# Voltage sense using summing amplifier

1. ## Voltage sense using summing amplifier

Hello

does anyone hava any idea how to solve this problem.

so, V1=Vin1 ~~ Vin2; V2=Vref in fig below

In my case Vin1=5V Vin2=5V, Vref=1.2V
I want to get Vout1=1.5V, Vout2=3.5V

but I am getting negative R4.
my question is that, is it possible to reduce Vin1 and Vin2? and how?

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2. ## Re: Voltage sense using summing amplifier

Hi,

Please use the same voltage naming as in the schematic. (Why do you make it more complicated than necessary?)
Please use the same resistor naming as in the schematic. (There is no R4 at all)

Usually when one uses "VRef" then it is when you want other voltages to be "with respect to it". If you want this, then you don't need a suming amplifier, but a difference amplifier.

Show your math, so we are able to correct it.

Klaus

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3. ## Re: Voltage sense using summing amplifier

I am sorry I made a mistake, I mean Rf2 is a negative value I am getting. when I use voltage drop at the input. I got a positive value from Rf2.

In my case Vin1=5V Vin2=13V, Vref=1.2V
I want to get Vout1=1.5V, Vout2=3.5V

When Vin is 5V V1 is 1.2V
When Vin is 13V V1 is 3V, in this case, Rf2 is positive.

is my circuit correct?

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4. ## Re: Voltage sense using summing amplifier

You don't say it's mandatory to use the math oriented analytical approach... but one trick you can try is to substitute a potentiometer for Rf1 & Rf2. This gives you a vast adjustable range of gain settings (lower than one and greater than one).

Also substitute a potentiometer for R1 & R2. This provides weighting for your input voltages V1 & V2. If you wish you can introduce a third voltage of the same or opposite polarity (through a suitable potentiometer), to provide level shifting.

Also consider you may need to rearrange things so as to feed your source voltages to the inverting input instead of the non-inverting.

5. ## Re: Voltage sense using summing amplifier

Hi,

Now two new resistors.
They make things complicated, because they involve new source impedance, that needs to be added to the input resistance R1.
Additionally the suming_point_voltage depends on the other input voltages.

The more simple circuit is the usual (inverting) suming amplifier, where all the input resistors are connected to the inverting_Opamp_input..(which usually has a constant voltage = 0V).

But at first give unambiguous informations:
This can be a complete formula like
V_out = A1 x V_in1 + A2 x 1.2V (replace A1 and A2 with your desired values)

Or give clear informations for different input situations, like:

Situation1:
* V_in1 = xxx V
* V2 = 1.2 V (as far as I understand this is always the case. Please correct, if necessary)
* Expected Vout = yyyy V

Situation2:
* V_in1 = xxx V
* V2 = 1.2 V
* Expected Vout = yyyy V

A simple situation for calculation is V_in1 = 0V..

Klaus

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