NEW !!! Full differential PI front end.

[simonbaker]

Hi Moodz:

Which "non-differential" circuit would be the closest to compare to help understand the benefits and workings of your differential design? Can you provide a schematic or link to one?

Regards,

-SB

[Carl-NC]

Paul,

This kind of circuit often works extremely well in sims but not in real life. Ferinstance, you would have to run the sim a long, long time to see the effect of leakage currents on the floating node. Also, the fact that it comes up to a decent DC value means little unless you run a transient sim that includes parasitics and ramping up the power supplies, and ramping them up at different rates. That floating node can easily end up railed out.

Are you actually running a real circuit this way? I would suggest a high-resistance voltage divider on the + input to guarantee a correct bias point.

- Carl

[moodz]

Hi Carl ... I never as a rule rely on spice sims ... when I ran the sims above I fully expected it not to work as I was expecting the results I was getting on the real circuit to be dependent on something in the front end of CA30XX devices ... like the protective diodes on the mosfet gates at the +/- inputs. However it does seem to work ... and I run the prototype circuits for as long as my battery pack has charge .. 2 - 4 hours while I poke around with the CRO and burn my hand on the soldering iron. There is no noticeable drift in the bias point. None of the schems I have published are based on spice or cut and paste circuits ... I have built all of them ... I could publish 10 times as fast if I just spiced everything.
I ran some sims on older devices like the LM740 and these work but very poorly ... bias point near zero.

I am mainly concerned with getting the coil bit right ... the diff amp is not rocket science .. there are much better amp designs ... Aziz ?? If I can contribute to improving the front end then I will be happy.

Regards,

moodz.

[moodz]

Hi Moodz:

Which "non-differential" circuit would be the closest to compare to help understand the benefits and workings of your differential design? Can you provide a schematic or link to one?

Regards,

-SB

Probably the Goldpic by Trevor Hill .... however on second thoughts having looked at the MineLab SD2000 ... you could compare it with that ... sort of a 'gold standard' ( no pun intended ) to aim for. I would want at least that level of sensitivity or better. I think you could easily google detail of both.
In terms of SNR the differential circuit is at least twice as good as a single ended design ... have a look at pulse amps used on disk drive heads etc. Now that I have split the Tx path from the Rx path by moving the "clamping and damping" ... hey Im a poet ... to the coil head .. this means the TX and RX circuits can be well isolated from each other and the design of each optimised. I was tempted to move the RX actives to the coil as well ... but there is the law of diminishing gains ( no pun intended ha ha ) .. and I would like the RX in the box so to speak.

Regards,

moodz

[moodz]

Hi Paul,

excellent work! I also would like to try out some full differential PI front end's soon. I will do this with bipolar power supplies due to its simplicity (biasing) and increased noise immunity performances. But it will take a few weeks to start with.

The differential stage could also be done with discrete solution with highly matched transistor/FET pairs (see SD2000 schematics). This should lower the noise further.

You did really a good job. Thanks for sharing your work.

Regards,
Aziz

Thanks Aziz .... can you work on the amp ... I need one

Regards,

moodz.

[moodz]

Paul,

This kind of circuit often works extremely well in sims but not in real life. Ferinstance, you would have to run the sim a long, long time to see the effect of leakage currents on the floating node. Also, the fact that it comes up to a decent DC value means little unless you run a transient sim that includes parasitics and ramping up the power supplies, and ramping them up at different rates. That floating node can easily end up railed out.

Are you actually running a real circuit this way? I would suggest a high-resistance voltage divider on the + input to guarantee a correct bias point.

- Carl

Hi Carl ... you are right ... the amplifier does not work in practice ... I reboarded the circuit on the v5 schematic and the opamp refused to bias at all on the new layout. Then I was thinking about my previous comment regarding the protective diodes on this type of amp that are present at the inputs to protect the sensitive mos gate. With previous layouts the first amp was very close to the switching mosfet ... and it's the high voltage transient spikes that were getting into the front end of the opamp causing the protective diodes to avalanche and basically conduct to V+ and charge the cap thus provide a sort of 'switched' bias. I never saw the transients beacause they appeared as common mode and were too fast for the amp to respond to. With version 5 the high voltage no longer gets near the reciever because of the separate shielded pair pickup from the clamping and damping which are now located at the coil. So there are no transients to avalanche the protective diodes. So the amp never biases. To test this theory I connected a 1M resistor from the hot side of Q1 to the non inverting input and sure enough the bias cap charged in couple of seconds ( not via the 1M but via the avalanche diodes ) and the amp worked like a charm. It keeps working after the resistor is removed because the offset current in these amps is 5 pico amps so the cap would take maybe some hours to discharge ... and I only work the circuit for an hour or two.
Well now I know three things ...

