# Problem with an exercise on the Differential Amplifier

Discussion in 'Homework Help' started by Helljoke, Nov 2, 2017.

1. ### Helljoke Thread Starter New Member

Nov 2, 2017
13
1
Hi guys! Sorry for my english wrote .
I don't understand why the value of the Opamp's Vout is too different from the calculate value.
Vout = (R7/R5) * (Vpos - Vneg), where Vpos=Vneg=2.5V. I know that, in the really case, there isn't the complete rejection of common mode, but i see that the different between ideal and really result is important. There's a Vout about of 2.40 mV, it's impossible!

2. ### WBahn Moderator

Mar 31, 2012
23,974
7,425
What is it that you are disturbed by? You seem to be trying to achieve an output of 0 V by applying the same voltage to both inputs. You aren't quite getting matched inputs and you aren't quite getting 0 V out.

Have you explored what output you get if you truly drive both opamp inputs with the same voltage? Sweeping a hard common mode signal across the entire input range might give you some insight.

And remember, this is all in simulation. When you start looking at effects down at this level, simulation and reality can differ by quite a bit.

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3. ### crutschow Expert

Mar 14, 2008
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6,267
No, it's quite possible.
Look at the input offset of the 741 op amp in its data sheet.
It's not that great and is likely what you are seeing at the output.
If you connect the 741 as a non-inverting follower, you should see a similar offset at the output.

If you want ideal results then you should use an ideal op amp.

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4. ### Helljoke Thread Starter New Member

Nov 2, 2017
13
1
Thank very much guys!
I'm studying an application report of TI to better understand this problem and solve it.