Disadvantages of Non-Inverting Summing Op Amp

[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.

Thanks in advance :)

 

[Averagesupernova]

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

 

[Poptart]

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

 

[jegues]

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.

 

[Poptart]

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

 

[jegues]

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.

 

[Poptart]

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 :)

 

[jegues]

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.

Non-inverting amplifiers and inverting amplifiers have both their disadvantages and advantages.

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