# Disadvantages of Non-Inverting Summing Op Amp

• Poptart
In summary: For example, if you are trying to save board space, an inverting amplifier may be a better choice. On the other hand, if you are looking for a more stable signal, an non-inverting amplifier may be a better choice.
Poptart
Hi,

I was wondering what the disadvantage of using a non-inverting summer was rather than using an inverting summer. I was told that inverting summers are better, but not why they are.

I ask because I'm making a circuit that performs a center of mass calculation:

(V1 + V2) - (V3 + V4) = Vout
(V1 + V2 + V3 + V4)

and the IC that does the division step requires that the denominator is greater than the numerator, so using a non-inverting summer on the denominator seems to make the most sense to me.

The main advantage that comes to mind for using an inverting summer is that the input impedance is constant compared to a non-inverting.

Okay. Does that noticeably affect the output? What kind of precision can I expect with using one versus using the other?

Poptart said:
the IC that does the division step requires that the denominator is greater than the numerator, so using a non-inverting summer on the denominator seems to make the most sense to me.

You could use 2 stage op-amp design to get the job done with an inverting summer.

The second stage would simply be a unity gain inverting amplifier.

Last edited:
Oh, excellent. Such a simple solution and works perfectly since the AD706 I'm using for the weighting sum has two op amps on the chip as it is.

I'd still like to know what the functional difference is between the two summers is, but that's more of a curiosity thing than a necessary question :) Thanks a ton to both of you

Poptart said:
I'd still like to know what the functional difference is between the two summers is

The only thing that I can think of is exactly what Averagesupernova has already mentioned.

For a non-inverting summer, the input resistors do not connect to a virtual ground. Thus a current flows in each input resistor that is a function of the voltage at all inputs.

This makes it impossible to define the input resistance for anyone input unless all other inputs are grounded.

jegues said:
The only thing that I can think of is exactly what Averagesupernova has already mentioned.

For a non-inverting summer, the input resistors do not connect to a virtual ground. Thus a current flows in each input resistor that is a function of the voltage at all inputs.

This makes it impossible to define the input resistance for anyone input unless all other inputs are grounded.
That makes sense. So then (last question) that reasoning wouldn't apply to a non-inverting amplifier, correct? Since it only has one input tied directly to the op amp's positive terminal, a non-inverting amplifier is just as good as an inverting amplifier? Intuitively it seems to me like it would be, otherwise an inverting unity gain amplifier would need to follow it to achieve the same result (but I'll be the first to admit that my intuition's not perfect).

Thanks again :)

Poptart said:
That makes sense. So then (last question) that reasoning wouldn't apply to a non-inverting amplifier, correct? Since it only has one input tied directly to the op amp's positive terminal, a non-inverting amplifier is just as good as an inverting amplifier? Intuitively it seems to me like it would be, otherwise an inverting unity gain amplifier would need to follow it to achieve the same result (but I'll be the first to admit that my intuition's not perfect).

Thanks again :)

The non-inverting input of the operational amplifier will still need a path for DC to ground. This is usually obtained through the signal source, but nonetheless if this isn't the case, or be it that the source requires a certain load impedance, the circuit will require another resistor from the input to ground.

The preference to one or the other will depend on the specific design you are trying to achieve.

## 1. What is a non-inverting summing op amp?

A non-inverting summing op amp is an electronic component that combines multiple input signals and produces an output signal that is the sum of all the inputs. It is commonly used in circuit designs to add or average signals from multiple sources.

## 2. What are the disadvantages of using a non-inverting summing op amp?

One of the main disadvantages of using a non-inverting summing op amp is that it requires a dual-polarity power supply, which can add complexity and cost to the circuit design. Additionally, the op amp introduces noise and distortion, which can affect the accuracy of the output signal.

## 3. How does a non-inverting summing op amp differ from an inverting summing op amp?

A non-inverting summing op amp produces an output signal that is the sum of all the inputs, while an inverting summing op amp produces an output signal that is the negative sum of all the inputs. Additionally, the non-inverting configuration offers a high input impedance, while the inverting configuration offers a low input impedance.

## 4. Are there any ways to mitigate the disadvantages of a non-inverting summing op amp?

Yes, there are a few ways to mitigate the disadvantages of a non-inverting summing op amp. One way is to use a single-supply op amp instead of a dual-supply op amp, which eliminates the need for a dual-polarity power supply. Another way is to use precision op amps with low noise and distortion characteristics to reduce the impact on the output signal.

## 5. In what applications are non-inverting summing op amps commonly used?

Non-inverting summing op amps are commonly used in audio systems, such as mixers and amplifiers, to combine multiple input signals. They are also used in data acquisition systems to measure and process multiple sensor inputs. Additionally, they can be found in control systems and power supply circuits.

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