or 12inch which is not possible indoors due to surrounding metal. Hope to test on the beach in due course but too many people there at the moment disregarding the safe distancing rule to get out in the sun. I have to be especially careful as in the very high risk category.

Eric.

[green]

By the end of today I will have a detector using long sample pulses ready to test in the field outdoors. Timing is 350uS TX pulse; 2A peak current; p.r.f. 520; first delay 15uS, sample 1 & 2, 150uS; intersample delay 500uS (end of 1 to start of 2). Workshop range test on quarter looks good. Equivalent size cupro-nickel shilling a bit less range, but still good. Tests so far on 8in coil, but need to use 10inch or 12inch which is not possible indoors due to surrounding metal. Hope to test on the beach in due course but too many people there at the moment disregarding the safe distancing rule to get out in the sun. I have to be especially careful as in the very high risk category.

Eric.

Does coil size effect indoor testing if coil is stationary and coin is swept over coil? Interested in your test results.

[6666]

Do the long samples still eliminate EFE ? I am avoiding people as much as possible, damn virus

[Carl-NC]

Does coil size effect indoor testing if coil is stationary and coin is swept over coil?

Static metal can deflect the decay curve enough to overload the demods. You can compensate with a manual offset adjust. Larger coils also couple in more EMI.

Do the long samples still eliminate EFE ?

As long as they are equal width.

[6666]

As long as they are equal width.

Thanks, should have known.

[6666]

Hello Eric, just received a bunch of J113 fets, I have a transistor tester that tests jfets, what is the most important parameter to look for when trying to match them IDss ? thanks.

[Ferric Toes]

Hello Eric, just received a bunch of J113 fets, I have a transistor tester that tests jfets, what is the most important parameter to look for when trying to match them IDss ? thanks.

What I measure is VGS(off) which is the gate source cutoff voltage. This appears to have a direct relationship to the drain source on resistance, which is what we are really interested in. So if we have two devices which have the same VGS(off) then the on resistance should be a reasonable match. The National data book I have only gives a maximum value for rDS (on) of 100 ohms. No minimum or typical. Data gives VGS(off) as between -0.5V and -3V. I have just measured one of my J113's and got a reading of -2.6V.

Eric.

[dbanner]

[6666]

What I measure is VGS(off) which is the gate source cutoff voltage. This appears to have a direct relationship to the drain source on resistance, which is what we are really interested in. So if we have two devices which have the same VGS(off) then the on resistance should be a reasonable match. The National data book I have only gives a maximum value for rDS (on) of 100 ohms. No minimum or typical. Data gives VGS(off) as between -0.5V and -3V. I have just measured one of my J113's and got a reading of -2.6V.

Eric.

Hello Eric, thanks for the reply, thats an interesting approach using VGS(off), most of the testers I have seen used IDSS for matching, I have a digital transistor tester and looking at the display it gives three readings, one is Zero current @ XX volts, one j113 tests at 2.7 volts so I guess that is VGS off, I dont have a handbook for it.
But IDSS is all over the place.
I will build this manual simple tester , I think it will give a better indication of VGS, we are getting down to zero temperatures over night now , hard to get into work shop