Make sense to switch to bipolar operational?

Thread Starter

samudavid

Joined Jan 4, 2018
28
Hi everyone and thanks for your time.

I have an issue that is driving me mad and consuming the time my doctor says I should use to exercise...

I use this circuit in an inductive sensor, is a kind of low speed differentiator. First we have JFET a peak detector, with low leakage and then a comparator made with an instrumental amplifier (NOT THE EXACT PART IN SCHEMATIC - LT1001->INA 826, AD795 ->TL082).
It works well, but from time to time, the output signal "jumps" lets say a volt.
I've tried almost everything. I think that the issue is related to a bit of charge in the JFET gates due to the high impedance of R2. Do you think that it could be solved changing the JFET amplifiers for bipolar amplifiers?. To my knowledge, they are more immune to electrostatic noise although they have more leakage current (I can assume it). There is something really curious, if I touch the board, it jumps, even if I blow over the board!. Once I saw a signal conditioning circuit for the same sensor made out of bipolar transistors, and it worked smoothly.
Also, if you have any bipolar amplifier suggestion pin compatible with TL082 it would be great.

Any comments or ideas are welcome.

Thanks in advance

upload_2019-7-9_19-29-48_samudavid.png
 

KeithWalker

Joined Jul 10, 2017
456
Hi everyone and thanks for your time.

I have an issue that is driving me mad and consuming the time my doctor says I should use to exercise...

I use this circuit in an inductive sensor, is a kind of low speed differentiator. First we have JFET a peak detector, with low leakage and then a comparator made with an instrumental amplifier (NOT THE EXACT PART IN SCHEMATIC - LT1001->INA 826, AD795 ->TL082).
It works well, but from time to time, the output signal "jumps" lets say a volt.
I've tried almost everything. I think that the issue is related to a bit of charge in the JFET gates due to the high impedance of R2. Do you think that it could be solved changing the JFET amplifiers for bipolar amplifiers?. To my knowledge, they are more immune to electrostatic noise although they have more leakage current (I can assume it). There is something really curious, if I touch the board, it jumps, even if I blow over the board!. Once I saw a signal conditioning circuit for the same sensor made out of bipolar transistors, and it worked smoothly.
Also, if you have any bipolar amplifier suggestion pin compatible with TL082 it would be great.

Any comments or ideas are welcome.

Thanks in advance

View attachment 181326
There is no ground reference for the input signal. You probably need a 1 Mohm resistor between the input and circuit common.
 

Thread Starter

samudavid

Joined Jan 4, 2018
28
Thanks for your kind answer.

Sorry, but I should have add more detail to the schematic. There is a 100K resistor from input to ground and a 47K resistor from U2 output to ground and from U3 output.

I think that the problem is the FET opamp, just want to know if it makes sense and if could be solved with a bipolar.
 

Analog Ground

Joined Apr 24, 2019
169
I see nothing in the circuit to cause your "jump" symptom. Perhaps you have an intermittent or bad solder joint or a cracked trace. Is there only one board? If more than one, does another board have the problem? BTW, 1 Megohm for R2 is really not a large value.
 

Bordodynov

Joined May 20, 2015
2,419
I'd advise you to remove (shorten) the 100 Ohm resistor and put it in series with the capacitor. In my case, the electronic circuit will be more stable (less phase shift in the feedback loop). I always do this when building peak detectors. Perhaps your circuit is sensitive to external influences due to excessive phase shift in the feedback loop.
 

Audioguru

Joined Dec 20, 2007
11,251
Samudavid, your schematic shows opamps WITH NO SUPPLY VOLTAGE!
The supply voltage is VERY important for a TL082 because its minimum supply is 10V and if an input gets within about 4V from its negative supply voltage then its output goes as high as it can (called Opamp Phase Inversion). Therefore a TL082 always needs a negative supply in addition to its positive supply.
 

Attachments

Thread Starter

samudavid

Joined Jan 4, 2018
28
Thank you everyone for your kind answers.

I see nothing in the circuit to cause your "jump" symptom. Perhaps you have an intermittent or bad solder joint or a cracked trace. Is there only one board? If more than one, does another board have the problem? BTW, 1 Megohm for R2 is really not a large value.
As when order PCBs from my favourite chinese supplier it cost the same, 1 or 5 I tested 3 boards.


