741 Op-amp summer circuit not working as expected

Reverse the output rails? You mean connect the opamp output to ground?

eagle1109 said:

I'm working on 741 op-amp summer circuit. I followed the circuits on google.

I did the circuit both on lab and simulation. But the results aren't as expected, I thought it will sum any voltage and add that to the outptut with setting the gain to be 1.

Here are the circuits I did:

1. Lab:

View attachment 197983

2. Simulation:

View attachment 197984


What I'm doing wrong ?

Click to expand...

1) You are paralleling V sources, not a good idea.
2) You have no virtual ground to perform the summing operation
3) You have wired shorts across the resistors, they no longer matter

To name a few.....

barry said:

Reverse the output rails? You mean connect the opamp output to ground?

Click to expand...

I'm sorry I take what I said back, I thought it's OK to supply a negative voltage to any device by just swithcing the rails of the signal and problem is solved.

But it turned out that in other situations it won't work; like, most common case for me is to send the signal to a microcontroller when I searched about reading negative voltages by ADC unit in a microcontroller isn't possible.

So I must invert the negative signal by using another op-amp:




Thanks to this YouTube tutorial: Using an Arduino to capture negative analog voltages.

You could always add a - DC input to a 4'th input to summer, and use that V
to level shift the ouput so that it is in + range on ouput of OpAmp. Then
in processor simply subtract thaut out oif reading. And in processor math
handle the sign inversion problem because of inverting summer behavior.

Just a thought, save an OpAmp.

eagle1109 said:

Could it be that the op-amp is summing the currents ?

Click to expand...

Yes ! But the input bias current needs to significantly less. All input current goes to the output, and the output voltage goes inverted to whatever it can, to null the input differential to the fixed reference on VIN+.

The VIN+ determines the value of VIN- when it is used for negative feedback. When the VIN+ is fixed, the inverting input is the only version to "sum current" with independent gain control of voltage. The opposite does not sum current as in independent voltage summer, and that’s the fundamental difference, because the Vin+ moves with each change whereas using Vin- for "Sum input" remains fixed Vref=Vin+

danadakk said:

You could always add a - DC input to a 4'th input to summer, and use that V
to level shift the ouput so that it is in + range on ouput of OpAmp. Then
in processor simply subtract thaut out oif reading. And in processor math
handle the sign inversion problem because of inverting summer behavior.

Just a thought, save an OpAmp.

Click to expand...

OK, I think I got you, but the problem the input voltages are working the other way around.

When I cancel one of the inputs, the output voltage decreases. Is that what you mean but doing the math in the code the same inversion going on the circuit ?



So, when I enable one of the inputs, the output voltage decreases.

Yes the solution is inverting, the extra input just to bias the single supply OpAmp
configuration into its output operating range. So with all outputs at 0 the bias value
raises the output to its max design value. Then the other inputs, as they increase, drive
the output negative from that high design point. To a lowest value, for processor
compatibility, to no lower, more negative, than ground.