1. Carl knows what he is talking about.
2. My old layout suffered from transients.
3. Must ring the patent office and put a hold on that Novel OPAMP biasing technique.

... its nice to know why it was happening.

Aziz .... I am waiting for that amplifier design ha ha

moodz

[Aziz]

Hi Paul,

Aziz .... I am waiting for that amplifier design ha ha

moodz

It will take some time. I am not ready at the moment due to lack of time and other things to do.

Why not operating the differential amplifier with bipolar power supply? This would increase the performance.

Aziz

[Aziz]

Hi Paul,

I have some true instrumentation amplifier chips at home (INAxxx). I would like to try out these first.

Aziz

[moodz]

Hi Paul,

I have some true instrumentation amplifier chips at home (INAxxx). I would like to try out these first.

Aziz

Cool ... you will be able to give them a good work out with the new CRO

moodz

[simonbaker]

Question

I think this has been asked before, but not sure...

With differential design, we put current through half the coil, but use whole coil to detect target signal. Advantage is less flyback voltage across MOSFET, right?

But.... what about magnetic field? Is it half of that with non-differential coil? Or do we try to pump twice the current into it?

What I'm asking is: is our magnetic field weaker with differential design, so we get less target stimulus?

You might say: yes it is weaker, but with non-differential design it was wasted anyway because MOSFET breakdown.

Reply to that is -- use slower cut-off to avoid MOSFET breakdown, and we'll get longer (therefore stronger) target stimulus -- at least for targets that are not extremely "fast".

So does differential design have weaker transmitted magnetic field pulse?

Regards,

-SB

[Carl-NC]

With previous layouts the first amp was very close to the switching mosfet ... and it's the high voltage transient spikes that were getting into the front end of the opamp causing the protective diodes to avalanche and basically conduct to V+ and charge the cap thus provide a sort of 'switched' bias.

I would not have guessed that was happening... at least you found the cause. Good to know!

- Carl

[moodz]

I think this has been asked before, but not sure...

With differential design, we put current through half the coil, but use whole coil to detect target signal. Advantage is less flyback voltage across MOSFET, right?

But.... what about magnetic field? Is it half of that with non-differential coil? Or do we try to pump twice the current into it?

What I'm asking is: is our magnetic field weaker with differential design, so we get less target stimulus?

You might say: yes it is weaker, but with non-differential design it was wasted anyway because MOSFET breakdown.

Reply to that is -- use slower cut-off to avoid MOSFET breakdown, and we'll get longer (therefore stronger) target stimulus -- at least for targets that are not extremely "fast".

So does differential design have weaker transmitted magnetic field pulse?

Regards,

-SB

Hi Simon ... at the risk of kicking off a bun fight .. here goes.

1. The coil is an N turn bifilar wound coil. When the the mosfet turns on the current starts increasing in the coil at a rate Ldi/dt depending on the inductance and peaks if the mosfet is on long enough at a value that is only dependant on the resistance of the coil , the feed wire and mosfet ON resistance.This current only flows in one of the bifilar conductors ( there is a small current due to the damping resistance across the other bifilar conductor )

Proposition 1. During TX the differential coil behaves the same as a 'standard' mono coil of N turns.

2. When the mosfet turns OFF however the magnet flux collapses through BOTH coils making up the bifilar coil and because of the series connection of the two bifilar conductors the damping resistor sees a 2N coil. ( twice the number of windings ). The voltage is also doubled as the flux to windings for the same current is directly proportional. The mosfet however is only reference to the centre connection to the series connected coil so it only sees half of the fly back voltage.

Proposition 2. The peak current in a differential coil at mosfet turn off is the same as a mono coil of N turns however the peak fly back flux is twice as great because the current in the differential coil flows through 2N turns. Target is stimulated with twice the flux at peak flyback.

3. When the fly back starts to decay the diodes ( now located at coil ) connected to mosfet and battery isolate the coil as they reverse bias and the target response is picked up by a 2N coil.

Proposition 3. The target is stimulated by a back EMF flux that is twice that of an N turn monocoil and the target eddy current flux is picked up by a coil that is twice the number of turns of a standard monocoil. Two by two is four ....

The short answer is that the 'charging' flux ie the 100 odd uS MOSFET ON is the SAME as a monocoil.However when the mosfet turns off the flyback flux is double. The RX coil response is double.

