# Is it possible to make a non-inverting difference amplifier?

For a crude current sense application I am wondering whether it is possible to build a non-inverting difference amplifier without common mode offset on the output.

Reason: I would like to avoid having to create a negative supply voltage just for this function.

I.e. I am looking for the following output: Vout = gain * (V2 - V1) where V2 > V1

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"Inverting" and "non-inverting" make no sense applied to a difference amplifier. It always takes one input minus the other times the gain. You can change the polarity by swapping the inputs. – Olin Lathrop May 2 '12 at 21:50
Make sure you have a look at current sense amps like ZXCT1009. – markrages May 3 '12 at 0:44
@markrages sorry, I've involountarily "copied" your comment in my answer, but without even reading it: the content was coming from an old question of mine...and the answer there was yours XD – clabacchio May 3 '12 at 11:28

I don't see any problem. Connect your V1 and V2 as shown here

and make

gain$=\dfrac{R_f}{R_1}=\dfrac{R_g}{R_2}$.

Use precision resistors, to have an acceptable common-mode rejection ratio. If you need a high CMRR, use an integrated difference amplifier.

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If what you need is just high CMRR, an instrumentation amplifier may be the best choice. Despite the name, you can use it everywhere and it behaves like a fully differential amplifier (usable also in cases where the common mode input is hundred times bigger than the signal).

The conceptual schematic of that amplifier is this:

$$V_{OUT} = \left(1 + \dfrac{2 \cdot R_1}{R_{GAIN}}\right) \dfrac{R_3}{R_2} (V_2 - V_1)$$

As you can see, the symmetric structure gives a great performance in rejecting common mode, and the offset of the third op-amp is made less important by the gain in the previous stages.

Most (if not all) the amplifiers have offset correction, which can improved also using an external resistor; they have also programmable gain, also using a resistor.

Here you have a table from Analog Devices where you can choose the proper one.

### Update

For measuring currents, you have also the choice of dedicated current sense amplifiers or, even more appropriate, high-side current monitors (like markrages suggested in the comment and here).

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TI's INA21x family is a very low offset high-side current monitor that I've had a great deal of success with. – Adam Lawrence May 3 '12 at 12:39

I agree with clabacchio's instrumentation amplifier solution to get a good CMRR. You can roll your own, but integrated InAmps are available at reasonable prices. Integrates ones will also have better resistor matching than you get with discrete components.

A word of warning, though, and for that we need the schematic:

Chances are that you use a high side current sensor, where $V_2$ is connected to $V_+$. If that same $V_+$ is the positive supply for your InAmp there's a problem. If $V_2$ > $V_1$ then there will flow a current through $R_1$/$R_{GAIN}$/$R_1$ from the bottom opamp to the top opamp. To make the inverting input of the bottom opamp equal to the non-inverting input ($V_2$) the output has to to go higher than that, and if $V_2$ = $V_+$ it can't do that. So the InAmp won't accept input voltages all the way to the rails.

The common difference amplifier from Telaclavo's answer doesn't have this limitation since the resistor divider $R_2$/$R_g$ will bring $V_2$ down from $V_+$.
For minimum offset error choose $R_1$ = $R_2$ and $R_f$ = $R_g$.

The AD820 is rail-to-rail output and has offset null inputs.

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Whoops I didn't notice markrages' comment, I've taken the components from here :) nice catch anyway! – clabacchio May 3 '12 at 11:25
@clabacchio - Always read the comments, you never know you'll find something interesting in them. Let this be a lesson to you! :-) – stevenvh May 3 '12 at 11:40
The amazing thing is that I've taken the component name from a post of him to an old question of mine :) – clabacchio May 3 '12 at 11:50
@clabacchio - Yes, I know, I was just reading it. Oddly enough he mentions the Zetex, but comes with a more evolved solution of his own. That Zetex looks sexy! – stevenvh May 3 '12 at 11:54