I'd advise you to remove (shorten) the 100 Ohm resistor and put it in series with the capacitor. In my case, the electronic circuit will be more stable (less phase shift in the feedback loop). I always do this when building peak detectors. Perhaps your circuit is sensitive to external influences due to excessive phase shift in the feedback loop.
I also tried this, and It didn't get better, anyway I'm going to try again as I also changed the original R2=7Mohm to 1Mohm and it got better. Maybe the two things combined could solve the issue.

Samudavid, your schematic shows opamps WITH NO SUPPLY VOLTAGE!
The supply voltage is VERY important for a TL082 because its minimum supply is 10V and if an input gets within about 4V from its negative supply voltage then its output goes as high as it can (called Opamp Phase Inversion). Therefore a TL082 always needs a negative supply in addition to its positive supply.
Thank you for your observation. I'm using a 0-24V power supply and the input signal is about 6 volts minimum. I also tried with an OPA2172 which is rail to rail.

The only thing left that occurs to me is to change the fet for a bipolar amplifier. I also tried with the lab power source, tried to isolate the input signal and applied a fixed voltage from a battery and a function generator... Do you think that the bipolar amplifier could be better?
 
Last edited:

ebeowulf17

Joined Aug 12, 2014
2,945
Thank you everyone for your kind answers.


As when order PCBs from my favourite chinese supplier it cost the same, 1 or 5 I tested 3 boards.



I also tried this, and It didn't get better, anyway I'm going to try again as I also changed the original R2=7Mohm to 1Mohm and it got better. Maybe the two things combined could solve the issue.


Thank you for your observation. I'm using a 0-24V power supply and the input signal is about 6 volts minimum. I also tried with an OPA2172 which is rail to rail.

The only thing left that occurs to me is to change the fet for a bipolar amplifier. I also tried with the lab power source, tried to isolate the input signal and applied a fixed voltage from a battery and a function generator... Do you think that the bipolar amplifier could be better?
Just guessing here, but what's the input impedance on the bipolar circuit that you said has worked well for this sensor? Is it possible that the 100k input resistor you're using is too high? What happens if you drop it to 10k?
 

Thread Starter

samudavid

Joined Jan 4, 2018
28
Hi and thank you for your help,

the output impedance of the source circuit is quite high and so if I lower the 100K resistor the signal will lower a bit too much. I aready tried it and didn't make a difference.
 

ebeowulf17

Joined Aug 12, 2014
2,945
Hi and thank you for your help,

the output impedance of the source circuit is quite high and so if I lower the 100K resistor the signal will lower a bit too much. I aready tried it and didn't make a difference.
Ok, sorry. I'm all out of ideas. It can't think of any reason why a bipolar op amp would solve this problem, but I'm no expert. My gut feeling is that the op amp isn't your problem, but I could be wrong.

Have you looked at the input and output with an oscilloscope? Can you confirm that these glitches are coming from the op amp, and not from the sensor or its associated wiring?

I don't see any harm in trying a bipolar system, but I'd hate to see you waste time and money if that's not the real problem.
 

BR-549

Joined Sep 22, 2013
4,938
Is the ground of the proceeding and succeeding circuits, the same physical ground as the imaged circuit?

Physically isolated/separated ground planes can acquire transient potentials.
 

Thread Starter

samudavid

Joined Jan 4, 2018
28
Thank you for all your effort,

The big deal with trying a bipolar is that I don't have any and the shipping costs are high. Anyway, just to discard, I'm going to try as soon as possible, if the problem persists, I don't know what else to try. I know that the problem is the circuit and not the sensor because I tried using a stable signal (a battery) and it jumped a few times.

There aren't separate ground planes, is a small board with a plane on the bottom layer.
 

Alec_t

Joined Sep 17, 2013
10,451
I'm using a 0-24V power supply
So the supply is not bipolar. Since you don't show how your "inductive sensor" is connected to the circuit I'm guessing that the sensor has an output which swings negative, driving the opamp input well below Vss. That can cause problems.
 
Top