NEW !!! Full differential PI front end.

[moodz]

... and now with single supply 10 volts ... and cap .... and the o/p self biases to approx V supply ( ie 5 volts ) and the pulse train looks OK.

... any comments welcome.

regards

moodz.

[moodz]

now with single supply and NO cap .... doh lost half the signal on the o/p


moodz

[moodz]

... and with the cap removed the amp no longer self biases but clamps the output to average ground ... which would not be good in a single supply amp as half the waveform would be below ground so to speak ... thats why I have the cap.

moodz

[moodz]

Hi Carl .... you could be right .... I will do some specific tests and post results.

cheerz moodz.

Well ... I did a quick sim on LTspice ... and it works OK. The amp self biases to mid supply range ... in this case near 0 volts because of + / - 5 volt supply ... and it handles AC no problemo. ( There is a small DC offset in the ouput however if I feed the next stage through a cap this will be removed.) In this case a 100 mv pulse at 100 uS ... unity gain. The caps are all ideal ... no leakage simulated. Only the amp is not ideal.

I am pretty sure this type of amplifier is used in biomedical applications where there must be no DC path to the patient ... I just cant seem to google an example of it apart from some stuff I have built.

note this is just an amp I picked from the LT lineup ... nothing special ... GBW = 18 Mhz.

Any comments ?

[moodz]

Meant to follow up before... are you sure about this? Even a CMOS opamp needs a DC path to ground for the inputs. I think you've gotten lucky, probably C5 has enough leakage (effective parallel R) to do the job. A different cap might not work. I think this is highly risky.



Yes, but the 1M resistor offers a DC path to the input. Add a cap in series, and this trick doesn't work.

- Carl

Hi Carl .... you could be right .... I will do some specific tests and post results.

cheerz moodz.

[Carl-NC]

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.

Meant to follow up before... are you sure about this? Even a CMOS opamp needs a DC path to ground for the inputs. I think you've gotten lucky, probably C5 has enough leakage (effective parallel R) to do the job. A different cap might not work. I think this is highly risky.

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.

Yes, but the 1M resistor offers a DC path to the input. Add a cap in series, and this trick doesn't work.

- Carl

[moodz]

version 5

More changes ...

0. Renamed from moodzPI to ZPi because the idea really started with Aziz so the Z is common letter to our names.

1. MOSFET driver turns on soft but shuts off harder. However now must invert tx pulse ... oh well .. changed one line in dspic code.

2. Got a separate drive cable and balanced receive cable from search head ... I think you can figure out what the big circle is The TX circuit and the RX circuit can now live in separate places on the PCB .... no naaasty volts and amps near the rx actives.

3. The two diodes D3 & D6 'float' the coil away from the cable / mosfet capacitance during the decay .... take one or both out and see what happens to your decay time ... and your noise.

4. Obviously dont put the coil head components on a PCB .. well not a PCB with big areas of copper. Suggest you air wire then pot in epoxy. Keep all leads short as poss. Place outside edge of loop.

This circuit provides excellent noise and hum resistance whilst providing good sensitivity and improved decay.

here it is ....



cheerz from moodz.

[moodz]

Hi Moodz,

Try changing the value of R5 from 1M down to 270k maybe as high as 500k and C7 down to 2.7pf (or remove) then diode D6 to BYV95C (an oldy but goody).

These changes will help the settling time of the opamp. I lashed up the circuit and used a CA3140 ran a simulation which matched closely with actual prototype.
Also used IRF840 for switching FET. Coil was 2x 300uH and 1K damping resistor.

I am playing with some more ideas and will post once I test them out.


Cheers

Brian K

Thanks Brian ... I will give that a go .... the 3130 becomes more sensitive at low gains as the GBW goes up.

Regards,

Moodz.

[Muntari]

Hi Moodz,

Try changing the value of R5 from 1M down to 270k maybe as high as 500k and C7 down to 2.7pf (or remove) then diode D6 to BYV95C (an oldy but goody).

These changes will help the settling time of the opamp. I lashed up the circuit and used a CA3140 ran a simulation which matched closely with actual prototype.
Also used IRF840 for switching FET. Coil was 2x 300uH and 1K damping resistor.

I am playing with some more ideas and will post once I test them out.


Cheers

Brian K

[Alexismex]

This happen many time they name it Syncronicity

[moodz]

Thanks ...

Hi moodz,



Please keep on good working. You have the instruments for this and you are quick in doing this... And you made more improvements on the idea.

I am looking forward to your results..
Regards,
Aziz

Thanks Aziz I honestly had not seen the circuit you published and I have come up with almost the identical design ... unbelievable !! I read somewhere that if you come up with an original design then someone somewhere else in the world will be working on similiar thoughts at the same time ... this happens in science many times ...I dont know if anyone has studied it ... it would make a good thesis. Maybe there is a collective consciousness ..
I will continue to work on this but the credit for any innovation must go to you .

regards,

paul.

[Aziz]

Hi moodz,

Aziz ... I had not seen this thread till now .... you already proposed the idea of differential front end.

http://goldprospecting.invisionplus.net/?mforum=goldprospecting&showtopic=2227

However the simple diode clamping proposed works ok in SPICE but not in practice as the +ve and -ve flybacks are not exactly balanced.

Good work ... I think that this should be pursued anyway and I agree with all your analysis.

Regards,

Moodz.

Please keep on good working. You have the instruments for this and you are quick in doing this... And you made more improvements on the idea.

I am looking forward to your results..
Regards,
Aziz

[moodz]

ooh ... it was already invented by Aziz

Aziz ... I had not seen this thread till now .... you already proposed the idea of differential front end.

http://goldprospecting.invisionplus.net/?mforum=goldprospecting&showtopic=2227

However the simple diode clamping proposed works ok in SPICE but not in practice as the +ve and -ve flybacks are not exactly balanced.

Good work ... I think that this should be pursued anyway and I agree with all your analysis.

Regards,

Moodz.

[moodz]

Oh yeh ...
Both targets 10 inch square.
Tx pulse = 100uS
Coil the same one as per schem.

moodz

[moodz]

descrim pics for moodzPI v400a

well here they are ...

First one below is NO TARGET. o/p from amp in v400a schematic.


Second below is copper target. Note the base response but the peak is a little lower ( not much change )


Third is below with steel target. Note BOTH the base and the peak get 'fatter' ... now you should be able to guess where the two places where the post processor will be sampling.


cheerz from moodz.