TINKERERS PREAMPLIFIER

[Tinkerer]

Differential input preamplifier.

Today I breadboarded the differential input preamplifier of post #6.
It works great.

I also tried the new 8" coils. Not so good.
There is a relationship between the diameters of the TX coil and the RX coil.
for greatest sensitivity, the Bucking coil should have the minimum turns.
As the coils get closer together, the amount of turns of the bucking coil has to increase and this reduces the sensitivity of the coil assembly.

Lesson learned: For optimal sensitivity, the coil assembly needs to be of optimal proportions.

Tinkerer

[hobbes_lives]

Today I breadboarded the differential input preamplifier of post #6.
It works great.

It seems that a differential RX has proven itself to be a good design around here recently.

It's hard to see the schematic that you provided. If I read the values right, and calculated correctly, is the gain of the circuit 3150? How's the noise? We should calculate the bandwidth as well, if you haven't done that already.

Nice work.

Edit: Can you post a better picture, or just send it to my email address?

[Tinkerer]

Differential input preamp

It seems that a differential RX has proven itself to be a good design around here recently.

It's hard to see the schematic that you provided. If I read the values right, and calculated correctly, is the gain of the circuit 3150? How's the noise? We should calculate the bandwidth as well, if you haven't done that already.

Nice work.

Edit: Can you post a better picture, or just send it to my email address?

thanks for the feedback, hobbes.

attached is the breadboarded version, except, the opamps on the breadboard are TLC72. Better, low noise opamps should improve the results.

The gain is variable with the R8 pot for experimenting purpose. The R and C's are the ones used. The value of the 150p capacitors depend a bit on the coil capacitance and choice of Opamp.

Tinkerer

[Aziz]

Hi Tinkeres,

a resistor is missing in the intrumentation amplifier design. The +input of IC3 should be connected with a resistor of same as R6 to ground.
Check this out.

Aziz

[Tinkerer]

Hi Tinkeres,

a resistor is missing in the intrumentation amplifier design. The +input of IC3 should be connected with a resistor of same as R6 to ground.
Check this out.

Aziz

Thanks Aziz, I added the 33k resistor from the +input IC3 to ground.

Tinkerer

[Aziz]

Thanks Aziz, I added the 33k resistor from the +input IC3 to ground.

Tinkerer

But don't forget the compensation capacitor too for the symmetry (I myself did forget to mention this).

Good work to start with such a differential pre-amplifier. Note: OP37 needs gain > 5 for stability. But they are well compensated with capacitors anyway.

Aziz

[Tinkerer]

But don't forget the compensation capacitor too for the symmetry (I myself did forget to mention this).

Good work to start with such a differential pre-amplifier. Note: OP37 needs gain > 5 for stability. But they are well compensated with capacitors anyway.

Aziz

Thanks again.
It was interesting to see how the resistor did not make any difference at all, but the capacitor made quite a difference in the signal shape.

Tinkerer

[Aziz]

Hi Tinkerer,

have a look at this: INA163
http://focus.ti.com/lit/ds/symlink/ina163.pdf

Very low noise (1nV/sqrtHz) and wide BW (sufficient enough for us). This could replace the above circuit totally with increased performance. The frequency compensation with capacitors can also be done due to pin outputs of the chip. Operates from +/-4.5V to +/-18V. Only SO-14 package. No DIP.

I want to try out this chip next time for a full differential balanced coil.

Aziz

[Tinkerer]

Hi Tinkerer,

have a look at this: INA163
http://focus.ti.com/lit/ds/symlink/ina163.pdf

Very low noise (1nV/sqrtHz) and wide BW (sufficient enough for us). This could replace the above circuit totally with increased performance. The frequency compensation with capacitors can also be done due to pin outputs of the chip. Operates from +/-4.5V to +/-18V. Only SO-14 package. No DIP.

I want to try out this chip next time for a full differential balanced coil.

Aziz

That looks like an excellent choice. There is still a lot to learn about using a differential input amplifier for the PI front end, but it definitely seems to be the way to go.

Tinkerer

[Tinkerer]

FINAL PREAMP

It seems that the preamp is fine the way it is now.
The noise level is good.
The sensitivity is good, so I move on to the further signal processing.
Attached is the final version. It has been tested on the breadboard, but not in the field yet.

COMMENTS AND SUGGESTIONS ARE STILL WELCOME.

Tinkerer

[nick_f]

COMMENTS AND SUGGESTIONS ARE STILL WELCOME.
Tinkerer

Hi Tinkerer,

The diagram looks very interesting, I hope to try it one day. Thank you very much for making it available.
I think the Sample & Hold chips are not marked in the diagram.
Regards,
Nicolae

[Monolith]

Hi Tinkerer,

The diagram looks very interesting, I hope to try it one day. Thank you very much for making it available.
I think the Sample & Hold chips are not marked in the diagram.
Regards,
Nicolae

Hi Nicolae,

The S&H used are NE5534. LF398 are equivalent.

Monolith

[Kev]

Hi Tinkerer,
I've been trying to understand what the characteristics of the differential front end are.

Does the centre tapped coil provide double the signal level of a single coil but 180 degrees out of phase?

Does the diff amp sum both of these in quadrature signals and subtract all others, so are you effectively getting double the gain and high frequency noise exclusion before the preamp?

Thank you for any clarification you can provide.

Cheers
Kev

[Tinkerer]

Hi Tinkerer,
I've been trying to understand what the characteristics of the differential front end are.

Does the centre tapped coil provide double the signal level of a single coil but 180 degrees out of phase?

Does the diff amp sum both of these in quadrature signals and subtract all others, so are you effectively getting double the gain and high frequency noise exclusion before the preamp?

Thank you for any clarification you can provide.

Cheers
Kev

Hi Kev,

I missed your post, but here is an answer.
To tell the truth, I do not know exactly all the differential preamp does, but try the following, it may give you a better answer than I can give you.

Take a very noisy differential signal input, invert one side and add it to the other side and observe what happened to the noise and the amplitude.

Aziz illustrated this experiment with scope pictures in a post some time ago.

All the best

Tinkerer

[moodz]

I am not Tinkerer but will provide my 2 cents ...

Does the centre tapped coil provide double the signal level of a single coil but 180 degrees out of phase?

Yes the signal is 180 out of phase but not double amplitude. The amplitude is only dependant on the coil turns and field strength. The centre tap acts as a voltage reference point if you need to measure the signal relative to a real device like an amplifier.

Does the diff amp sum both of these in quadrature signals and subtract all others, so are you effectively getting double the gain and high frequency noise exclusion before the preamp?

A quadrature signal is a phase diff of 90 degrees not 180. The signals at the coil consist of common mode and difference ( 180 degree signals ) . Common mode signals are typically coupled via capacitance ...diff signals produced by magnetic fields inducing currents in the coil. A differential amplifier will usually have a property called common mode rejection ... for example if the signal at the two inputs was 1.1 and 1.0 volts and the gain = 1 then the output of the amplifier is 0.1 volts because the common mode 1.0 volt on the input signal has been rejected. So voltage coupled ( capacitive ) noise coupled to the coil will be rejected. How does the centre tap assist. It provides a voltage ( or ground ) reference and helps limit the floating voltage that might exceed the common mode rejection capability of the amplifier. In some amplifier configurations it provides bias for the amplifier to work correctly.

Thank you for any clarification you can provide.