NEW !!! Full differential PI front end.

[Alexismex]

This happen many time they name it Syncronicity

[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

[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.

[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.

[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]

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.

[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]

... 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]

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


moodz

[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]

... 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.

That should read ... and the o/p self biases to approx HALF V supply ( ie 5 volts ) ...

moodz

[moodz]

v5 ommission

...eek .. should be a 1k resistor from top of Q1 to ground. Play with the value a bit ... depends on your MOSFET capacitance ... will improve the decay some.
Will publish in next schem update.

moodz.

[moodz]

v5 errata 2.

...eek .. should be a 1k resistor from top of Q1 to ground. Play with the value a bit ... depends on your MOSFET capacitance ... will improve the decay some.
Will publish in next schem update.

moodz.

D1, 2, 4, and 5 are 1n4148 ... not schottky as illustrated.

moodz

[moodz]

amp with ideal op amp

.. amp simulation with single supply, cap on bias leg and IDEAL opamp. Notice that the bias point is EXACTLY vsupply/2.

Supply = 10 volts.
Bias point = 5 volts.
Pulse input = 100uS duty cycle
Period = 200uS
Pulse amplitude =100 mv pp

I am happy enough that this self biasing is a valid design technique for AC amplifiers.

real opamps will exhibit a slight DC offset but if everything is AC coupled ... no problem.

Any comments welcome.

moodz

[Aziz]

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