OP-AMP Summing Circuit

Hi Nathan,

I think what are you after is a summing opamp in non-inverting configuration, where you want to be able to set the overall gain to 2/3. To achieve that, you need to add an extra 1K resistor, connected from ground to the inverting input of the opamp (pin 2). I'm not sure what is the purpose of R5 in your original circuit, anyway here is a version I made that I hope clarifies the circuit:

As for the gain, it can be calculated with the following formula:

   Vout=(1+R4R5 )(V1R1 +V2R2 +V3R3 1R1 +1R2 +1R3 )

where V1,V2,V3 are the inputs. Choosing R1=R2=R3=1K, semplifies the formula, leaving us with:

   Vout=(1+R4R5 )(V1+V2+V33 )

Now, making R4=R5=1K, we obtain the gain of 2/3 we were after. This way, cosidering the V1, V2 and V3 all range from 0 to +5V, their sum ranges from 0 to +15V, giving us a range from 0 to +10V for Vout.

I hope this answer your question.

Fabio.

This is a non ideal circuit - even after Fabio's suggestions, because the gain of each input is affected by the source impedance at the other inputs. It may be that this doesn't matter and the advantage of using only one op amp outweighs the problems.

Fabio shows the 741 powered by +/- 18V - you should NEVER do this - it is designed to work from +/- 15V and the 18V is the absolute maximum (for LM741C from TI) that should ever be applied, expect smoke at 18.01V.

But you should not be using a 741 in 2017 - the design is now 49 years old and there are better parts.

If you use a CMOS input op amp you could use Fabio's design but increase the value of R1,2 and 3 to 1M, you'll still need to keep the source impedance below 10k for reasonable errors but that would be a lot better.  You may as well increase R4 and R5 to 680k to reduce power consumption. You can't use resistors this big with the 741 because the input bias current is so high (spec worst case offset current = 200nA would give 140mV error at output).

I suppose if you MUST use the 741 you could use 100k input resistors and 68k for R4 and R5, but the combined effect of input bias current and input offset current could result in 24mV error on the output (worst case 25C, increases to 45mV over temperature).

Remember you must short any unused inputs to ground.

MK

Thanks Michael for the information I'm using a an  UA741CPUA741CP op-amp and when purchased from the supplier he told me that the max supply voltage the chip could handle was 25VAC I hope I didn't blow this chip up Looking at the spec sheet now I see a max voltage of 18V and then somewhere else in the data sheet saying 15V Anyways I was testing the circuit I posted in a simulation program called"iCircuit and using 1k resistors across the board with the three 5V inputs the output came to 10V but using another simulation program I got different results As you were saying about spikes in mV on the output I don't believe that low of voltage change will be an effect on what I'm using it for Many thanks for your suggestions If you say R5 isn't needed then I will delete it from the design I was just looking at some examples on the internet and it had R5 for some reason Is there anything else you may think I should know Thanks again

I also just tested both inverting and non-inverting methods and can't seem to get anything working. Its probably my chip thats fried.

Attached is an example of a few simulations I did.

The inverting configuration you used gives you Vout=-10.5V  because the ratio between the feedback resistor (7K) and the resistor used for the inputs (10k) is not exactly 2/3. If, for instance, you used 3K for the input resistors and 2K for the feedback, you would have got exactly -10V for Vout. For completeness, below you will find the formula to calculate Vout in case of an inverting configuration, so you can play with the resistors' values, to match your needs:

Vout=−(V1R1 +V2R2 +V3R3 )·Rƒ eedback

where R1 is the resistor on input 1, R2 on input 2 and R3 on input 3, Rfeedback is the resistor connecting opamp output pin to the inverting input.

P.S: I hope you haven't fried your 741 because of my example...

Thanks for your response Fabio,

No it wasn't because of your example. I went to an electronics store to buy some parts and asked specifically for a 25VAC op-amp. He told me this was rated for Max 30V and I thought to myself "perfect" because I have a 25VAC transformer at this other circuit I was going to be using this op-amp at. Unfortunately its not actually rated for the 30V and have to build a step-down transformer on the circuit board now

The project is to take the three 5V DC inputs (0-5V is a scaled voltage from a air flow measuring device). The circuit takes this voltage sums up the voltages and spits out a total voltage. Then in a computer program looks at the total voltage and scales it to a total airflow of the three inputs.

Sounds interesting. I take you will use some ADC to read the output of the summing circuit and convert in for the computer program. I would be tempted to suggest to avoid the summing circuit altogether, and use a microcontroller board like Arduino instead (for example even a basic Arduino Uno R3 would do), which can handle multiple 0-5V analog inputs signals. Then you would only need to interface your airflow measuring devices outputs with the microcontroller board analog inputs (keeping an eye on max current allowed) and write a small program that reads the voltage from the analog inputs and convert them to digital using the internal ADC, and just add the program logic to do the sum and trigger whatever action it is needed.