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Problem with adder circuit in opamp

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jjros

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Hi,
I have problems with adder circuit in opamp, with two input of 0,45V and 0,22V the output is 0,56V, what is the problem?, my circuit is suppled with +- 12VDC.
I thing the output should be 0,67V, is correct?, please is possible a circuit tested.
Thanks.
 

Re: Adder opamp

Most important: dc or ac signals?
(I am always surprised that some guys expect complete and correct answers without giving complete and sufficient information).
 

Re: Adder opamp

I have not clear the circuit!,
but the idea is,
two signals 0.22mVDC and 0.45mVDC

and I take out 0.67mVDC.

Thanks in advance.
 

Adder opamp

Unless we see your circuit, how can we help?

Your voltages seem to have changed to millivolts! That could be part of the problem.

Circuit please!

Keith.
 

Re: Adder opamp

keith1200rs said:
Unless we see your circuit, how can we help?

Your voltages seem to have changed to millivolts! That could be part of the problem.

Circuit please!

Keith.
Click to expand...

I am sure, that IS the problem! These voltages are in the order of the offset voltages at the opamp output. DC amplifiers are always problematic.
 

Re: Adder opamp

Thanks for yours answers, I checked the offset and as usual, the testing I've done with TL071 and TL082, the offset has not influenced.

This is schematic in gEDA.
 

Adder opamp

You need to fill in the resistor values and clarify whether it's volts or millivolts, but you real problem is that isn't an adder! An adder would sum the currents to the virtual earth on the INVERTING input.

You probably have a signal averager there.

Keith
 

Re: Adder opamp

hi
simple adder circuit first stage is adder second inverter
 

Re: Adder opamp

keith1200rs said:
You need to fill in the resistor values and clarify whether it's volts or millivolts, but you real problem is that isn't an adder! An adder would sum the currents to the virtual earth on the INVERTING input.
You probably have a signal averager there.

Keith
Click to expand...

Sorry, KEITH. But you can add voltages with this circuit. An adder does not necessarily need virtual earth.
The principle is as follows: All input resistors are equal and the ratio of both feedback resistors is unity (for two input signals) or two (for three input signals), respectively.
 

Adder opamp

Yes, I understand. It is an averaging circuit but if you multiply the average of 3 values by 3 then you have the sum. My mistake, sorry.

To come back to the original problem, offsets may be the issue if the voltages are now mV. I don't know the offset of the opamp but I suspect it is several mV.

Keith
 

Re: Adder opamp

keith1200rs said:
................
To come back to the original problem, offsets may be the issue if the voltages are now mV. I don't know the offset of the opamp but I suspect it is several mV.

Keith
Click to expand...

Yes, I agree. And - more than that - offset effects due to the dc currents are certainly not negligible.
 

Re: Adder opamp

Hi,

Ok, perfect, tested in circuit maker is exact, thank you very much.

Instead, test in Isis (Proteus), the result is disastrous.
 

Re: Adder opamp

jjros said:
Hi,

Ok, perfect, tested in circuit maker is exact, thank you very much.

Instead, test in Isis (Proteus), the result is disastrous.
Click to expand...

jjros, are you aware that simulators not always tell you the truth?
I've got the impression you were searching around until you have found a program with satisfying results. Look first into the opamp model if it has incorporated all the important features (input current, output impedance,...).
Be careful if other programs and other models show different results.
Otherwise you will be faced with surprising effects in reality (hardware).
 

Re: Adder opamp

Thank you , but I find it curious that the results are so different.
obviously with the same characteristics.
 

Re: Adder opamp

jjros said:
Thank you , but I find it curious that the results are so different.
obviously with the same characteristics.
Click to expand...

Results are different with the same characteristic? THIS sounds curious to me.
 

Re: Adder opamp

Yes, same characteristics, diferent results.
 

Re: Adder opamp

I don`t know if somebody is able to understand this sentence. Me not. Sorry.
 

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