I'm so confused with my OPAMP BUFFER malfunction.

Thread Starter

earlycastle

Joined Apr 18, 2021
5
Sorry for my bad english but,..

I'm trying to sensor current amount flow through my BLDC motor.

So I've configured the circuit like this.

1.png

And..

From +24V power source, the current flows through BLDC motor, and the current flows out through its gnd line.
2.png
And it connected to my current sensing resistor.

And the voltage difference between the resistor, I've configured instrumentation amplifier.


But,.. FROM THE START,

Look at the voltage of the buffer (U1D)
3.png

I SWEAR, I've check the voltage from my U1D pin12, with my FLUKE 15B Digital Multimeter, and it was 0.087V

BUT,

The voltage shown at pin13(U1.13), it was 4.33V (3.72V when I've run BLDC motor).

How can it be????


I'm very confused. And I have nowhere to ask about the problem like this. So I've googled about it, and I've found here.
I beg your help badly.

Thanks for reading.
 

Audioguru again

Joined Oct 21, 2019
3,505
The TL07x and TL08x opamps have a minimum supply of 7V.
They have a problem called Opamp Phase Inversion where the opamp output goes as high as it can if an input gets within a few volts from its negative supply. So when your opamp input goes lower than about 3V or 4V the output goes high.
Here is what happens with a signal:
 

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Thread Starter

earlycastle

Joined Apr 18, 2021
5
The TL07x and TL08x opamps have a minimum supply of 7V.
They have a problem called Opamp Phase Inversion where the opamp output goes as high as it can if an input gets within a few volts from its negative supply. So when your opamp input goes lower than about 3V or 4V the output goes high.
Here is what happens with a signal:
Didn't know about that. Great. Thank you. I'll try to find out more information about that.
 
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