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Acquire ouput signal from a uA741 op. amplifier

Hi.

I'd like to study this circuit that use a uA741 Op amplifier powered by a single power source (12V).

For this purpose I use as input sinusoidal signal generated by means a function generator.

Then I'd like to acquire the output (amplified) signal in labview by means an USB 6000 interface:

Circuit.png

Because the operational amplifier is powered by a single DC source (12V) the two 10 k resistors create a fictitious ground so that I can power my uA741 with the requested +6 V (Vcc) and -6 V (-Vcc).

The circuit works fine in Multisim.

The problem is that in the real world, when I try to acquire the output signal I can't see any signal.

Trying to troublesoot the circuit I've noted that when I plug the USB cable of my USB 6000 device in the PC, the dual source (+6 and -6V) no longer exists and I can see only +12 trough the first resistor and 0 volts trough the second resistor.

Any idea?

Thanks in advance.

 

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Sounds like a question that an undergraduate Electrical Engineer could help diagnose.  I notice there was no schematic attached, and no way to tell if the wiring in the schematic matched the wires and unlabeled parts in the picture.

 

When voltages shift from ±6v to 0 - 12v, it's often because "Ground" has been re-defined by having Ground on device 1 be connected to, say, -6v on a "floating" Device 2.

 

Bob Schor

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Hi Bob.

Thanks for your reply.

Here is the full circuit where the green arrows indicate the ficticious ground (due to the single power source to power the Op. Amplifier):

Circuit MS.png

 

To troubleshoot purposes the previous immage of the breadboard is without the resistor R2, R3 and R6, but even with this resistors the result is the same.

I don't have in this Covid19 time an available real Oscilloscope (I'm a Teacher and our schools are currently closed) but I think that is anything related to the USB 6000 device connected to the output of the circuit.

 

 

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So when you connect the USB-6002 to the PC's USB port, that is when the voltages change?

 

Are you using single ended or differential input mode to the USB-6002?

Is the PC a desktop PC or a laptop?

If a laptop, is it plugged in?

If a laptop, and you don't plug it in, but run it off of its battery, do you still have this voltage issue?

 

Going back to that Bob said, I wondering if you have drawn down your artificial ground that it is 6V above earth ground by connecting it to the PC.  That via the 6002's ground to USB ground, to earth ground via the PC's power supply system.

 

If you don't connect the USB-6002 to the circuit, and take a voltmeter to measure the potential difference between your 6002's negative input and where you would be attaching it to the circuit, do you see about 6V?  If this is a laptop and unplug it from the power cord so it runs off of battery, is it still 6V or something else?  Because I'm thinking that a laptop running off of a battery has a ground potential that floats relative to earth.

 

I'd be careful with all of this, you don't want to risk blowing up the 6002 or a PC's USB port.

 

If you aren't using differential input, you definitely should be.

Maybe you could also use some sort of opto-isolator circuit between the circuit and the USB.

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Hi RavensFan.

Thanks for your reply.

I've my USB 6000 (not 6002) connected via USB port to a desktop PC.

I've not understand why you say "...I'd be careful with all of this, you don't want to risk blowing up the 6002 or a PC's USB port..."

Actually I'm using an RSE input (AI0 and AI GND):

USB 6000.png

Reading the user guide I haven't really understood how to configure another kind of input.

 

 

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@mforbiz wrote:

Hi RavensFan.

Thanks for your reply.

I've my USB 6000 (not 6002) connected via USB port to a desktop PC.

I've not understand why you say "...I'd be careful with all of this, you don't want to risk blowing up the 6002 or a PC's USB port..."

Actually I'm using an RSE input (AI0 and AI GND):

USB 6000.png

Reading the user guide I haven't really understood how to configure another kind of input.

 

 


Sorry about that. I'm not sure why I thought it was a 6002.  Maybe that was the link I stumble don first when I searched for the 6000 manual.  It looks like the 6000 does not have differential inputs.

 

As for "I've not understand why you say "...I'd be careful with all of this, you don't want to risk blowing up the 6002 or a PC's USB port..."" ,  the issue is you've moved the ground 6V so it is no longer the same potential as the PC ground.  Imagine what happens if you shorted a 6V battery, a lot of current is going to flow from the 6V to the ground.  Well, something like that could be happening to the USB port.

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