NEW !!! Full differential PI front end.

[moodz]

descrimination for moodzPI

... well the adventure continues ...

The last frontend v303b was perfectly good as a frontend for a standard PI however the ability to analyse the conductivity of the target would be very handy ..... some might call it descrimination.

So ... v400a is posted below. Note that the 'clamping diodes' have been replaced by two 5.6 volt zeners. The amplifier now has a gain of approx one. ( yes 1 ) and for those who build this circuit and examine the waveform out of the amp you will see very interesting behaviour at the peak for ferrous vs more conductive targets.
The post processing will now take place at two points ... at the peak and the base. ( ie two main channels to the cpu )
For those who cant be bothered building the circuit yet well ... Ok ... Ill post some scope pics on the next post. ( gotta get my points up for Carl's comp after all. )
cheerz all.

[moodz]

Hi Moodz:

What is the value of the damping resistor? It looks like 470R....?

Regards,

-SB

Simon ... 470 ohms.

I have also tried placing the damping resistor right at the coil .... seems to have a marginal benefit.

moodz.

It reads 470R to me

[simonbaker]

Hi Moodz:

What is the value of the damping resistor? It looks like 470R....?

Regards,

-SB

[moodz]

You still need a DC path on the + input.

Hi Carl .... its buried away in the data sheet for the CA3130 ( page 7 on the intersil specs ) ... to quote ...


DIFFERENTIAL DC VOLTAGE
(ACROSS TERMINALS 2 AND 3) = 0V
OUTPUT VOLTAGE = V+ / 2

If I isolate all inputs with low loss caps then this works.
So i dont need any biasing to ground. The DC value across the inputs is 0.
So this amp has no DC gain and the caps appear as very high impedances DC but low impedances to the pulse waveforms.

I have used this trick in a number of places.

Another example of this type of self biasing is to take a hcmos inverter and put a 1M feed back resistor from the output to the input. The output will settle to VCC / 2 and you can use the device as a type of crude amplifier.

Regards,

moodz.

[Carl-NC]

Notice that the opamp is fully isolated from DC at the inputs.

You still need a DC path on the + input.

[moodz]

There have been some changes as previously mentioned ....

Isolating diodes have been added. There is a gotcha with differential circuits and that is when noise gets into one side of the circuit and the will not reject it as it is not common mode. The 1n4004 diode on the mosfet will reverse bias as the flyback pulse falls ... why because ... the MOSFET looks like a capacitance to ground when it is off and this will charge via the diode ... when the flyback falls lower thant the peak charge the diode will reverse and the junction capacitance will isolate the relatively huge transfer capacitance of the MOSFET.

The CPU I am using ( dsPIC ) can easily drive the MOSFET directly ... however there is a relatively huge transfer capacitance from the gate to the drain and this will couple RF hash on the gate lead to the driven side of the differential circuit again unbalancing it and causing differential noise. NOTE ... this would also impact standard PI designs ... so check for noise entering via this path. The LM311 is a quick and dirty way to achieve high voltage ... high current drive and no RF hash from the CPU drive line.

Likewise D6 provides isolation of the coil after the Tx pulse ends. It also provides a bias point for the clamping diode system on C4

Notice that the opamp is fully isolated from DC at the inputs. After all we are amplifying pulses ( AC ) here. The opamp will bias nicely to approx mid v+ and you can see this in the scope pics in the previous post.
When the circuit is first turned on it may take 10 seconds or so to stabalise as C5 has to charge via R6 to provide the bias point.

At the moment the only voltage regulator is the +5 volt reg for the microprocessor. ( not shown in the schematic )

The coil I am using is fairly ordinary so I will be looking to speed this up next .... however I feel that the output is already good enough to feed into the A2D converter in the CPU for processing.

will post more info ....

any suggestions welcome.

moodz

[moodz]

updated circuit ....

v303a

enjoy ... moodz.



NO TARGET

20 cent coin

[moodz]

http://www.hayselectronics.com/parts.htm

Vasster .. your the man.

thanks very much .. moods

Why are you so aggressive!
I just wanted to learn moodz intentions about the future of his work. I am not the one promising to publish a field trial ready circuit.
BTW thanks a lot Moodz and Bugwiskers. You shared a lot in this forum.

If you have something as good as he has and you have spent a a lot of time and money why shouldn't he go commercial and get something back for his efforts?
I think he has shared a lot already don't you?
And there are those that don't want it to go commercial for obvious reasons!!!
I think that he and moodz should get together and each use the advantages of each others technology.

Really will you share your work here or go commercial like Bugwhiskers?

If you have something as good as he has and you have spent a a lot of time and money why shouldn't he go commercial and get something back for his efforts?
I think he has shared a lot already don't you?
And there are those that don't want it to go commercial for obvious reasons!!!
I think that he and moodz should get together and each use the advantages of each others technology.

So I will be publishing a field trial ready circuit when I get the following changes somewhere towards completion ...

Really will you share your work here or go commercial like Bugwhiskers?

[vasster]

http://www.hayselectronics.com/parts.htm

[moodz]

'I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.'

That means you dropped the caps C1, C3 and possibly IC2?
Can you please post your final circuit?

Hi Vasster ... I am currently making daily .. ( nearly hourly ) changes to the circuit ... at the backend. So I will be publishing a field trial ready circuit when I get the following changes somewhere towards completion ...

This is list of changes.

dPI (differential PI ) build status

1. Amp front end ( completed )

2. Remove all switching supplies ... everything runs from single DC supply anywhere between 8 and 40 volts. Actually I have not tried it at 40 volts ... have gone as far as 16 volts .... generating 800 volts differential on coil.

3. Reduced damping resistor to 500 ohms due to slight ringing of coil at 1000 ohms. ( completed )

4. Improved MOSFET drive. ( completed )

5. dsPIC code to sample and process front end amp o/p ( 90 % )

6. Layout PCB ( 10 % )

7. Source coil form, wand and electronics housing ( 0 % )
Does anyone know where I can source coil housing 'blanks' anywhere ??

8. Field trials ( 0 % )

Regards,

moodz

[vasster]

'I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.'

That means you dropped the caps C1, C3 and possibly IC2?
Can you please post your final circuit?