Regards,

moodz.

[simonbaker]

Hi Simon ... at the risk of kicking off a bun fight .. here goes.

1. The coil is an N turn bifilar wound coil. When the the mosfet turns on the current starts increasing in the coil at a rate Ldi/dt depending on the inductance and peaks if the mosfet is on long enough at a value that is only dependant on the resistance of the coil , the feed wire and mosfet ON resistance.This current only flows in one of the bifilar conductors ( there is a small current due to the damping resistance across the other bifilar conductor )

Proposition 1. During TX the differential coil behaves the same as a 'standard' mono coil of N turns.

2. When the mosfet turns OFF however the magnet flux collapses through BOTH coils making up the bifilar coil and because of the series connection of the two bifilar conductors the damping resistor sees a 2N coil. ( twice the number of windings ). The voltage is also doubled as the flux to windings for the same current is directly proportional. The mosfet however is only reference to the centre connection to the series connected coil so it only sees half of the fly back voltage.

I agree with that, but it doesn't say anything about what the target sees. The target only sees the magnetic field from N turns. If we put the current through 2N turns (non-differential design), we'd have twice the magnetic field if we had the same current. Now, the resistance of 2N turns is twice that of N turns, so maybe we have half the current. If we accept the premise that our TX pulse is long enough and our battery strong enough to reach steady state, then I agree the differential design seems advantageous. If we cannot achieve twice the current through the N turns compared to 2N turns, then the advantage seems to be lost. (Well... see below as I begin to maybe see your point...)

Proposition 2. The peak current in a differential coil at mosfet turn off is the same as a mono coil of N turns however the peak fly back flux is twice as great because the current in the differential coil flows through 2N turns. Target is stimulated with twice the flux at peak flyback.

That one I'm not sure about and had not considered. Maybe that is what I was missing. It implies that during TX pulse turn-off, the magnetic field actually grows before it diminishes. In other words, although the current starts decreasing in the first N turns, the second N turns has current start flowing so that the total current * turns increases before it decreases. I guess I need to study that.

3. When the fly back starts to decay the diodes ( now located at coil ) connected to mosfet and battery isolate the coil as they reverse bias and the target response is picked up by a 2N coil.

Yes that's good, but no better than a 2N TX coil.

Proposition 3. The target is stimulated by a back EMF flux that is twice that of an N turn monocoil and the target eddy current flux is picked up by a coil that is twice the number of turns of a standard monocoil. Two by two is four ....

That is the crux I have to accept, that the magnetic field doubles the instant the current is turned off. I guess it makes sense if the "flyback voltage" drives the current (which does not change instantaneously) through the second set of N windings.

The short answer is that the 'charging' flux ie the 100 odd uS MOSFET ON is the SAME as a monocoil.However when the mosfet turns off the flyback flux is double. The RX coil response is double.
Regards,

moodz.

Ok, thanks for going over that for me.

Now -- assuming the above is correct, what is the optimum ratio for the "center tap"? Should we have N:3N or N:10N?

Regards,

-SB

[Tinkerer]

Simonbaker & moods,

the idea of the fully differential front end is intriguing. Experimenting only with the differential input preamp already got my head spinning. Things are just not the same anymore.

About stimulating the target:
What counts are ampere turns. Trade turns for amperes or the other way around, it is still the same.
Now, when we have a finite amount of energy stored in the inductor, we need to apply this energy in a way that is most efficient in stimulating the target.

Simonbaker has compared the target with a bell before.

What makes the bell ring better, a very short, hard blow, or a softer longer blow? (considering the same amount of energy)
I think it will depend on the size of the bell. Large bells ring at a lower frequency (TC?) The ideal bell stroke has to be adjusted to that frequency.

Now, what happens when you hit the large bell in quick succession twice?
You kill the ringing.

Analyse the Flyback. Does it not go in one direction first and then in the other direction? Which way stimulates and which way kills the eddy currents?

Tinkerer

[Aziz]

Hi Paul,

I made a quick & dirty balanced coil drive and had a look into the flyback voltages (no differential frontend yet).
The differential coil damping needs to be splitted or compensated to the common (ground) due to the inbalance of the coil halves due to the production tolerances (coil to coil capacitance, coil capacitances, inductivities). Particularly, the coil capacitances may differ and needs individual coil damping.

So I can confirm, what Carl has already found this earlier (month ago).

There will be slightly inductivity inbalance of the coils too. So probably a mixture of compensation adjustment of damping resistors and capacitors would be convenient.

Regards,
Aziz