Preamp

[Qiaozhi]

It's FULLY fledged and FREE! Or if you google "sonsivri" you can get any full version CAD package. MultiSim and Ultiboard etc, Eagle, Sprint and even Protel, but the setup for that takes about three hours. Let me know.

Sean - I have registered a request to join the Sonsivri forum, but it requires someone to actually invite me to join for free. Otherwise it costs $20.
Please can you go to the Invitation Request section of their site and click on "Invite this person"?
Thanks.

[Sean_Goddard]

Alright I'll do the maths and the electronics design, you do the PCB layout and prototyping. Deal?

I can do the programming as well unless we have a more experienced volunteer.

Sounds like a "goer" to me. As soon as you have done the schematic I'll start on the PCB. Now, what design package are we going to use as we need a commonality. Has anyone tried DesignSpark from RS? Mechanical AND now Electrical too (that was my idea).

http://www.rs-online.com/designspark/electronics/

It's FULLY fledged and FREE! Or if you google "sonsivri" you can get any full version CAD package. MultiSim and Ultiboard etc, Eagle, Sprint and even Protel, but the setup for that takes about three hours. Let me know.

[Teleno]

The typical surf cct uses 100n cap between preamp and int

Yes, it's 100n in series with a 10K resistor. The time constant is 1ms.

The late sample (EF) is taken at about 240us after the main sample. This means the late EF is 20% lower than the early EF (it decays to 80% of the initial value after 240us). It can be compensated by a gain coefficient, though.

A better solution is to use a preamp (gain 10 - 20) and couple it capacitively to a JFET-input Op-Amp which, in turn, is DC coupled tot he integrator. In this case you can use C = 100n and R=1M for a time constant of 100ms. The decay after 240us is a mere 0.3%.

[6666]

The typical surf cct uses 100n cap between preamp and int

[Teleno]

I'm thinking Teleno's amp needs to be capacitor coupled to the integrator. I've been direct coupling with my test circuit. Wondering what the advantages and disadvantages are. EF cancelling works because the two samples are the same. With capacitor coupling they won't be, is that a problem?

Surfmaster is already capacior coupled, so EF cancelling must be iffy at best.

Could you please post a link to it ?

This is it: http://www.geotech1.com/forums/showt...mit-time/page3

[Old cart]

I'm thinking Teleno's amp needs to be capacitor coupled to the integrator. I've been direct coupling with my test circuit. Wondering what the advantages and disadvantages are. EF cancelling works because the two samples are the same. With capacitor coupling they won't be, is that a problem?

Only if the cap decays during the TX off time. If it is large enough it should not be problem

[6666]

I have posted my novel GB scheme in another thread, should be tested in Aussie goldfields. If it works then gain shouldn't be a problem.

Could you please post a link to it ? or a clue where to find it . thanks

[green]

I'm thinking Teleno's amp needs to be capacitor coupled to the integrator. I've been direct coupling with my test circuit. Wondering what the advantages and disadvantages are. EF cancelling works because the two samples are the same. With capacitor coupling they won't be, is that a problem?

[green]

No problem, at the time of sampling the avalanche is long gone together with its noise.

That has been my thought, but I'm not certain it's correct. Need to see if I can see a difference in post amp output noise with and without the snubber.

[Teleno]

I agree, but am told the avalanche is noisy and it's better to snub the volts. Not much difference in time, 470 snub volts vs 500 avalanche volts.

No problem, at the time of sampling the avalanche is long gone together with its noise.

[green]

A snubber makes no sense because it's better to go above the MOSFET's breakdown voltage, which then becomes a faster snubber (higher voltage). Just make sure you don't surpass the MOSFET's avalanche ratings which are quite generous.

I agree, but am told the avalanche is noisy and it's better to snub the volts. Not much difference in time, 470 snub volts vs 500 avalanche volts.

[Teleno]

The snubber effects the decay at clamp volts and then drops out. I/T=E/L, 500volts/300uH equals 1.67amps/usec. Snubber adds to decay time. [But then you get flyback twice as long as in the first example, reply #108]

A snubber makes no sense because it's better to go above the MOSFET's breakdown voltage, which then becomes a faster snubber (higher voltage). Just make sure you don't surpass the MOSFET's avalanche ratings which are quite generous.

[green]

I believe it would be the same, after all a snubber is reducing the R as well.

The snubber effects the decay at clamp volts and then drops out. I/T=E/L, 500volts/300uH equals 1.67amps/usec. Snubber adds to decay time. [But then you get flyback twice as long as in the first example, reply #108]

[Teleno]

What is the advantage of reducing circuit resonance vs using a snubber to keep the voltage below avalanche volts? The coil decay time is faster with a snubber.

Once you surpass the MOSFET's breakdown voltage the decay rate is constant no matter what you do, the L is as good as short-circuited.

A snubber has to reduce R anyway in order to avoid reaching the breakdown voltage. I believe there's no tradeoff.

[Teleno]

The LT is not JFET type like the AD (not sure?!). Descrete solution like cutting edge standalone op amp offers "repeat-ability" - it's all there in the package.
If you take 10 different op amps they will perform the same.
While the other solution may give unexpected results - lots of connections may pick up some noise long the way, component tolerance, etc.

The discrete solution does not require tight tolerances because an MCU will adjust the 2 offsets required (via gates of J1 and J3). That's all.

If you look at the circuit, all the connections except the input one are low impedance, carrying currents in the order of mA. Parasitic noise stands little chance.

It will be interesting to see a real working example, if it will prove better / worse.

Here's a version with LT1034 as a second stage. Noise-wise is the same, but the op-amp version is a bit slower to settle and the output swing is limited. Then there's the added overhead of a negative rail.