# Differential Amplifier help

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#### bubble_d

##### Junior Member level 3
Hi everyone.

So need help finding the voltage diffrence between V1 and V2 where the voltages across a series of 4 3.7 voltage cells. So essentially If I am looking at the first cell, the voltage at V1 terminal of the op-amp is 14.8 = 3.7*4 and the voltage at v2=11.1=3.7*3. I want the output voltage to show me 3.7 Volts, but All I am getting is 11.1!

Also note, its a single source 15 volt supply.

Here is the multisim design I am working with.

[EDIT]

WOW, EPIC FAIL ON MY PART

I used A LM318 OP-AMP instead and I got it to give me a gain of 1.

All done, I just need to choose between a 741 or 318. I know 318 is much better, but will it be sufficient for my needs.

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#### keith1200rs

##### Super Moderator
Staff member
I cannot work out what you are doing with the INA105. It is a precision unity gain instrumentation amplifier with built in resistors, you don't have direct access to the internal +/-IN inputs so you cannot ignore the internal resistors. You either need to connect the device as it is intended to be used or use a different device. An opamp would probably do what you need - you don't really need the attributes of an instrumentation amplifier.

Keith

#### FvM

##### Super Moderator
Staff member
Why are you setting the resistors for an amplifier gain of 10 when you want a gain of 1? Why do you place external resistors instead of using the internal resistors provided with INA105? https://www.ti.com/lit/ds/symlink/ina105.pdf

#### bubble_d

##### Junior Member level 3
I was trying everything I can, Even when I connect the two voltages directly to the 2 inputs I get the same 11.1 V output. I am unsure what I am doing wrong, Does someone have instructions to setting up A INA105KP? Or is there some other op-amp I can use that will give me unity gain and show me the diffrence between two voltages with a single supply?

#### FvM

##### Super Moderator
Staff member
Does someone have instructions to setting up A INA105KP?
It's all in the data sheet.

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